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bio100-Ch2-Pt2.pptxChapter 2: Chemistry part 2
Raw materials and fuel for our bodies
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Hi and welcome to the lecture corresponding to Chapter 2, chemistry.
Four Types of Macromolecules
Carbohydrates: fuel, some structures
Lipids: fuel, insulation, hormones, membranes
Proteins: structural and functional roles (enzymes, transporters, antibodies, contractile fibers ect.etc.)
Nucleic acids: genetic information
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The are four types of macromolecules—large molecules made up from smaller building blocks—found in living organisms: carbohydrates, lipids, proteins, and nucleic acids.
Carbohydrates
C, H, and O
Primary fuel for organisms: store energy in their bonds
Cell structure
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Carbohydrates are molecules that contain mostly carbon, hydrogen, and oxygen: they are the primary fuel for running all of the cellular machinery and also form much of the structure of cells in all life forms.
Sometimes they contain atoms of other elements, but they must have carbon, hydrogen, and oxygen to be considered a carbohydrate.
Further, a carbohydrate generally has approximately the same number of carbon atoms as it does H2O units.
For instance, the best-known carbohydrate, glucose, has the composition C6H12O6 (6 carbons and a little math will show us that it also has 6 H2O units; notice that 6 H2 = H12 and 6 O = O6).
Simple sugars: monosaccharides
Three to six carbon atoms.
Example: glucose, fructose, galactose, ribose
Glucose:
Fuel for cellular activity
Stored temporarily as glycogen in liver/muscle (think “carbo loading”)
Converted to fat
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Carbohydrates are classified into several categories, based on their size and their composition. The simplest—monosaccharides or simple sugars—contain anywhere from three to seven carbon atoms.
The carbohydrate of most importance to living organisms is glucose. This simple sugar is found naturally in most fruits, but most of the carbohydrates that you eat, including table sugar (called sucrose) and the starchy carbohydrates found in bread and potatoes, are converted into glucose in your digestive system. The glucose then circulates in your blood at a concentration of about 0.1%. Circulating glucose, also called “blood sugar,” has one of three fates it can be used directly as cellular fuel, it can be stored as a complex polysaccharide called glycogen (this is what you do when you eat a lot of carbs before a race), or can be also converted to fat.
What is “carbo-loading”?
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“Carbo-loading” is a method by which athletes can, for a short time, double or triple the usual amount of glycogen that is stored in their muscles and liver, increasing the amount of fuel available for extended exertion and delaying the onset of fatigue during an endurance event.
Carbo-loading is usually done in two phases: a depletion phase and a loading phase.
The depletion phase begins six or seven days before a competition.
In this phase, the combination of a super-low carbohydrate intake and exhaustive exercise depletes glycogen in the muscles.
The loading phase takes place during the two days before the competition.
During this phase, a super-high carbohydrate diet is combined with reduced exercise in order to achieve a higher blood glucose than is necessary, so that much of the excess glucose is stored as glycogen.
2.9 Complex carbohydrates: di- and polysaccharides
More than 1 sugar (monosaccharide) unit
Disaccharides
sucrose
lactose
Polysaccharides
starch
cellulose
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In contrast to the simple sugars, complex carbohydrates contain more than one sugar unit.
For example, two simple sugars can be joined together into a disaccharide, such as sucrose (table sugar) and lactose (the sugar found in milk).
When many simple sugars—sometimes as many as ten thousand—are joined together, the resulting molecule is called a polysaccharide Depending on how the simple sugars are bonded together, they may function as “time-release” stores of energy or as structural materials that may not be digestible to animals at all.
An example of such a structural material is the polysaccharide cellulose—the primary component of plant cell walls. Like simple sugars, many disaccharides and polysaccharides are important sources of fuel.
Unlike simple sugars, however, disaccharides and polysaccharides must undergo some preliminary processing before the energy can be released from their bonds.
Let’s look at what happens when we eat some sucrose, common table sugar.
Sucrose is the primary carbohydrate in plant sap.
It is a disaccharide composed of two simple sugars, glucose and fructose, linked together.
Because humans can’t directly utilize sucrose, we first must break the bond linking the glucose and fructose.
Only then can the individual monosaccharides be broken down into their component atoms and the energy from the broken bonds be harvested and used. Similarly, lactose is a disaccharide made up of a molecule of glucose and a molecule of galactose bound together.
As with sucrose, we must break the bond before we can extract any usable energy from a molecule of lactose.
