BIOCHEMISTRY DISCUSSION 6
Leahh
16Glycolysis.pptxBiochemistry: A Short Course Fourth Edition CHAPTER 16 Glycolysis
Tymoczko • Berg • Gatto • Stryer
© 2019 Macmillan Learning
Created by Brett Barbaro
All right, so now we’re going to get down to the real nuts and bolts here - GLYCOLYSIS. Now glycolysis is, of course, an extremely important function. There are a lot more functions that take place in the cell, but we’re going to go into glycolysis in great detail because it is so important, and also we need to go into something in great detail, and glycolysis is a good candidate for that.
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Chapter 16: Outline
16.1 Glycolysis Is an Energy-Conversion Pathway
16.2 NAD+ Is Regenerated from the Metabolism of Pyruvate
16.3 Fructose and Galactose Are Converted into Glycolytic Intermediates
16.4 The Glycolytic Pathway Is Tightly Controlled
16.5 Metabolism in Context: Glycolysis Helps Pancreatic Beta Cells Sense Glucose
Created by Brett Barbaro
Glycolysis is an energy conversion pathway. It basically converts the energy that was stored in glucose into ATP. Or actually, glycolysis is more about just the initial breakdown of this glucose, but we’ll get into more a bit later. NAD, which we introduced in the last chapter, is regenerated from the metabolism of pyruvate and that’s in the glycolytic cycle. Pyruvate is one of the end products of glycolysis. Fructose and galactose, which are very similar to glucose - and remember, because this is glycolysis, this is just the breaking apart of glucose - fructose and galactose can be actually converted into intermediates that are part of the glycolytic cycle. We’re going to talk about regulation of the glycolytic pathway, and then we’re going to talk about some of the ways that glycolysis affects the hormone release in cells.
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Glucose
Why is glucose such a prominent fuel in all life forms?
Glucose may have been available for primitive biochemical systems because it can form under prebiotic conditions.
Glucose is the most stable hexose.
Glucose has a low tendency to nonenzymatically glycosylate proteins.
Created by Brett Barbaro
So - why glucose? Glucose is important for pretty much all life forms. And there are some theoretical ideas that it may have been available in the very beginning, before life existed, just because it is so stable {it can be made from formaldehyde under prebiotic conditions}. It’s a very stable hexose, and a hexose is a very stable form of sugar because it’s circular and it therefore has fewer points that it can be attacked. And a 6-membered ring is one of the most stable rings that you can make in organic chemistry. So glucose is just something that is very easy and useful. But it also has a low tendency to attach to proteins just randomly - there are other sugars that exist that do modify proteins just randomly, and you wouldn’t want those in your body because then you wouldn’t be able to control the regulation of these proteins using glycolytic modification.
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ANOMERS
ANOMERIC CARBON
Many Common Sugars Exist in Cyclic Forms
A solution of glucose at equilibrium contains about one-third α anomer, two-thirds β anomer, and 1% open chain. BECAUSE IN BETA, THE HYDROXYL GROUPS ARE ALL EQUATORIAL AND THEREFORE IT’S MORE STABLE! THAT’S WHY IT’S USED IN CELLULOSE INSTEAD OF ALPHA! AND WHY WE CAN’T DIGEST IT!
Created by Brett Barbaro
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Figure 10.3 Pyranose formation. The open-chain form of glucose cyclizes when the C-5 hydroxyl group attacks carbon atom C-1 of the aldehyde group to form an intramolecular hemiacetal. Two anomeric forms, designated α and β, can result.
So on the left we see an open chain form of glucose. And that red hydroxyl group attacks the blue carbonyl group up at the top, and that would produce a ring formation. This ring formation could either be “alpha” or “beta”. The beta form is more stable, because in the beta form all of the hydroxyl groups are equatorial and therefore they’re farther away from each other. And so, in a mixture at equilibrium, you’d find about ⅔ beta and ⅓ alpha, and very little open chain because the cyclical forms are more stable. But this is a good point - first of all, there is more of the beta going around, and that’s why it’s used in cellulose instead of the alpha form. There’s a lot more of it and it’s more stable. But because it’s more stable, it’s also more difficult to digest.
Polysaccharide Structures
Created by Brett Barbaro
Figure 10.14 Glycosidic bonds determine polysaccharide structure. The β-1,4 linkages favor straight chains, which are optimal for structural purposes. The α-1,4 linkages favor bent structures, which are more suitable for storage.
Here again is the picture of cellulose and all of these glucoses are connected with beta-1,4 linkages. And they form a very tight network which is difficult for enzymes to penetrate.
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Polysaccharide Structures
Created by Brett Barbaro
Figure 10.14 Glycosidic bonds determine polysaccharide structure. The β-1,4 linkages favor straight chains, which are optimal for structural purposes. The α-1,4 linkages favor bent structures, which are more suitable for storage.
Alpha-1,4 linkages, like you find in starch and glycogen, however, produce a helical, more open conformation that can be easily hydrated and is also more accessible to digestive enzymes.
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Section 16.1 Glycolysis Is an Energy-Conversion Pathway
Learning objective 1: Describe how ATP is generated in glycolysis.
Glycolysis converts one molecule of glucose to two molecules of pyruvate with the generation of two molecules of ATP.
Glycolysis can be thought of as occurring in two stages:
Stage 1 traps glucose in the cell and modifies it so that it can be cleaved into a pair of phosphorylated 3-carbon compounds.
Stage 2 oxidizes the 3-carbon compounds to pyruvate while generating two molecules of ATP.
Created by Brett Barbaro
So after you’ve released your glucose from your glycogen, it’s time to start converting it into energy. And with glycolysis, you’re able to take a molecule of glucose and end up with two molecules of ATP for every glucose that you put in there. And we’ll talk about glycolysis as going through two separate stages. The first stage basically is to capture the glucose and to modify it so it can be broken down into smaller parts, and it cleaves it into a pair of identical phosphorylated 3-carbon compounds. And then the second stage is the breakdown of these three carbon compounds to pyruvate, and that’s when you generate the 2 molecules of ATP. Interestingly, the first stage actually requires the breakdown of ATP because you’re doing some non-favorable modifications.
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Diagram of the Stages of Glycolysis
Created by Brett Barbaro
Figure 16.1 Stages of glycolysis. The glycolytic pathway can be divided into two stages: (1) glucose is trapped, destabilized, and cleaved into two interconvertible three-carbon molecules, generated by the cleavage of six-carbon fructose and (2) the three-carbon units are oxidized to pyruvate, generating ATP.
Here’s an overall picture of all of the things that are going to happen in glycolysis. I think the images are probably too small for you to see clearly, but we’ll zoom in on them. But overall, just want you to notice there are 11 different substrates and there are 10 different enzymes that are going to be involved in this process.
