Metalloproteins- Roles of Metal Ions

Metalloproteins: Roles of Metal Ions in Protein Structure, Function, and

Regulation

Metalloproteins are proteins that contain metal ions. Metal ions play essential roles in the

structure, function and regulation of many metalloproteins. The specific metal ions bound and

their coordination geometry are important determinants of a metalloprotein's function.

Metalloproteins are ubiquitous and metal ion cofactors and clusters are present in nearly

every cell and play critical roles in processes such as oxygen transport, DNA synthesis, gene

regulation, metabolic catalysis and more.

Some important classes of metalloproteins and the roles of their metal ions include:

Zinc finger domains contain zinc ions that are tetrahedrally coordinated by cysteine and

histidine residues in the domains. These structural zinc binding modules allow proteins to

fold into distinct three-dimensional structures and are important protein-protein interaction

domains found in transcription factors where they mediate specific DNA binding. Variations

in residue sequence in different zinc finger domains allow them to recognize different DNA

sequences.

Ribonucleotide reductases catalyze the reduction of ribonucleotides to deoxyribonucleotides,

a key step in DNA synthesis and repair. They contain catalytic metal centers important for

substrate binding and redox activity including labeled tyrosyl radical cofactors. Class Ia

enzymes contain two subunits, one with a diferric tyrosyl radical cofactor critical for catalysis

mitochondria and bacteria. Cytochrome P450 enzymes act as monooxygenases that catalyze

regio- and stereospecific hydroxylations of organic substrates using reduced O2 and

performing important roles in oxidative drug metabolism, steroidogenesis, and fatty acid ω-

hydroxylation.

Carbonic anhydrase contains a zinc ion tetrahedrally coordinated by three histidine residues

and a water molecule. It catalyzes the reversible hydration of carbon dioxide to bicarbonate

and a proton. Humans have at least 16 isozymes which differ in subcellular localization and

catalytic properties but all utilize a binuclear Zn cluster for catalysis. Carbonic anhydrase is

essential for pH and CO2 homeostasis and electrolyte secretion in many tissues including the

kidneys, stomach, pancreas, and lungs.

sequesters and positions the metal cofactors for catalysis through histidine ligand

coordination. Their metal cofactors are important structural elements that enable distinct

redox chemistry during the dismutation reaction.

Nitrogenase enzymes contain the most complex metalloclusters known and are responsible

for biological nitrogen fixation, the conversion of atmospheric nitrogen to ammonia. They

catalyze the ATP-dependent reduction of N2 via a reducing substrate. The molybdenum-iron

(Mo-Fe) nitrogenase contains two metal clusters - the [8Fe-7S] P-cluster and [7Fe-9S-Mo-C-

homocitrate] FeMo-co factor, which are the active sites that coordinate substrate binding and

electron transfer during reduction. The vanadium-iron (V-Fe) nitrogenase contains similar

clusters with vanadium instead of molybdenum which allows it to function under

incorporation into copper/zinc superoxide dismutase or cytochrome c oxidase.

Metallochaperones utilize selective metal binding sites and protein-protein interactions to

safely traffic metals to their proper destinations.

Metallopeptidases contain catalytic zinc ions that activate water molecules for peptide bond

hydrolysis through coordination and polarization. Bacterial leukoproteases contain two zinc

ions in their active sites which act cooperatively during the catalytic mechanism. The

thermolysin family of neutral metallopeptidases utilizes a single zinc ion and two glutamate

residues to perform catalysis. Astacin family zinc metalloproteinases contain a single zinc

bound by three histidines. Their metal ion and coordinating side chains positions substrates in

the active site for catalysis. Metalloproteinases play diverse roles in pathogenic virulence,

As illustrated above, metal ions perform a wide variety of critical structural, catalytic and

regulatory functions in metalloproteins through distinct coordination geometries and

electronic properties. The specific metal ion identity and ligand coordination sphere it adopts

within the protein environment are important determinants of a metalloprotein's biological

activity and functionality. Let's take a deeper look at some molecular-level aspects of how

metal ions influence protein structure and function.

domain and other regions of the protein stabilize associations that maintains the precise

cofactor environment needed for activity.

cycle between different oxidation states. This redox flexibility is often linked to their ability

to perform electron transfer reactions in electron transport, metabolic catalysis, and small

molecule activation/activation. For example, heme iron cycling between Fe2+ and Fe3+

couples oxygen binding to redox chemistry in hemoglobin. Other metalloproteins like

nitrogenases, ribonucleotide reductase, cytochromes, hydrogenases and methane

monooxygenase all reversibly change the oxidation state of their transition metal cofactors to

drive multi-electron redox reactions.

backbonding to activate it for substrate monooxygenation. Computational studies predict

metal ions lower transition state energies of catalyzed reactions through stabilizing charge

development along reaction coordinates.