Starch and glycogen
> 100’s of glucose molecules joined together
Barley, wheat, rye, corn, and rice
Glycogen—“animal starch”
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Energy can also be stored in a complex carbohydrate called starch, which consists of a hundred or more glucose molecules joined together in a line.
In plants, starch is the primary form of energy storage, found in their roots and other tissues (Fig. 2-24).
Commonly cultivated grains such as barley, wheat, and rye are high in starch content, and corn and rice are more than 70% starch.
Although it is composed exclusively of glucose molecules linked together, starch does not taste sweet.
Because of its shape it does not stimulate the sweetness receptors on the tongue.
Because the glycogen that stores energy in your muscles and liver is a complex carbohydrate, it is sometimes referred to as “animal starch” (although it is more branched than starch and carries more glucose units linked together).
How carbs affect blood sugar
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The relative amounts of complex carbohydrates and simple sugars in foods cause them to have very different effects when you eat them.
Oatmeal (along with rice and pasta), for example, is rich in complex carbohydrates.
Fresh fruits, on the other hand, are rich in simple sugars such as fructose.
Consequently, although the fruit will give a quick burst of energy as the sugars are almost immediately available, the fuel will soon be gone from the bloodstream.
The simple sugars in the oatmeal will only gradually become available as the complex carbohydrates of the oats are slowly broken down into their simple sugar components (see Figure 2-25 Short-term versus long-term energy).
2.10 Not all carbohydrates are digestible.
Chitin: rigid skeleton of insects and crustaceans
Cellulose: trees, wood, cotton, etc.
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Two different complex carbohydrates—both indigestible by humans—serve as structural materials for invertebrate animals and plants: chitin (pronounced kite’ in) and cellulose
Chitin forms the rigid skeleton of most insects and crustaceans (such as lobsters and crabs).
Cellulose forms a huge variety of structures that are visible all around us.
We find cellulose in trees and the wooden structures we build from them, in cotton and the clothes we make from it, in leaves, and in grass. In fact, it is the single most prevalent compound on earth.
Fiber
“Roughage”
Colon cancer prevention/reduction
Termites ecological role
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Although it is not digestible, cellulose is still important to human diets.
The cellulose in our diet is known as fiber. It is also appropriately called “roughage” because as the cellulose of celery stalks and lettuce leaves passes through our digestive system, it scrapes the wall of the digestive tract. Its bulk and the scraping stimulate the more rapid passage of food and the nasty byproducts of digestion through our intestines.
That is why fiber reduces the risk of colon cancer and other diseases (but it is also why too much fiber can lead to diarrhea.)
Unlike humans, termites have some microorganisms living in their gut that are able to break down cellulose. That’s why they can chew on wood and, with the help of the cellulose-digesting boarders in their gut, actually break down the cellulose and extract usable energy from the freed glucose molecules—energy that is then made available through the food web.
Why does a salad dressing made with vinegar and oil separate into two layers shortly after you shake it?
Hydrophobic= “water-hating”
Nonpolar molecules such as lipids (ex.oils) are not soluble in water : hydrophobic
Hydrophilic = “water-loving”
Polar molecules are souble in water: hydrophilic
Hydrophilic and hydrophobic molecules
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Lipids are insoluble in water because, in sharp contrast to water, they consist mostly of hydrocarbons, which are nonpolar.
Nonpolar molecules (or parts of molecules) tend to minimize contact with water and are considered hydrophobic.
Instead, lipids cluster together when mixed with water, never fully dissolving. Molecules that readily form hydrogen bonds with water, on the other hand, are considered hydrophilic (meaning “water loving”).
2.11 Lipids
Lipids are non-soluble in water and greasy to the touch.
They are valuable to organisms in long-term energy storage and insulation, membrane formation, and as hormones.
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One familiar type of lipid is fat, the type most important in long-term energy storage and insulation. Lipids also include sterols, which include cholesterol and many of the sex hormones that play regulatory roles in animals, and phospholipids, which form the membranes that enclose cells.
2.12 Fats are tasty molecules too plentiful in our diets.
Glycerol: “head” region
Fatty acid “tails”
Triglycerides
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All fats have two distinct components: they have a “head” region, and two or three long “tails.”
The head region is a small molecule called glycerol. It is linked to “tail” molecules that are called fatty acids.