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http://www.rcsb.org/pdb/education_discussion/molecule_of_the_month/poster_full.pdf
These are SOME examples of enzymes in the glycolytic pathway. Many of these are from bacteria. Each organism has its own structural variations, but all organisms perform the same functions.
Enzymes in the glycolytic pathway
Created by Brett Barbaro
And here’s a quick look at some of the enzymes that are responsible for the glycolytic pathway. And these are primarily from bacteria, but you know, they’re all kind of similar in their form and function. An interesting point, just like to point out it’s not really necessarily something that you need to understand for glycolysis but take a look at these molecules - I think you’ll notice that all of them seem to have some sort of symmetry. And that is just very common in biochemistry. When you have enzymes, they tend to group together to form these complexes that are made up out of more than one subunit. I think that’s the case in all of these except for phosphoglycerate kinase.
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Glycolysis - Stage 1
Created by Brett Barbaro
All right, so the first stage of glycolysis. You start with the glucose molecule up on the top, and the first thing you do is you add a phosphate group to it. That phosphate group will help keep it in the cell. The second step is a simple isomerization. It’s taking the 6 membered ring and turning it into a 5 membered ring called fructose 6-phosphate. And now another molecule of ATP needs to be hydrolyzed to attach another phosphate to the structure creating fructose 1,6-bisphosphate. And at this point, aldolase breaks down the fructose 1,6-bisphosphate into two 3 carbon members called dihydroxyacetone phosphate (or DHAP) and glyceraldehyde 3-phosphate. These structures are isomers of each other and can be interconverted with one another using the enzyme triose phosphate isomerase.
So just quickly note - it’s taken the input of two ATPs to get you to this point. But now we’re ready to break them down even further.
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Glycolysis - Stage 2
At this point there are 2 of these - one has been converted from DHAP.
Created by Brett Barbaro
Glyceraldehyde 3-phosphate is the form that continues on with glycolysis. And so some DHAP is around, but it needs to be changed to glyceraldehyde 3-phosphate before it can be used in glycolysis anymore.
The next stage is glyceraldehyde 3-phosphate dehydrogenase, which means the removal of some hydrogens. It also includes the addition of an inorganic phosphate, so you don’t need to sacrifice an ATP for this one. It creates 1,3-bisphosphoglycerate - which as you might imagine, having two phosphate groups on it, is quite unstable. Instability means that there is energy in there that can be harvested.
So the next step harvests some of that energy, converting ADP into ATP, and that’s phosphoglycerate kinase. Now an interesting point about the word, “phosphoglycerate kinase” - what does a kinase do? It adds a phosphate group to things. But what are we doing here? We’re removing a phosphate group from things. Phosphoglycerate kinase catalyzes both of those reactions, and you can see the arrows going back and forth. And that’s the case for almost all of these steps. These enzymes can go back and forth, so the actual flow of molecules through this process is governed in large part by the concentrations of the substrates. And why is it named “phosphoglycerate kinase” instead of “1,3 bisphosphoglycerate phosphatase”? It probably has to do with, historically speaking, when it was discovered - it was probably first identified as something that can add a phosphate group to 3-phosphoglycerate. And so, it got named by whoever discovered that, and he called it a kinase, and that name has stuck. And since then, people have been using it over and over again. That is how a lot of things get named in biochemistry, and it can be confusing.
But the overall effect here is that you’ve got, starting at the top with the glyceraldehyde 3-phosphate, a carbon-hydrogen bond and a carbon{-oxygen} double bond. And now, it’s a carbon with two partial double bonded oxygens on to it. So the carbon’s been further oxidized - and if you remember from the chart that we looked at previously, that would produce over a 100 kilojoules per mole of energy. So that energy is what’s been used in this case, to create the ATP. Now, remember ATP only has about 30 kilojoules per mole of energy. So where did all of the rest of energy go? Well, as it turns out, that NADH up at the top, actually, is holding a lot of energy as well. The hydrogen and the electrons that are associated with it are used later in the citric acid cycle to generate more ATP, but we’ll talk about that later.
The next step in the process is a simple isomerization. And that doesn’t require much energy at all. And then there is the removal of a water, generating a carbon double bond. That is also more or less energetically neutral.
Then the final step, conversion of phosphoenolpyruvate to pyruvate, is mediated by pyruvate kinase. You’re taking that central carbon and oxidizing it further by changing it to a carbon double-bonded with an oxygen, and that is enough energy to create a second molecule of ATP. And remember, there’s 2 of these glyceraldehyde 3-phosphates for every glucose {one converted from DHAP}. So in stage two, each one of those is generating two ATP. So you have a net generation of 4 ATP minus the 2 that were needed to get it to this point. So that’s the net generation of 2 ATP.
And that’s pretty much glycolysis.
http://isite.lps.org/sputnam/AdvancedChem/Metabolism/ch15_E-landscape_glycolysis.jpg
Energy Levels of Intermediates
Created by Brett Barbaro
Here’s a slightly different way of looking at glycolysis - starting with glucose in the top left and ending at the bottom with pyruvate and {lactate}. And what this is showing is the energy level of these intermediates. So the very first thing that happens is the conversion of ATP to ADP and the addition of that phosphate group to {create} glucose 6-phosphate. And that releases over 30 kilojoules per mol. So this step is what they would call “irreversible”. It is of course, actually reversible, but the energy required to change G6P back into glucose is so great that it doesn’t really happen to any appreciable extent.
The next step, you lose a little energy, but not much. But in the third step you lose quite a bit of energy. And that’s the phosphofructokinase and that’s a important regulatory step, which is also considered to be irreversible. Now once you’ve gotten to that point, you kind of dither around at that same energy level until you get to phosphoenolpyruvate. And then the conversion of phosphoenolpyruvate to pyruvate involves another large loss of energy, and is another irreversible step. You’ll notice also here, that the NAD is turned into NADH in the middle and then regenerated in that final step as pyruvate is converted to {lactate}.
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Hexokinase Traps Glucose in the Cell and Begins Glycolysis
Upon entering the cell through a specific transport protein, glucose is phosphorylated at the expense of ATP to form glucose 6-phosphate.
Hexokinase catalyzes the reaction.
Hexokinase, like most kinases, employs substrate-binding induced fit to minimize hydrolysis of ATP.
The phosphate group gives glucose 6-phosphate a -2 charge, making it almost impossible to diffuse across the membrane, and no longer a substrate for the transporter.
Created by Brett Barbaro
So now to get into a little bit more detail on these steps. The first step is the addition of a phosphate group to glucose by hexokinase. Remember, this happens after the glucose has been transported into the cells. And the hexokinase wraps around the glucose, so that the glucose is isolated from the rest of the cytoplasm, and it helps create this environment where this reaction can occur without any interruptions. And now that this phosphate group is attached, the glucose 6-phosphate is negatively charged {and a bit bigger} and cannot get out of the cell.