Nature has evolved clever strategies for manipulating metal ion coordination preferences and

ligand field stabilization energies through protein environments. This allows activation of

"reluctant" metals for function that otherwise would not be catalytically active. For example,

non-heme iron centers in enzymes like methane monooxygenase precisely coordinate Fe2+ to

perform oxygenase chemistry normally only associated with "harder" first row transition

metals like copper. In nitrogenase, the coordination of FeMo-cofactor creates an electronic

structure that allows breaking of the very strong triple bond in molecular nitrogen.

Protein environments also modulate redox potentials of bound cofactors to control electron

transfer reactivity. For instance, cytochrome c exchanges electrons with other redox partners

in respiration by precisely tuning the Fe2+/Fe3+ couple to an optimized potential through

noncovalent interactions with surrounding residues. Similarly, the tetraheme cytochrome c3

from sulfate reducing bacteria sequentially reduces four hemes to shift redox potentials 100

like Nif, Suf and ISC systems involved in iron-sulfur cluster biogenesis. Metal-sensing

transcription factors sense intracellular metal levels and tightly modulate expression of

import, efflux and storage proteins. Metal-dependent protein conformational changes also

CsoR are activated or deactivated upon binding copper. Nature employs intricate mechanisms

to precisely govern transition metal homeostasis and trafficking via metalloregulatory

proteins and pathways.

There are some key universal considerations that govern metal binding to biomolecules. All

metals bind organic ligands through donated electron pairs from heteroatoms like nitrogen,

oxygen, sulfur or phosphorus. Ligand donor atoms are generally Lewis bases that are

Elucidating metalloprotein functions also often requires reconstitution of metal centers

followed by characterization or activity assays. This is challenging due to protein structural

techniques are also complemented by computational bioinorganic studies that model

electronic structures, ligand interactions and redox properties to gain insight into reaction

mechanisms. Continued development of techniques that provide high resolution structures of

metal clusters and centers within intact metalloproteins will advance our understanding of

structure-function relationships in these complexes biological catalysts. Defining metallome

complements of organisms also enhances ability to design new biological functions by

targeted metallocofactor insertion or substitution.

In conclusion, this discussion illustrates how metal ions inserted into proteins serve as

indispensable cofactors for diverse biological functions from oxygen transport to metabolic

catalysis to gene regulation. The specific metal ion identity, ligands, coordination geometry

and redox properties it assumes integrated within the protein matrix dictate a metalloprotein's

activity. Nature has evolved intricate and often unexpected strategies for constructing and

regulating metallocofactor environments. Continued investigation using complementary

structure, function and regulation of many metalloproteins. The specific metal ions bound and

their coordination geometry are important determinants of a metalloprotein's function.

Metalloproteins are ubiquitous and metal ion cofactors and clusters are present in nearly

regulation, metabolic catalysis and more.

Some important classes of metalloproteins and the roles of their metal ions include:

Hemoproteins such as hemoglobin contain iron protoporphyrin IX (heme) groups which

allow them to bind and transport oxygen and carbon dioxide via redox cycling between

ferrous and ferric iron states. The precise octahedral coordination geometry of ferrous iron

domains found in transcription factors where they mediate specific DNA binding. Variations

in residue sequence in different zinc finger domains allow them to recognize different DNA

sequences.

Ribonucleotide reductases catalyze the reduction of ribonucleotides to deoxyribonucleotides,

a key step in DNA synthesis and repair. They contain catalytic metal centers important for

substrate binding and redox activity including labeled tyrosyl radical cofactors. Class Ia

enzymes contain two subunits, one with a diferric tyrosyl radical cofactor critical for catalysis

and the other with a dinuclear manganese(III/IV) cluster or diiron(III/IV) cluster for long-

range electron transfer during the reaction.

states. Cytochrome c functions in electron transport in respiration by shuttling electrons

between Complex III (cytochrome bc1 complex) and Complex IV (cytochrome c oxidase) in

mitochondria and bacteria. Cytochrome P450 enzymes act as monooxygenases that catalyze

regio- and stereospecific hydroxylations of organic substrates using reduced O2 and

performing important roles in oxidative drug metabolism, steroidogenesis, and fatty acid ω-

hydroxylation.