A fatty acid is simply a long hydrocarbon, that is, a chain of carbon molecules, often a dozen or more, linked together with one or two hydrogen atoms attached to each carbon.
The fats in most foods we eat are triglycerides, which are fats having three fatty acids linked to the glycerol molecule.
For this reason, the terms fats and triglycerides are often used interchangeably.
Triglycerides that are solid at room temperature are called fats, while those that are liquid at room temperature are called oils. Fat molecules contain much more stored energy than carbohydrate molecules.
That is, the chemical breakdown of fat molecules releases significantly more energy.
A single gram of carbohydrates stores about 4 calories of energy, while the exact same amount of fat stores about 9 calories—not unlike the difference between a $5 bill and a $10 bill.
Because fats store such a large amount of energy, animals have evolved a strong taste preference for fats over other energy sources. Organisms evolving in an environment of uncertain food supply will build the largest surplus by consuming molecules that hold the most amount of energy in the smallest mass.
This feature helped humans to survive millions of years ago, but today puts us in danger from the health risks of obesity now that fats are all too readily available.
Saturated and Unsaturated Fats
# of bonds in the hydrocarbon chain in a fatty acid
Health considerations
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An important distinction is made between saturated and unsaturated fats. These terms refer to the hydrocarbon chain in the fatty acids.
If each carbon atom in the hydrocarbon chain in a fatty acid is bonded to two hydrogen atoms, the fat molecule carries the maximum number of hydrogen atoms and is said to be saturated. Most animal fats, including those found in meat, cheese, and eggs, are saturated. They are not essential to your health and, because they accumulate in your bloodstream and can narrow the vessel walls, can cause heart disease and strokes. A fat is unsaturated if any of its carbon atoms are bound to only a single hydrogen.
Most plant fats are unsaturated.
Unsaturated fats may be mono-unsaturated (if the hydrocarbon chain has only one carbon in an unsaturated state) or polyunsaturated (if more than one carbon is unsaturated).
Unsaturated fats are still high in calories, but because they can lower cholesterol, they are generally preferable to saturated fats. Foods high in unsaturated fats include avocados, peanuts, and olive oil. Relative to other animal fats, fish tend to have less saturated fat.
Hydrogenation and trans fats
Many snack foods contain “partially hydrogenated” vegetable oils.
Adding hydrogen atoms to a vegetable oil improves the food’s texture, taste, and half life
Creates trans fats => increase the risk of heart disease
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The ingredient list for many snack foods lists “partially hydrogenated” vegetable oils.
The hydrogenation of an oil means that a liquid, unsaturated fat has had additional hydrogen atoms added to it, so that it becomes more saturated.
This can be useful in creating a food with a more desirable texture because increasing a fat’s degree of saturation changes its consistency and makes it more solid at room temperatures. Hydrogenation of unsaturated fats is problematic from a health perspective, however, because it creates trans fats, referring to the unusual orientation of the hydrogen atoms added, which differs from other dietary fats, which have their hydrogens in an orientation called cis.
Trans fats cause your body to produce more cholesterol, further raising the risk of heart disease, and they also reduce your body’s production of a type of cholesterol that protects against heart disease.
Other lipids
Not all lipids are fats
The sterols: cholesterol, steroid hormones (estrogens, testosterone etc.)
Phospholipids: important components of membranes
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Not all lipids are fats, nor do they necessarily function in energy storage. Other important lipids are cholesterol and steroid hormones, as well as phospholipids.
2.13 Cholesterol and steroids
Sterols:
Cholesterol:
Component of cell membranes
Precursor of steroid hormones
Steroid hormones:
estrogen
testosterone
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Sterols, plays an important role in regulating growth and development.
Cholesterol, estrogen, and testosterone are all lipids.
This group includes some very familiar lipids: cholesterol and the steroid hormones such as testosterone and estrogen. These molecules are all modifications of one basic structure formed from four interlinked rings of carbon atoms.
Phospholipids and Waxes
Phospholipids are the major component of the cell membrane.
Waxes are strongly hydrophobic.
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Phospholipids and waxes are also lipids.
Phospholipids are the major component of the membrane that surrounds the contents of a cell and controls the flow of chemicals into and out of the cell. They have a structure similar to fats, but with two differences: they contain a phosphorous atom (hence phospholipids) and they have two fatty acid chains rather than three. We will explore the significant role of phospholipids in cell membranes in the next chapter.