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Permeability Coefficients of Ions and Molecules in a Lipid Bilayer
N2
O2
CO2
NO
some steroids
Glucose 6-phosphate
Created by Brett Barbaro
Figure 12.4 Permeability coefficients of ions and molecules in a lipid bilayer. The ability of molecules to cross a lipid bilayer spans a wide range of values. The permeability coefficient (P), expressed in cm s1, provides a quantitative estimate of the rate of passage of a molecule across a membrane.
If you remember, previously we were talking about permeability of various molecules to the cell membrane. We talked about how sodium and potassium are highly impermeable because they’re positively charged ions. Well, Glucose 6-phosphate is even worse. It has two charges, and it’s very large. And you also remember the rest of the molecule is also covered with hydroxyl groups, so it’s very hydrophilic. So basically, there’s no way it’s getting across.
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Hexokinase Reaction
OVERALL, ALL CHARGES ARE BALANCED
Created by Brett Barbaro
Here’s a diagram of that reaction, and you can see the ATP is being converted to ADP and the phosphate group is added to the glucose. Now, I want to make a note here - if you look at the charges on the left, you don’t see any charges. On the right, you see 2 negative charges on the phosphate group, and a positive charge on the hydrogen. Well, that looks to me like you’ve had an overall loss of one charge. But if that were the case, then you would be building up charge throughout every glycolysis, which is not the case. So it’s important to note that ATP is charged – it has 4 negative charges. ADP has 3 negative charges. And so the charges are actually balanced.
This used to drive me crazy when I was studying this at first, because I would look at it and say, “How could that be happening?” It’s just kind of sloppy work - they’re putting charges on some of the elements and not on others. But remember that these are all sort of balanced based on charge.
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Structures of ATP, ADP, and AMP
-4
-3
-2
Created by Brett Barbaro
Figure 15.4 Structures of ATP, ADP, and AMP. These adenylates consist of adenine (blue); a ribose (black); and a tri-, di-, or monophosphate unit (red). The innermost phosphorus atom of ATP is designated Pα, the middle one Pβ, and the outermost one Pγ.
If you count the charges on ATP, you got the 2 minuses on the gamma phosphate, then one on the beta, and one on the alpha, for a total of 4 negative charges. And ADP has negative 3, AMP has negative 2. So when you see these, it’s something to keep in mind.
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Model of Induced Fit in Hexokinase
http://chemwiki.ucdavis.edu/Textbook_Maps/Organic_Chemistry_Textbook_Maps/Map%3A_Bruice_6ed_%22Organic_Chemistry%22/26%3A_The_Organic_Chemistry_of_Metabolic_Pathways/26.03%3A_The_“High-Energy”_Character_of_Phosphoanhydride_Bonds
Transition state is VERY UNSTABLE – will react with anything.
Must keep water away!
Created by Brett Barbaro
Figure 16.2 Induced fit in hexokinase. The two lobes of hexokinase are separated in the absence of glucose (left). The conformation of hexokinase changes markedly on binding glucose (right). Notice that two lobes of the enzyme come together, creating the necessary environment for catalysis. [After RSCB Protein Data Bank; drawn from PDB yhx and 1hkg by Adam Steinberg.]
Now if you’ll recall, there is a transition state that occurs between two molecules that are reacting, and that transition state is very unstable. So when you’re trying to add a phosphate group from ATP to glucose you want to keep that away from water because any water that might be involved in that would probably end up just hydrolyzing the ATP, and you wouldn’t end up with glucose 6-phosphate - you’d just lose that energy.
So in order to protect the reaction from water, the two lobes of hexokinase (you can see clearly on the left, it’s kind of like a C shaped thing) close in around the active site when glucose is bound, and that is able to protect this reaction from water.
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Hexokinase
Hexokinase - Great Loss of Energy
http://isite.lps.org/sputnam/AdvancedChem/Metabolism/ch15_E-landscape_glycolysis.jpg
Created by Brett Barbaro
And just remember - this first step, hexokinase, is a great loss of energy for the system.
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Fructose 1,6-bisphosphate Is Generated from Glucose 6-phosphate (1/2)
The conversion of glucose 6-phosphate to fructose 6-phosphate is catalyzed by phosphoglucose isomerase. The reaction is readily reversible.
Created by Brett Barbaro
The second reaction, which is catalyzed by phosphoglucose isomerase, is an isomerization, and basically is just a reorganization of the molecule from a glucose into a fructose format. This is something that can happen just naturally based on the energetic fluctuations in the cell, and doesn’t require any special input of energy.
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Fructose 1,6-Bisphosphate Is Generated from Glucose 6-Phosphate (2/2)
The carbohydrate is trapped in the fructose form by the addition of a second phosphate to form fructose 1,6-bisphosphate.
This irreversible reaction is catalyzed by the allosteric enzyme phosphofructokinase (PFK).
DID YOU KNOW?
The prefix bis- in bisphosphate means that two separate monophosphoryl groups are present, whereas the prefix di- in diphosphate (as in adenosine diphosphate) means that two phosphoryl groups are present and are connected by an anhydride linkage.
Created by Brett Barbaro
But this next step does require energy. And that is the addition of another phosphate group to the fructose 6 phosphate. And the carbohydrate is then trapped in the fructose form because it requires a lot of energy to get it into this form, and it's very unlikely for it to go back to its original form spontaneously. There is also a note here – “bisphosphate” means that there’s two separate phosphate groups, whereas “diphosphate” means that there’s two that are attached. Also note here that this reaction is catalyzed by an allosteric enzyme so, as you might expect, there is some regulation of this enzyme by other molecules.
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Phosphofructokinase
Phosphofructokinase – big loss
http://isite.lps.org/sputnam/AdvancedChem/Metabolism/ch15_E-landscape_glycolysis.jpg
Created by Brett Barbaro
And once again, on our energy landscape, this is a great loss of energy, making it one of those “irreversible” reactions.
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Clinical Insight: The Six-Carbon Sugar
CLINICAL INSIGHT
The Six-Carbon Sugar Is Cleaved into Two Three-Carbon Fragments
The second phase of glycolysis begins with the cleavage of fructose 1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP). This readily reversible reaction is catalyzed by the enzyme aldolase.
GAP can be processed to pyruvate to yield ATP, whereas DHAP cannot. The enzyme triose phosphate isomerase interconverts GAP and DHAP, allowing the DHAP to be further metabolized.
Interestingly, triose phosphate isomerase deficiency is the only glycolytic enzymopathy that is lethal.