Superoxide dismutases are metalloenzymes that catalyze the disproportionation of the

superoxide radical anion into hydrogen peroxide and molecular oxygen. They protect cells

different metal cofactors including manganese (Mn-SOD in mitochondria), iron (Fe-SOD in

chloroplasts and some bacteria), or copper-zinc (Cu/Zn-SOD in cytosol and extracellular

space). Each class has a characteristic eight-stranded Greek key beta-barrel fold which

sequesters and positions the metal cofactors for catalysis through histidine ligand

coordination. Their metal cofactors are important structural elements that enable distinct

redox chemistry during the dismutation reaction.

molybdenum-deficient conditions. The electronic properties of the metal clusters underlie

nitrogenase's ability to break the very stable triple bond in dinitrogen gas.

specific recipient metalloproteins and prevent toxicity from free ions. They direct metalation

of recipient apoproteins, facilitate metal insertion or cluster assembly, and regulate

intracellular metal homeostasis. For example, copper chaperones like CCS deliver copper for

incorporation into copper/zinc superoxide dismutase or cytochrome c oxidase.

Metallochaperones utilize selective metal binding sites and protein-protein interactions to

safely traffic metals to their proper destinations.

ions in their active sites which act cooperatively during the catalytic mechanism. The

thermolysin family of neutral metallopeptidases utilizes a single zinc ion and two glutamate

residues to perform catalysis. Astacin family zinc metalloproteinases contain a single zinc

bound by three histidines. Their metal ion and coordinating side chains positions substrates in

the active site for catalysis. Metalloproteinases play diverse roles in pathogenic virulence,

coagulation, degradation of extracellular matrix, and protein processing.

Metal ion cofactors often bind within well-defined domains or motifs in metalloproteins and

their structures precisely position ligands for metal binding. These "metal binding domains"

structure. Examples are the four-helix bundle of cytochrome c, copper binding motifs like

cupredoxins, zinc finger domains, heme binding motifs in hemoproteins like myoglobin, and

iron-sulfur cluster binding motifs. Often non-covalent interactions between the metal binding

domain and other regions of the protein stabilize associations that maintains the precise

cofactor environment needed for activity.

Beyond just structural roles, metal ions undergo reversible redox reactions that allow them to

Lewis acid-base interactions between metal ions and protein ligands are also fundamental to

the catalytic activity of many metalloenzymes. Metal ions polarize atom and group

positively charged Zn2+ ion in carbonic anhydrase's active site attracts and positions the

negatively charged carbon dioxide molecule for hydration. The Fe2+ in ribonucleotide

reductase and oxygenating cytochrome P450s weakly binds molecular oxygen through pi-

backbonding to activate it for substrate monooxygenation. Computational studies predict

metal ions lower transition state energies of catalyzed reactions through stabilizing charge

development along reaction coordinates.

"reluctant" metals for function that otherwise would not be catalytically active. For example,

non-heme iron centers in enzymes like methane monooxygenase precisely coordinate Fe2+ to

perform oxygenase chemistry normally only associated with "harder" first row transition

metals like copper. In nitrogenase, the coordination of FeMo-cofactor creates an electronic

structure that allows breaking of the very strong triple bond in molecular nitrogen.

Manipulation of metal coordination through protein interactions is a key design principle that

mV per heme to efficiently transfer electrons down an energy gradient. Control of metal ion

redox potentials is vital for mediating electron transfer steps in biological energy conversions.

activity. Metallochaperones maintain strict control over free metal ion concentrations by

selectively delivering specific metal cofactors like copper or iron to metalloprotein targets.

Synthesis of metal clusters also involves coordination of metal insertion by assembly factors

like Nif, Suf and ISC systems involved in iron-sulfur cluster biogenesis. Metal-sensing

transcription factors sense intracellular metal levels and tightly modulate expression of

import, efflux and storage proteins. Metal-dependent protein conformational changes also

underlie regulatory strategies, such as how copper sensing transcription factors AztA and

nucleophilic and polarizable. The stronger the donating ability of a ligand, the more likely its

ligand substituent effect will stabilize higher formal oxidation states of transition metal

cations. Spectroscopic techniques like electron paramagnetic resonance (EPR) and X-ray

environments, oxidation states and spin states of metal ions in metalloproteins. These

techniques have revealed nature innovatively generates new coordination preferences and

geometries versus inorganic complexes.

Elucidating metalloprotein functions also often requires reconstitution of metal centers

followed by characterization or activity assays. This is challenging due to protein structural

changes from disrupting native metal binding sites. Biochemical and spectroscopic

mechanisms. Continued development of techniques that provide high resolution structures of

metal clusters and centers within intact metalloproteins will advance our understanding of

structure-function relationships in these complexes biological catalysts. Defining metallome

complements of organisms also enhances ability to design new biological functions by

targeted metallocofactor insertion or substitution.

structure-function mysteries and expand upon the remarkable catalytic diversity they exhibit.

Understanding metalloproteins' molecular design principles also stimulates efforts in

bioinspired catalyst design for important applications in energy, medicine and biotechnology.

Metalloproteins are proteins that contain metal ions. Metal ions play essential roles in the

structure, function and regulation of many metalloproteins. The specific metal ions bound and

their coordination geometry are important determinants of a metalloprotein's function.