Waxes resemble fats but have only one long-chain fatty acid, linked to the glycerol head of the molecule. Because the fatty acid chain is highly nonpolar, waxes are strongly hydrophobic; that is, these molecules do not mix with water but repel it. Their water resistance accounts for their use as a natural coating on the surface of many plants and their use in the outer coverings of many insects. In both cases the waxes prevent the plants and animals from losing water essential to their life processes. Many birds, too, have a waxy coating on their wings, keeping them from becoming water-logged when they get wet.
2.14 Proteins are bodybuilding macromolecules.
Chief building blocks of life
Structural and functional roles
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You can’t look at a living organism and not see proteins. Inside and out, proteins are the chief building blocks of all life. They make up skin and feathers and horns. They make up bones and muscles.
In your bloodstream, proteins fight invading microorganisms and stop you from bleeding to death from a shaving cut.
Proteins control the levels of sugar and other chemicals in your bloodstream and carry oxygen from one place in your body to another.
And in just about every cell in every living organism, proteins called enzymes initiate and assist all chemical reactions that occur.
Proteins are made of Amino Acids
20 different amino acids
Strung together to make proteins
All have 1 amino (NH2) and 1 carboxyl (COOH) group
Side chain determines chemical properties
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Although proteins perform several very different types of functions, they are all built in the same way and from the same raw materials in all organisms. Proteins, are constructed from a sort of alphabet. Instead of 26 letters, there are 20 molecules, known as amino acids. Unique combinations of these 20 amino acids are strung together, like beads on a string, and the resulting protein has a unique structure and chemical behavior. They all have the same basic two-part structure: one part is the same in all 20 amino acids, and the other part is unique, differing in each of the 20 amino acids.
Proteins contain the same familiar atoms as carbohydrates and lipids—carbon, hydrogen, and oxygen—but differ in an important way: they also contain nitrogen.
At the center of every amino acid is a carbon atom, with four covalent bonds (Figure 2-37 Amino acid structure). One bond attaches the carbon to something called a carboxyl group, which is a carbon bonded to two oxygen atoms. The second bond attaches the central carbon to a single hydrogen atom. The third bond attaches the central carbon to an amino group, which is a nitrogen atom bonded to three hydrogen atoms. These components are the foundation that identifies a molecule as an amino acid and, as multiple amino acids are joined together, form the “backbone” of the protein. The fourth bond attaches the central carbon to a functional group or side chain.
This side chain is the unique part of each of the 20 amino acids. In the simplest amino acid, glycine, for example, the side chain is simply a hydrogen atom. In other amino acids, the side chain is a single CH3 group or three or four such groups. Most of the side chains include both hydrogen and carbon, and a few include nitrogen or sulfur atoms. The side chain determines an amino acid’s chemical properties, such as whether the amino acid molecule is polar or nonpolar.
2.15 Proteins are an essential dietary component.
Proteins are needed for
Growth
Repair
Replacement
Complete proteins: have all essential amino acids (essential: we cannot make them)
Incomplete proteins do not
Complementary proteins supply all amino acids
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The atoms present in the plant and animal proteins we eat—especially nitrogen—are essential to the constant growth, repair, and replacement that take place in our bodies.
As we eat protein and break it down into its parts through digestion, our bodies are collecting the amino acids needed for various building projects. Proteins also store energy in their bonds and, like carbohydrates and lipids, they can also be used to fuel living processes.
The amount of protein we need depends on the extent of the building projects underway at any given time. Most individuals need 40 to 80 grams of protein per day. Bodybuilders, however, may need 150 grams a day or more to achieve the extensive muscle growth stimulated by their training; similarly, the protein needs of pregnant or nursing women are very high as well. Many foods, called complete proteins, have all of the essential amino acids.
Animals products such as milk, eggs, fish, chicken, and beef tend to provide complete proteins.
Most vegetables, fruits, and grains, on the other hand, more often contain incomplete proteins, which do not have all of the essential amino acids.
If you consume only one type of incomplete protein in your diet, you may be deficient in one or more of the essential amino acids. Two incomplete proteins (called complementary proteins) eaten together, however, can provide all essential amino acids. Traditional dishes in many cultures often include such pairings. Examples are corn and beans in Mexico, rice and lentils in India, and rice and black-eyed peas in the southern United States.