Created by Brett Barbaro
At this point, the molecule is split into two 3-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP). This can actually also be put back together. They’re pretty happy either way, but only the glyceraldehyde 3-phosphate (or GAP) can be further processed to make ATP. So there is the enzyme, triose phosphate isomerase, which will change DHAP into GAP and GAP into DHAP (it goes both ways) - but then of course the GAP gets sent down to make more ATP, and that creates an imbalance such that the equilibrium shifts and you end up converting all of it to GAP, pretty much. The enzyme that catalyzes this is triose phosphate isomerase. And as noted at the bottom, a deficiency of TPI (triose phosphate isomerase) is the only glycolytic enzymopathy that is lethal. So for all these other enzymes you could have something go wrong with them, but the cell can counteract it somehow, but with TPI, you really need it.
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Aldolase Reaction
Created by Brett Barbaro
Well, this is a diagram showing the breakdown of fructose 1,6 bisphosphate (F-1,6-BP) to dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP). Basically just splits it down the middle, and the resulting products are very similar (they’re isomers of each other), but slightly different.
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Triose Phosphate Isomerase Reaction
Created by Brett Barbaro
And then triose phosphate isomerase will rearrange them so that they can become the same.
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The Oxidation of an Aldehyde Powers the Formation of a Compound Having High Phosphoryl-Transfer Potential
In the next reaction of glycolysis, a compound with high phosphoryl-transfer potential, 1,3-bisphosphoglycerate, is generated by the oxidation of GAP in a reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
Created by Brett Barbaro
All right, so now we start to get some energy out of the system. The next step will create a molecule and release a lot of energy through the oxidation of an aldehyde.
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(HPO4-2)
117
180
238
285
Oxidation of aldehyde powers the reaction
(lots of energy released!)
GAP to 1,3-BPG
ENERGY RELEASED:
(kJ per mol)
Created by Brett Barbaro
So going from GAP to 1,3-BPG, you’re taking that blue hydrogen on the upper left corner {AND ITS 2 ELECTRONS!} and transferring {them} to NAD+ to create NADH. And in the process turning an inorganic phosphate into a phospho group on that molecule (this is an oxidation). And as you see at the chart on the bottom, remember we had looked at this before, the transformation of a formaldehyde to a formic acid type structure releases 238 kJ/mol of energy (and that’s a lot). So where does all of this energy go? Well, it goes in some part to the addition of the phosphate, which creates a very high energy molecule. But also, the reduction of NAD+ to NADH, and that energy is then carried on to other parts of the system.
Free-energy profiles for glyceraldehyde oxidation followed by acyl-phosphate formation (1/2)
The formation of 1,3-bisphosphoglycerate can be thought of as occurring in two steps: the highly exergonic oxidation of carbon 1 in GAP to an acid and the highly endergonic formation of 1,3-bisphosphoglycerate from the acid.
THIS DOES NOT HAPPEN
Created by Brett Barbaro
FIGURE 16.3 Free-energy profiles for glyceraldehyde oxidation followed by acyl-phosphate formation. (A) A hypothetical case with no coupling between the two processes. The second step has a large activation barrier, making the reaction very slow.
Let’s get a little bit into the mechanism of how this would happen. You might imagine that it happens in two steps. The first, you lose the hydrogen and oxidize that carbon and then add those electrons and a hydrogen to NAD+. And then, in the second step, you can add the inorganic phosphate and create the acyl phosphate. Well, the first of those reactions is the oxidation of an aldehyde - that releases a great deal of energy, is very favorable. The second reaction would be the addition of the phosphate, and that requires a lot of energy. That is an {endergonic} reaction. So if you did these one at a time, you would end up in a very deep energy pit as you can see in the diagram on the right. If you oxidize it first, then you would have a very large delta-G, and the remaining energy would be lost. It would be taken out to the cytosol and dispersed through heat - and you would need to get a great deal more energy in order to proceed to the next step, which is the attachment of the phosphate group. Well, that’s NOT how it happens.
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Free-energy profiles for glyceraldehyde oxidation followed by acyl-phosphate formation (2/2)
INSTEAD, these two reactions are linked by the formation of an energy-rich thioester in the active site of glyceraldehyde 3-phosphate dehydrogenase.
Created by Brett Barbaro
FIGURE 16.3 Free-energy profiles for glyceraldehyde oxidation followed by acyl-phosphate formation. (B) The actual case with the two reactions coupled through a thioester intermediate. The thioester intermediate is more stable than the reactant, and hence, its formation is spontaneous. However, the intermediate is less stable than the product, which forms spontaneously. Thus, the barrier separating oxidation from acyl-phosphate formation is eliminated.
What actually happens is that this reaction proceeds through the formation of a thioester bond. And that is a high energy bond. Its energy is between the reactant energy and the product energy and therefore acts as a kind of a stepping stone so that this reaction can proceed quickly. You never lose the energy as thermal heat. It is just stored in that bond until the next reaction takes place. And the overall reaction then is favorable, as well as each step in the reaction. This is just an interesting example of how energy is managed in the cell.
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ATP Is Formed by Phosphoryl Transfer from 1,3-Bisphosphoglycerate
The energy of oxidation of the carbon atom, initially trapped as 1,3-bisphosphoglycerate, is used to form ATP.
1,3-Bisphosphoglycerate has a greater phosphoryl transfer potential than ATP. Thus, it can be used to power the synthesis of ATP from ADP and Pi in a reaction-catalyzed by phosphoglycerate kinase.
Created by Brett Barbaro
The 1,3 bisphosphoglycerate that is formed is a very high energy compound. It has more energy than ATP, so by breaking it apart you can take that energy and use it to make ATP.
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Diagram Showing ATP Has a Central Position in Phosphoryl-transfer Reactions
Created by Brett Barbaro
Figure 15.7 ATP has a central position in phosphoryl-transfer reactions. The role of ATP as the cellular energy currency is illustrated by its relation to other phosphorylated compounds. ATP has a phosphoryl-transfer potential that is intermediate among the biologically important phosphorylated molecules. High-phosphoryl-transfer-potential compounds (1,3-BPG, PEP, and creatine phosphate derived from the metabolism of fuel molecules are used to power ATP synthesis. In turn, ATP donates a phosphoryl group to other biomolecules to facilitate their metabolism. [After D. L. Nelson and M. M. Cox, Lehninger Principles of Biochemistry, 5th ed. (W. H. Freeman and Company, 2009), Fig. 13-19.]
You see here on our energy chart, 1,3-BPG has quite a bit more energy than ATP. Or one would say {phosphoryl} transfer potential.
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Phosphoglycerate Kinase Reaction
Wrong in book!
Created by Brett Barbaro
So the overall reaction catalyzed by phosphoglycerate kinase takes 1 ADP and turns it into ATP.
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Additional ATP Is Generated with the Formation of Pyruvate
3-Phosphoglycerate is converted into 2-phosphoglycerate by phosphoglycerate mutase. A dehydration reaction, catalyzed by enolase, results in the production of phosphoenolpyruvate (PEP).
Phosphoenolpyruvate is a high phosphoryl-transfer compound because the presence of the phosphate traps the compound in the unstable enol tautomer.
ADP is phosphorylated at the expense of PEP, generating ATP and pyruvate, in a reaction catalyzed by pyruvate kinase.
Created by Brett Barbaro
So the result of this 3-phosphoglycerate then goes through a couple of changes to 2-phosphoglycerate and then a dehydration reaction to create phosphoenolpyruvate. And that ends up focusing a lot of that energy that’s in the molecule on that phosphate. And then that phosphate can then be used as a high phosphoryl-transfer compound to make more ATP.
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Phosphoglycerate Mutase, Enolase, and Pyruvate Kinase Reactions
Created by Brett Barbaro
And here’s a diagram of how those steps take place. The change from the PO3 on the top left to its location on the right in 2-phosphoglycerate doesn’t take a lot of energy. That’s an easy isomerization to take place.
And then by removing a water, you’re able to actually increase the electron density near that phosphate, giving it a higher phosphoryl transfer potential and the ability to combine with ADP, creating ATP -catalyzed by pyruvate kinase - and ending with the molecule pyruvate.
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Diagram Showing ATP Has a Central Position in Phosphoryl-transfer Reactions
Created by Brett Barbaro
Figure 15.7 ATP has a central position in phosphoryl-transfer reactions. The role of ATP as the cellular energy currency is illustrated by its relation to other phosphorylated compounds. ATP has a phosphoryl-transfer potential that is intermediate among the biologically important phosphorylated molecules. High-phosphoryl-transfer-potential compounds (1,3-BPG, PEP, and creatine phosphate derived from the metabolism of fuel molecules are used to power ATP synthesis. In turn, ATP donates a phosphoryl group to other biomolecules to facilitate their metabolism. [After D. L. Nelson and M. M. Cox, Lehninger Principles of Biochemistry, 5th ed. (W. H. Freeman and Company, 2009), Fig. 13-19.]
And you might recall - phosphoenolpyruvate (PEP) was actually our highest energy compound on this chart, so it’s rather easy to turn that into ATP.
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Pyruvate Kinase
Pyruvate Kinase releases energy
http://isite.lps.org/sputnam/AdvancedChem/Metabolism/ch15_E-landscape_glycolysis.jpg
Created by Brett Barbaro
In fact, it releases so much energy that it’s able to create ATP and at the same time lose a great deal of energy to the surrounding media to reach a lower energy state overall.
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Two ATP Molecules Are Formed in the Conversion of Glucose into Pyruvate
The net reaction for glycolysis is
Created by Brett Barbaro
So the net reaction, overall, is taking a glucose, 2 inorganic phosphates, and 2 ADP, and 2 NAD+, and changing it into 2 pyruvate, 2 ATP, 2 NADH (which are also high energy molecules), 2 hydrogens, and 2 waters. Getting from that glucose to pyruvate involves the oxidation of carbons, and that energy has been trapped or transferred into the ATP and NADH molecules.
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Table 16.1 Reactions of Glycolysis
Created by Brett Barbaro
Now, we’re going to talk a little bit here about delta G and delta G naught prime (that’s what that little circle is – “naught”), and I think these words here are too small for you to read, so, we’ll break it down.
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Table 16.1 Reactions of Glycolysis
Created by Brett Barbaro
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Here are all of the reactions of glycolysis, as we have just discussed.
Table 16.1 Enzymes of Glycolysis
Created by Brett Barbaro
And here are the enzymes that catalyze those reactions, as we also just discussed. Now, if you look at the delta G naught prime of these reactions, you see something interesting. You see that the operation of hexokinase is very [exergonic]. Phosphofructokinase is very [exergonic], and pyruvate kinase is very [exergonic], as you would expect. Those are the three reactions that cause the greatest overall loss in energy. But you also see phosphoglycerate kinase looks very [exergonic] and aldolase is extremely [endergonic]. So why do these not show up in our energy diagram? Well, remember the delta G is the result of the delta G naught prime (when everything is normal and in equal concentrations) - but delta G figures in the concentrations of these substrates as well. And once you’ve figured in the concentrations, you end up with the numbers that we saw on that other graph, with a great deal of loss of energy on the hexokinase, phosphofructokinase, and pyruvate kinase steps, and very little change in energy on the others.
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Three Largest Losses of Energy
http://isite.lps.org/sputnam/AdvancedChem/Metabolism/ch15_E-landscape_glycolysis.jpg
Created by Brett Barbaro
And here’s another look at that, which will show you that, overall, in cellular conditions, at cellular concentrations of these chemicals, you will have the three largest losses of energy in those three enzymes {reactions}.
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Quick Quiz 1
QUICK QUIZ 1
The gross yield of ATP from the metabolism of glucose to two molecules of pyruvate is four molecules of ATP. However, the net yield is only two molecules of ATP. Why are the gross and net values different?
Created by Brett Barbaro
Here’s a quick quiz you can take yourself. If the gross yield of ATP from metabolism of glucose to two molecules of pyruvate is four molecules of ATP, and the net yield is only two molecules of ATP, why are the gross and net values different?
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Section 16.2 NAD+ Is Regenerated from the Metabolism of Pyruvate
Learning objective 2: Explain why the regeneration of NAD+ is crucial to fermentations.
The conversion of glucose into pyruvate generates ATP, but for ATP synthesis to continue, NADH must be reoxidized to NAD+. This vital coenzyme is derived from the vitamin niacin (B3).
NAD+ can be regenerated by further oxidation of pyruvate to CO2, or by the formation of ethanol or lactate from pyruvate.
Created by Brett Barbaro
Now, when you take glucose and turn it into pyruvate, you make ATP - but you also make NADH. Where does this NADH go? It has to go somewhere. You need to regenerate the NAD+ in order for glycolysis to continue. Well, in an anaerobic situation, NAD+ can be regenerated by the fermentation of pyruvate to ethanol or lactate. And that’s why ethanol and lactate are byproducts of that reaction. In aerobic situations, you can take the NADH and get {a lot} more ATP out of it during the process of the further conversion of pyruvate to carbon dioxide.
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Diagram of the Diverse Fates of Pyruvate
Created by Brett Barbaro
Figure 16.4 Diverse fates of pyruvate. Ethanol and lactate can be formed by reactions that include NADH. Alternatively, a two-carbon unit from pyruvate can be coupled to coenzyme A (see Chapter 18) to form acetyl CoA.
So, starting with pyruvate at the top, it has basically three different directions that it can go. It can lose a carbon dioxide and become acetaldehyde, and then that acetaldehyde will be used to regenerate NAD+ and be converted to ethanol; or you can convert it directly to lactate by adding the NADH and reintroducing those electrons; or you can lose a carbon dioxide and attach the remaining two carbons to the coenzyme A, making acetyl CoA, and then you can regenerate your NAD+ farther down the line as you further oxidize the carbons.