2.16 A protein’s function is influenced by its three-dimensional shape.
Amino acids are bound by peptide bonds
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Proteins are formed by linking individual amino acids together with a peptide bond, in which the amino group of one amino acid is bonded to the carboxyl group of another Two amino acids joined together is a dipeptide, and several amino acids joined together is a polypeptide. Protein molecules have 4 structural levels, from primary to quaternary.
Primary Structure
The sequence of amino acids
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The sequence of amino acids in the polypeptide chain is called the primary structure of the protein, and can be compared to the sequence of letters that spells a specific word.
Secondary Structure
Hydrogen bonding between amino acids
The two most common patterns:
twist in a corkscrew-like shape
zig-zag folding
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Amino acids don’t remain in a simple straight line like beads on a string, though.
The chain begins to fold as side chains come together and hydrogen bonds form between various atoms within the chain.
The two most common patterns of hydrogen bonding between amino acids cause the chain to either twist in a corkscrew-like shape or into a zig-zag folding.
This hydrogen bonding between amino acids gives a protein its secondary structure.
Tertiary Structure
Folding and bending of the secondary structure
Due to bonds such as hydrogen bonds or covalent sulfur-sulfur bonds.
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The protein eventually folds and bends upon itself and additional bonds continue to form between atoms within the side chains of amino acids that are near each other.
Eventually, the protein folds into a unique and complex three-dimensional shape called its tertiary structure.
The exact form comes about as the secondary structure folds and bends, bringing together amino acids that then form bonds such as hydrogen bonds or covalent sulfur-sulfur bonds.
Quaternary Structure
When two or more polypeptide chains are held together by bonds between the amino acids on the different chains
Hemoglobin: 4 chains
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Some protein molecules have a quaternary structure in which two or more polypeptide chains are held together by bonds between the amino acids on the different chains.
Hemoglobin, the protein molecule that carries oxygen from the lungs to the cells where it is needed, is made from four polypeptide chains, two “alpha” chains and two “beta” chains.
Denaturation
Egg whites contain much protein.
Why does beating them change their texture, making them stiff?
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For proteins to function properly, they must retain their three-dimensional shape. When their shapes are deformed, they usually lose their ability to function. We can see proteins deformed when we fry an egg. The heat breaks the hydrogen bonds giving the protein its shape. The proteins in the clear egg white unfold, losing their secondary and tertiary structure. Disruption of protein folding is called denaturation. Almost any extreme environment will denature a protein. Take a raw egg, for instance, and crack it into a dish containing baking soda or rubbing alcohol. Both chemicals are sufficiently extreme to turn the protein white like fried egg whites.
2.17 Enzymes are proteins that initiate and speed up chemical reactions.
Enzymes are proteins that help initiate and speed up chemical reactions.
They are not permanently altered in the process but rather can be used again and again.
Biological catalysts
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Protein shape is particularly critical in enzymes, molecules that help initiate and accelerate the chemical reactions in our bodies. Enzymes emerge in their original form when the reaction is complete and thus can be used again and again.
Enzymes facilitate chemical reactions 1
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Here’s how enzymes work:
Think of an enzyme as a big piece of popcorn. Its tertiary or quaternary structure gives it a complex shape with lots of nooks and crannies. Within one of those nooks is a small area called the “active site” (Figure 2-42 part 1 Lock and key). Based on the chemical properties of the atoms lining this pocket, the active site provides a place for the reactants, called substrate molecules, to nestle briefly.
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Enzymes facilitate chemical reactions 2
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Enzymes are very choosy: they bind only with their appropriate substrate molecules, much like a lock that can be opened with one key (Figure 2-42 part 2 Lock and key).
The exposed atoms in the active site have electrical charges that attract rather than repel the substrate molecules, and only the substrate molecules can fit into the active site groove.
Once the substrate is bound to the active site, a reaction can take place—and usually does very quickly.
What is Activation Energy?
Chemical reactions occurring in organisms can either release energy or consume energy.
In either case, the reaction needs a little “push” in order to initiate the reaction―called activation energy.
Enzymes act as catalyst by lowering the activation energy.
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The chemical reactions that occur in organisms can either release energy or consume energy. But in either case, there is a certain minimum energy—a little “push”—needed to initiate the reaction, called activation energy.
And although enzymes don’t alter the amount of energy released by a reaction, they
act as catalysts by lowering the activation energy, which causes the reaction to occur more quickly.
How do enzymes do this? Go to next slide…
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An enzyme can reduce the activation energy in a variety of ways.