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Fermentations Are a Means of Oxidizing NADH (1/3)
Fermentations are ATP-generating pathways in which electrons are removed from one organic compound and passed to another organic compound.
The formation of ethanol from pyruvate regenerates NAD+. Pyruvate carboxylase requires the coenzyme thiamine pyrophosphate, which is derived from the vitamin thiamine (B1).
Created by Brett Barbaro
So if you take pyruvate and turn it into acetaldehyde, you’re losing a carbon dioxide, and therefore that reaction is fairly irreversible. Carbon dioxide gets transported out, and it is the most oxidized form of carbon, so you’re losing a lot of energy there, and you’re stuck with acetaldehyde. And then, you can reduce that acetaldehyde using the electrons and hydrogen from NADH. And that’s catalyzed by the alcohol dehydrogenase enzyme. Now, the second reaction is reversible, and I believe that’s what happens when you drink alcohol (ethanol), is that your alcohol dehydrogenase enzyme breaks it down creating NADH and acetaldehyde. And it’s the acetaldehyde that gives you the headache.
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Fermentations Are a Means of Oxidizing NADH (2/3)
The conversion of glucose into two molecules of ethanol is called alcoholic fermentation.
The NADH generated by glyceraldehyde 3-phosphate dehydrogenase is oxidized by alcohol dehydrogenase, regenerating NAD+.
(HPO4-2)
(-3)
(-4)
Created by Brett Barbaro
So this is the conversion of glucose into two molecules of ethanol, and that’s alcoholic fermentation. And that does not occur in our human bodies, that’s something that occurs in yeast, which is why they make alcohol and beer. Also occurs in other microorganisms, where you get the alcohol from wine. But they've got here this equation, the glucose plus 2 inorganic phosphate, etc. And if you notice, in the book when they have this equation, I've added a few things like the HPO4-2, and then there's a -3 charge on ADP, and a -4 charge on the ATP, and then there's the positive charge on hydrogen there in the middle. So I've added those three things because it's always bothered me, going through biochemistry texts, is that the charges did not seem to balance. You had two hydrogen positively charged on the left, and then there was nothing positively charged on the right, I was like, so where did those positive charges go? Well, it's because they're leaving things out. Remember, that the inorganic phosphate is actually HPO4, it's not just a phosphorus atom like it looks like. It's inorganic, which means it doesn’t have any carbon attached to it. But it's still a phosphate, and as such it has a -2 charge. And remember, earlier on in this presentation we saw the ADP has a -3 charge, ATP has a -4 charge, and if you take all of those into consideration, the charges do balance. So the charges always balance, even though sometimes it looks like they don't. It's usually because things are being left out but don't be alarmed by that. So the NADH that we've generated earlier on in the glycolytic cycle is oxidized by alcohol dehydrogenase, regenerating NAD+ so that it can participate in the next round of glycolysis. And note that also, the alcohol dehydrogenase is named for the reverse reaction, changing alcohol into acetaldehyde. But in this case, it’s changing the acetaldehyde into alcohol - so it was named for the reverse reaction, but it does go both ways.
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Diagram of Maintaining Redox Balance in Alcoholic Fermentation
Created by Brett Barbaro
Figure 16.5 Maintaining redox balance in alcoholic fermentation. The NADH produced by the glyceraldehyde 3-phosphate dehydrogenase reaction must be reoxidized to NAD+ for the glycolytic pathway to continue. In alcoholic fermentation, alcohol dehydrogenase oxidizes NADH and generates ethanol.
So here’s an example of the NAD, NADH cycle. The NADH is produced during glycolysis, and then regenerated in the generation of ethanol by alcohol dehydrogenase.
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Fermentations Are a Means of Oxidizing NADH (3/3)
NADH can also be oxidized by converting pyruvate to lactate in a reaction catalyzed by lactate dehydrogenase.
The conversion of glucose into two molecules of lactate is called lactic acid fermentation.
Created by Brett Barbaro
You can also regenerate the NAD by converting pyruvate into lactate, and that’s what happens in your muscles when they don’t have enough oxygen. {UPDATE – this is NOT responsible for muscle soreness.}
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Diagram of Maintaining Redox Balance in Lactic Acid Fermentation
Created by Brett Barbaro
Figure 16.6 Maintaining redox balance in lactic acid fermentation. In lactic acid fermentation, lactate dehydrogenase oxidizes NADH to produce lactic acid and regenerate NAD+.
So both lactate and ethanol formation result in the regeneration of {NAD+} and keep the glycolytic cycle going.
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Biological Insight: Fermentations
BIOLOGICAL INSIGHT
Fermentations Provide Usable Energy in the Absence of Oxygen
Obligate anaerobes cannot survive in the presence of oxygen. Some obligate anaerobic microorganisms are pathogenic.
Many food products, including sour cream, yogurt, various cheeses, beer, wine, and sauerkraut, result from fermentation.
DID YOU KNOW?
Fermentation is an ATP-generating process in which organic compounds act as both donors and acceptors of electrons. Fermentation can take place in the absence of O2. Louis Pasteur, who discovered fermentation, described it as la vie sans l’air (“life without air”).
Created by Brett Barbaro
Now usually you can get a lot more ATP out of your glucose by fully oxidizing it, but you can’t always do that. And in fact, there are some animals that, that would actually kill them. Those are called obligate anaerobes, and a lot of these obligate anaerobes will cause sickness.
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Table 16.2 Examples of Pathogenic Obligate Anaerobes
Created by Brett Barbaro
These get their energy entirely from glycolysis and do not produce anything further down the line. You can see for example, gas gangrene. There’s a reason that you get gangrene, and that’s because your circulation isn’t working, and your circulation is what’s bringing oxygen to your tissues. So if you don’t have any oxygen there, then the gangrene can proliferate because it’s an obligate anaerobe. So keep yourself oxygenated.
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Section 16.3 Fructose and Galactose Are Converted into Glycolytic Intermediates
Fructose from table sugar or high-fructose corn syrup and galactose from milk sugar are converted into glycolytic intermediates.
In the liver, fructose is metabolized by the fructose 1-phosphate pathway.
In other tissues such as adipose tissue, fructose is directly phosphorylated by hexokinase.
Created by Brett Barbaro
Now, we’ve talked about glucose. What about fructose and galactose, which are also very common sugars that we run into? Well, let’s talk about fructose first. In the liver, there’s actually a different pathway called the fructose 1-phosphate pathway. And fructose is processed through that. In other tissues fructose can be directly phosphorylated by hexokinase and introduced into the glycolytic pathway.