By stressing, bending, or stretching critical chemical bonds
By directly participating in the reaction
By creating a microhabitat that is conducive to the reaction
By simply orienting or holding substrate molecules in place so that they can be modified.
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If not for enzymes, it is possible that nothing would even get done. At least not in living organisms. Enzymes don’t alter the outcome of reactions, but without the chemical “nudge” they supply—often increasing reaction rates to millions of times their uncatalyzed rate—the processes necessary to sustain life could not occur.
An enzyme can help to bring about the reaction in a variety of ways. These include:
By stressing, bending, or stretching critical chemical bonds, increasing their likelihood of breaking.
By directly participating in the reaction, perhaps temporarily sharing one or more electrons with the substrate molecule, thereby giving it chemical features that increase its ability to make or break other bonds.
By creating a microhabitat that is conducive to the reaction. For instance, the active site might be a water-free, nonpolar environment, or it might have a slightly higher or lower pH than the surrounding fluid. Both of these slight alterations might increase the likelihood that a particular reaction occurs.
By simply orienting or holding substrate molecules in place so that they can be modified.
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Factors influencing the rate of chemical reactions: 1. enzyme concentration
Insert new figure 2-43, preferably broken into 4 stepped segments
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There are several factors that affect the activity of an enzyme and therefore the rate of chemical reactions. These factors include:
Enzyme and substrate concentration.
For a given amount of substrate, an increase in the amount of enzyme increases the rate at which the reaction occurs. Similarly, for a given amount of enzyme, an increase in the substrate concentration increases the reaction rate. In both cases, the increases occur only up to the point at which all of the enzyme molecules are bound to substrate, or vice versa, at which point additional enzyme or substrate no longer increases the reaction rate.
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Insert fig 2-43, pt 2
Factors influencing the rate of chemical reactions: 2. temperature
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2. Temperature.
Because increasing the temperature increases the movement of molecules, reaction rates generally increase at higher temperatures. This occurs only up to the optimum temperature for an enzyme. At higher temperatures, reaction rates decrease as enzymes lose their shape or even denature. Enzymes from different species can have widely differing optimum temperatures.
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Insert fig 2-43, pt 3
Factors influencing the rate of chemical reactions: 3. pH
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3. pH.
As with temperature, enzymes have an optimum pH. Above or below this pH, interactions between excess hydrogen or hydroxide ions and amino acid side chains in the active site disrupt enzyme function (and sometimes structure) and decrease reaction rates.
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Insert fig 2-43, pt 4
Factors influencing the rate of chemical reactions: 4. Activators & inhibitors
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4. Presence of inhibitors or activators.
One of the most common ways that cells regulate their metabolic pathways is through the binding of other chemicals to enzymes. This can alter enzyme shape in a way that increases or decreases the enzyme’s activity.
Inhibitors reduce enzyme activity and come in two types: competitive inhibitors bind to the active site, blocking substrate molecules from the site and thus from taking part in the reaction. Noncompetitive inhibitors do not compete for the active site but rather bind to another part of the enzyme, altering its shape in a way that changes the structure of the active site, reducing or blocking its ability to bind with substrate. Often, it is the very product of a metabolic pathway that acts as an inhibitor of enzymes early in the pathway, effectively shutting off the pathway when enough of its end product has been produced.
Just as a molecule can bind to an enzyme and inhibit the enzyme’s activity, some cellular chemicals act as activators. Instead of their binding to an enzyme “turning it off,” their binding to the enzyme “turns it on,” altering the enzyme’s shape or structure so that it can now catalyze a reaction.
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“Misspelled” Proteins
Incorrect amino acid sequence
Active site disruptions
Phenylketonuria, lactose
intolerance
Insert fig 2-44 to right
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Sometimes a protein “word” is misspelled in that the sequence of amino acids is incorrect.
If an enzyme is altered even slightly, the active site may change and the enzyme no longer functions.
Slightly modified, nonfunctioning enzymes are responsible for a large number of diseases and physiological problems, including the inability to break down the amino acid phenylalanine (phenylketonuria) among many others.
2.19 Nucleic acids are macromolecules that store information.
Insert fig 2-45
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We have examined three of life’s macromolecules: carbohydrates, lipids, and proteins.
We turn our attention now to the fourth: nucleic acids, macromolecules that store information and are made up of individual units called nucleotides. All nucleotides have three components: a molecule of sugar, a phosphate group (containing a phosphorous atom bound to four oxygen atoms), and a nitrogen-containing molecule.
Two Types of Nucleic Acids
Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)
Both play central roles in directing the production of proteins.
Insert fig 2-45 to right
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There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both play central roles in directing the production of proteins in living organisms, and by doing so play a central role in determining all of the inherited characteristics of an individual.
In both types of nucleic acids, the molecule has a consistent backbone: a sugar molecule attached to a phosphate group attached to another sugar, then another phosphate, and so on.
Attached to each sugar is one of the nitrogen-containing molecules, called DNA bases due to their chemical structure. A ten-unit nucleic acid therefore would have ten bases, one attached to each sugar within the sugar-phosphate backbone. But the base attached to each sugar is not always the same. It can be one of several different bases.
For this reason, a nucleic acid is often described by the sequence of bases attached to the sugar-phosphate backbone.
Information Storage
The information in a molecule of DNA is determined by its sequence of bases.
Adenine, guanine, cytosine, and thymine
CGATTACCCGAT
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Nucleic acids are able to store information by varying which base is attached at each position within the molecule.
At each position in a molecule of DNA, for example, the base can be any one of four possible bases: adenine (A), thymine (T), guanine (G), or cytosine (C).
Just as the meaning of a sentence is determined by which letters are strung together, the information in a molecule of DNA is determined by its sequence of bases. One molecule may have the sequence adenine, adenine, adenine, guanine, cytosine, thymine guanine—abbreviated as AAAGCTG. Another molecule may have the sequence CGATTACCCGAT. Because the information differs in each case, so too does the protein for which the sequence codes.
2.20 DNA holds the genetic information to build an organism.
Insert new fig 2-46
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A molecule of DNA has two strands, each a sugar-phosphate-sugar-phosphate backbone with a base sticking out from each sugar molecule. The two strands wrap around each other, each turning in a spiral.
Although each strand has its own sugar-phosphate-sugar-phosphate backbone and sequence of bases, the two strands are connected by the bases protruding from them. You can imagine a molecule of DNA as a ladder. The two sugar-phosphate-sugar-phosphate backbones are like the long vertical elements of the ladder that give it height. A base sticking out represents a rung on the ladder. Or, more accurately, half a rung. The bases protruding from each strand meet in the center and bind to each other (via hydrogen bonds), holding the ladder together.
DNA differs from a ladder slightly, in that it has a gradual twist. The two spiraling strands together are said to form a double helix (Figure 2-44 A gradually twisting ladder).
Base-Pairing: bases are complementary
A & T
G & C
What is the complimentary strand to this strand: CCCCTTAGGAACC?
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The two intertwining spirals fit together because only two combinations of bases pair up together. The base A always pairs with T and C always pairs with G. Consequently, if the base sequence of one of the spirals is CCCCTTAGGAACC, the base sequence of the other must be GGGGAATCCTTGG.
That is why researchers working on the Human Genome Project describe only one sequence of nucleotides when presenting a DNA sequence. With that sequence, we can infer the identity of the bases in the complementary sequence and thus we know the exact structure of the nucleic acid. The sequence of base pairs containing the information about how to produce a particular protein may be anywhere from a hundred bases to several thousand.
In a human, all of the DNA in a cell, containing all of the instructions for every protein that a human must produce, contains about three billion base pairs. This DNA is generally in the nucleus of a cell.
2.21 RNA is a universal translator, reading DNA and directing protein production.
Insert fig 2-47
Bio100 NU Phelan
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The process of building a protein from a DNA sequence is not a direct one. Rather, it incorporates a middle man, RNA, that is also a nucleic acid.
Segments of the DNA are read off, directing the production of short strips of RNA that contain the information from the DNA about the amino acid sequence in a protein. The RNA moves to another part of the cell and then directs the piecing together of amino acids into a three-dimensional protein.
We explore this in greater detail in Chapter 5.
RNA differs from DNA in 3 important ways.
The sugar molecule of the sugar-phosphate backbone
Single-stranded
Uracil (U) replaces thymine (T)
Bio100 NU Phelan
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RNA differs from DNA in three important ways:
First, the sugar molecule of the sugar-phosphate backbone differs slightly, containing an extra atom of oxygen.
Second, RNA is single stranded. The sugar-phosphate-sugar-phosphate backbone is still there, as is the base that protrudes from each sugar. The bases, however, do not bind with anything else.
Third, while RNA has the bases―A, G, and C―it replaces the thymine with a similar base called uracil (U).