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Diagram of Entry Points in Glycolysis for Galactose and Fructose
Created by Brett Barbaro
Figure 16.7 Entry points in glycolysis for galactose and fructose.
So remember - fructose and galactose are isomers of glucose. They’re very similar. So it doesn’t take a lot to get them into the process of glucose metabolism.
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Diagram of Fructose Metabolism
Created by Brett Barbaro
Figure 16.8 Fructose metabolism. Fructose enters the glycolytic pathway in the liver through the fructose 1-phosphate pathway.
This is the pathway that fructose follows in the liver. It’s first converted to fructose 1-phosphate by the enzyme fructokinase. And then, that is broken down into glyceraldehyde and dihydroxyacetone phosphate after which the glyceraldehyde is phosphorylated and the glycolytic reactions are able to proceed from this point.
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Galactose Is Converted into Glucose 6-Phosphate (1/2)
Galactose is converted into glucose 6-phosphate by the galactose–glucose interconversion pathway, which begins with the phosphorylation of galactose by galactokinase.
Created by Brett Barbaro
Galactose starts its journey by being phosphorylated by ATP and the enzyme galactokinase.
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Diagram of the Reactions of Galactose Metabolism
Created by Brett Barbaro
Figure 16.9 Galactose metabolism. Galactose 1-phosphate reacts with activated glucose (UDP-glucose) to form UDP-galactose, which is subsequently converted into UDP-glucose.
The next step of the journey is slightly discontinuous. Now, when I look at this diagram The first thing I ask myself is why don't they have an enzyme that isomerizes galactose 1-phosphate directly into glucose 1-phosphate, instead of going through this extra step?" And I'm not going to go into the details on that right now, but these other metabolites are important for other pathways, and this creates another control point where the activity of these pathways can be balanced. So the way it works is your [galactose] 1-phosphate combines with a UDP-glucose, which we encountered in Chapter 10 (UDP is just like an ADP, but with a uridine instead of an adenine on it), and these two combine to create UDP-galactose and glucose 1-phosphate - and this glucose 1-phosphate goes on toward the glycolytic pathway. The UDP-glucose is then regenerated by UDP-galactose 4-epimerase, which just flips this one hydroxyl group, and it can therefore participate in another round of this process.
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Galactose Is Converted into Glucose 6-Phosphate (2/2)
The sum of the reaction of the galactose–glucose interconversion pathway is
Glucose 1-phosphate can be converted into glucose 6-phosphate by phosphoglucomutase.
-4
-2
-3
+1
Created by Brett Barbaro
So the overall reaction for the galactose-glucose conversion pathway takes galactose and ATP and makes glucose 1-phosphate, ADP, and a hydrogen. And I’ve included the charges on these molecules just so that you’ll be able to see that they do in fact balance. The glucose 1-phosphate can then be converted to glucose 6-phosphate by another enzyme called phosphoglucomutase.
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Clinical Insight: Milk Intolerance
CLINICAL INSIGHT
Many Adults Are Intolerant of Milk Because They Are Deficient in Lactase
Lactose intolerance, or hypolactasia, occurs because most adults lack the enzyme to degrade lactose.
Created by Brett Barbaro
Now, the first step in digesting lactose is to break it apart into galactose and glucose. And that is done by the enzyme lactase. Lactose intolerance occurs when people don’t have the ability - they’re actually missing the lactase enzyme (or at least have low levels of it), and are therefore unable to process lactose.
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Clinical Insight: Galactose (1/2)
CLINICAL INSIGHT
Galactose Is Highly Toxic If the Transferase Is Missing
Classic galactosemia results if galactose 1-phosphate uridyl transferase activity is deficient.
Symptoms include failure to thrive, jaundice, and liver enlargement that can lead to cirrhosis. Cataract formation may also occur.
Created by Brett Barbaro
Having too much galactose in your body is also not so good. And the way that galactose is supposed to be processed is through that galactose 1-phosphate uridyl transferase pathway. So if there’s something wrong with that, it can cause problems like jaundice, liver problems, and also cataract formations.
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Clinical Insight: Galactose (2/2)
CLINICAL INSIGHT
Galactose Is Highly Toxic If the Transferase Is Missing
Cataracts, a clouding of the lens due to water accumulation, form because the galactose is converted into galactitol, which is poorly metabolized.
Galactitol is osmotically active and causes water to diffuse into the lens.
Created by Brett Barbaro
Cataracts form because galactose, in the eyes, is transformed into galactitol, which is not a substrate for anything. But it’s very osmotically active so that water will get into the lens of your eye. I don’t think this is a reaction that is very useful for anything, it’s probably just a side reaction for this aldose reductase. Aldose reductase is responsible for doing a lot of things in the cell - it just sort of happens to also turn galactose in to galactitol. And thus you have some unintended consequences of the presence of too much galactose in your system.
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Section 16.4 The Glycolytic Pathway Is Tightly Controlled
Enzymes catalyzing irreversible reaction in metabolic pathways are potential control sites – if you’re going to stop a pathway, stop it before it becomes irreversible!
In glycolysis, these enzymes are hexokinase, phosphofructokinase, and pyruvate kinase.
Created by Brett Barbaro
Now, since glycolysis is so important, it’s going to have to be regulated - turned on when it’s needed, turned off when it’s not needed. So the best places to regulate these pathways are the irreversible reactions. If you want to stop a pathway from proceeding, then you would best off to stop it before it becomes irreversible. So in that case, the irreversible reactions in glycolysis are hexokinase, phosphofructokinase, and pyruvate kinase. Those are the ones that result in the greatest loss of free energy - and therefore, those are the ones that are targeted during regulation of the pathway.
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Table 16.1 Enzymes
Created by Brett Barbaro
Remember, they are the three enzymes that result in the greatest delta G.
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Energy Levels of Intermediates
http://isite.lps.org/sputnam/AdvancedChem/Metabolism/ch15_E-landscape_glycolysis.jpg
Created by Brett Barbaro
And the three greatest drops in overall energy level.
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Glycolysis in Muscle Is Regulated by Feedback Inhibition to Meet the Need for ATP (1/2)
Phosphofructokinase is the key regulator of glycolysis in mammals. The enzyme is allosterically inhibited by ATP and allosterically stimulated by AMP.
When ATP needs are great, adenylate kinase generates ATP from 2 ADP.
AMP then becomes the signal for the low-energy state.
Created by Brett Barbaro
Phosphofructokinase is the most important regulator of glycolysis, in mammals at least. And it works by being allosterically inhibited by ATP and allosterically stimulated by AMP. So if there’s a lot of ATP in the cell, then it will attach to phosphofructokinase and prevent it from doing its work. If there’s a lot of AMP, which means usually that there’s not a lot of ATP, then that will encourage the phosphofructokinase to do more. When there is a lot of ADP, this enzyme adenylate kinase will make ATP out of ADP and produce AMP, which you can see in the reaction at the bottom, is just the transfer of one phosphate group from one molecule to the other. But then, you can tell if there’s a lot of AMP around, then you must be at a low energy state.
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Glycolysis in Muscle Is Regulated by Feedback Inhibition to Meet the Need for ATP (2/2)
Hexokinase is allosterically inhibited by glucose 6-phosphate.
Pyruvate kinase is inhibited by the allosteric signals ATP and alanine and stimulated by fructose 1,6-bisphosphate, the product of the phosphofructokinase reaction.
In muscle, glycolysis is regulated to meet the energy needs of contraction.
Created by Brett Barbaro
Figure 16.12 The allosteric regulation of phosphofructokinase. A high level of ATP inhibits the enzyme by decreasing its affinity for fructose 6-phosphate. AMP diminishes the inhibitory effect of ATP.
Hexokinase, the very first step in glycolysis, is allosterically inhibited by its own product, glucose 6-phosphate. So once it’s made a lot of glucose 6-phosphate, if that glucose 6-phosphate is not being processed, then it’ll build up and it’ll prevent the further phosphorylation of glucose. Obviously, if there is a lot of glucose 6-phosphate building up, you don’t really need any more glucose. So let it leave the cell and go somewhere else.
Pyruvate kinase, one of the last steps in glycolysis, is inhibited by ATP. So if you have a lot of ATP then pyruvate kinase won’t be doing much work. But if you have a lot of fructose 1,6-bisphosphate, which is higher up in the process, then you will stimulate the activity of pyruvate kinase. So it’s sort of a balancing game. If you have a lot of the reactants, the upstream elements of the glucose, then it will be trying to process those, but if you have a lot of the product then it will try to inhibit it.
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FEEDBACK/FEEDFORWARD INHIBITION/STIMULATION
VARIES ACCORDING TO TISSUE (E.G. LIVER)
Created by Brett Barbaro
Figure 16.13 The regulation of glycolysis in muscle. At rest (left), glycolysis is not very active (thin arrows). The high concentration of ATP inhibits phosphofructokinase (PFK) and pyruvate kinase, while glucose 6-phosphate inhibits hexokinase. Glucose 6-phosphate is converted into glycogen (Chapter 25). During exercise (right), the decrease in the ATP/AMP ratio resulting from muscle contraction activates phosphofructokinase and hence glycolysis. The flux down the pathway is increased, as represented by the thick arrows.
So this is how it works in your muscles. You can see that, starting at rest, if you’re not exercising, glucose gets processed to glucose 6-phosphate, which builds up and will inhibit hexokinase. So if you’ve got a lot of glucose 6-phosphate you don’t need any more. The phosphofructokinase is inhibited by ATP - so once again, if you have a lot of the product it will inhibit the activity of both phosphofructokinase and pyruvate kinase. That means you’re basically shutting down glycolysis. When you’re exercising however, then you need to ramp up glycolysis. And the AMP is the signal to increase the activity of phosphofructokinase, as well as fructose 1,6-bisphosphate increases the rate of pyruvate kinase. This is the case in the muscles, but it behaves differently in other tissues, so just something to keep in mind.
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A Family of Transporters Enables Glucose to Enter and Leave Animal Cells
Five glucose transporters, termed GLUT1-5, facilitate the movement of glucose across the cell membrane.
Created by Brett Barbaro
Glucose enters the cells and also leaves them through these glucose transporters, which we’ve already discussed to some extent.
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Table 16.3 Family of Glucose Transporters
Created by Brett Barbaro
GLUT1 and GLUT3 are available in all mammalian tissues and are your main glucose uptake. Muscles and fat cells are places where you need to take in extra glucose for doing exercise, and also for storage, so they use a GLUT4 transporter, which is a little bit faster than the GLUT1 or 3. And that can also be increased in your plasma membrane by doing endurance training - in the muscle. Now, your liver and pancreatic cells, they need to process a great deal of glucose. Liver for storing it in the form of glycogen, and pancreas for monitoring the glucose levels in the body. If there’s a lot of glucose in your blood, the pancreas will sense it and will send more insulin around to tell your body to start taking up more glucose.
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16.5 Metabolism in Context: Glycolysis Helps Pancreatic Beta Cells Sense Glucose
Insulin is secreted by β cells of the pancreas in response to high blood levels of glucose. This secretion is stimulated by the metabolism of glucose by the β cells.
Glucose enters β cells through GLUT2 and is metabolized to pyruvate, and the pyruvate is subsequently oxidized to CO2 and H2O.
The increase in ATP closes a K+ channel, which alters the charge across the cell membrane. This alteration in turn opens Ca2+ channels. The influx of Ca2+ ions stimulates the release of insulin.
Created by Brett Barbaro
The pancreas senses the amount of glucose in your blood by transporting it into the pancreatic cells, and then processing it all the way to pyruvate, carbon dioxide, and water. That creates a great deal of ATP. So the pancreas starts to get a lot of energy. It's sort of your front line glucose metabolic site. And when the pancreas gets a lot of ATP built up, it ends up closing a potassium channel, which changes the charge across the cell membrane, and that opens up calcium channels, which allows calcium into the cytosol. And remember, calcium is an important signaling molecule which will then combine with other enzymes and stimulate the release of insulin. So the pancreas eats first, and it says “hey, there’s a lot of food, we should get going on this thing,” - so it sends out insulin to the rest of the body saying, “all right let’s get this glucose!”
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Diagram of How Insulin Release is Regulated by ATP
Why so complex? Every step is an opportunity for regulation.
Created by Brett Barbaro
Figure 16.19 Insulin release is regulated by ATP. The metabolism of glucose by glycolysis increases the concentration of ATP, which causes an ATP-sensitive potassium channel to close. The closure of this channel alters the charge across the membrane () and causes a calcium channel to open. The influx of calcium causes insulin-containing granules to fuse with the plasma membrane, releasing insulin into the blood.
And here’s just a diagram of it. You can see the glucose coming in, being turned into ATP, shutting off the potassium channels (the psi symbol there just means a membrane potential), which then triggers calcium intake. and then, the calcium intake releases insulin. Kind of interestingly, this is very similar to what happens in the synapse. When calcium enters into the cytoplasm of the synapse, it releases neurotransmitters. It’s probably a very similar mechanism. So you might ask yourself - why is this whole process so complex? Well, it’s hard to say for sure - but you know, a lot of things have happened and this is the way that it’s all gotten sorted out, just this is the way it is. But an important thing to remember is that every time that you have a complex pathway then you can have lots of different possibilities to regulate it. And we’re not going to get into all of those different regulatory elements of the pancreatic beta cell insulin release, but it’s good to know that it’s very sensitive to a number of different changes and stuff in the body.
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