Metalloproteins are ubiquitous and metal ion cofactors and clusters are present in nearly

every cell and play critical roles in processes such as oxygen transport, DNA synthesis, gene

regulation, metabolic catalysis and more.

and hemoglobin to tune their oxygen binding affinity for different physiological purposes.

Zinc finger domains contain zinc ions that are tetrahedrally coordinated by cysteine and

fold into distinct three-dimensional structures and are important protein-protein interaction

domains found in transcription factors where they mediate specific DNA binding. Variations

in residue sequence in different zinc finger domains allow them to recognize different DNA

sequences.

Ribonucleotide reductases catalyze the reduction of ribonucleotides to deoxyribonucleotides,

a key step in DNA synthesis and repair. They contain catalytic metal centers important for

Lewis acid-base interactions between metal ions and protein ligands are also fundamental to

the catalytic activity of many metalloenzymes. Metal ions polarize atom and group

electronegativity differences in bound substrates to facilitate reactions. For example, the

positively charged Zn2+ ion in carbonic anhydrase's active site attracts and positions the

negatively charged carbon dioxide molecule for hydration. The Fe2+ in ribonucleotide

"reluctant" metals for function that otherwise would not be catalytically active. For example,

non-heme iron centers in enzymes like methane monooxygenase precisely coordinate Fe2+ to

perform oxygenase chemistry normally only associated with "harder" first row transition

structure that allows breaking of the very strong triple bond in molecular nitrogen.

Manipulation of metal coordination through protein interactions is a key design principle that

expands the catalytic repertoire of bioinorganic active sites.

Protein environments also modulate redox potentials of bound cofactors to control electron

transfer reactivity. For instance, cytochrome c exchanges electrons with other redox partners

in respiration by precisely tuning the Fe2+/Fe3+ couple to an optimized potential through

selectively delivering specific metal cofactors like copper or iron to metalloprotein targets.

Synthesis of metal clusters also involves coordination of metal insertion by assembly factors

like Nif, Suf and ISC systems involved in iron-sulfur cluster biogenesis. Metal-sensing

import, efflux and storage proteins. Metal-dependent protein conformational changes also

underlie regulatory strategies, such as how copper sensing transcription factors AztA and

CsoR are activated or deactivated upon binding copper. Nature employs intricate mechanisms

to precisely govern transition metal homeostasis and trafficking via metalloregulatory

proteins and pathways.

There are some key universal considerations that govern metal binding to biomolecules. All

nucleophilic and polarizable. The stronger the donating ability of a ligand, the more likely its

ligand substituent effect will stabilize higher formal oxidation states of transition metal

cations. Spectroscopic techniques like electron paramagnetic resonance (EPR) and X-ray

absorption spectroscopy (XAS) are instrumental for characterizing the coordination

environments, oxidation states and spin states of metal ions in metalloproteins. These

techniques have revealed nature innovatively generates new coordination preferences and

mechanisms. Continued development of techniques that provide high resolution structures of

metal clusters and centers within intact metalloproteins will advance our understanding of

structure-function relationships in these complexes biological catalysts. Defining metallome

targeted metallocofactor insertion or substitution.

In conclusion, this discussion illustrates how metal ions inserted into proteins serve as

indispensable cofactors for diverse biological functions from oxygen transport to metabolic

catalysis to gene regulation. The specific metal ion identity, ligands, coordination geometry

and redox properties it assumes integrated within the protein matrix dictate a metalloprotein's

activity. Nature has evolved intricate and often unexpected strategies for constructing and

structure, function and regulation of many metalloproteins. The specific metal ions bound and

their coordination geometry are important determinants of a metalloprotein's function.

Metalloproteins are ubiquitous and metal ion cofactors and clusters are present in nearly

regulation, metabolic catalysis and more.

Some important classes of metalloproteins and the roles of their metal ions include:

Hemoproteins such as hemoglobin contain iron protoporphyrin IX (heme) groups which

allow them to bind and transport oxygen and carbon dioxide via redox cycling between

ferrous and ferric iron states. The precise octahedral coordination geometry of ferrous iron

acid sequence around the heme binding pocket allow different hemoproteins like myoglobin

and hemoglobin to tune their oxygen binding affinity for different physiological purposes.

Zinc finger domains contain zinc ions that are tetrahedrally coordinated by cysteine and

histidine residues in the domains. These structural zinc binding modules allow proteins to

fold into distinct three-dimensional structures and are important protein-protein interaction

enzymes contain two subunits, one with a diferric tyrosyl radical cofactor critical for catalysis

and the other with a dinuclear manganese(III/IV) cluster or diiron(III/IV) cluster for long-

range electron transfer during the reaction.

Cytochromes contain heme B (iron protoporphyrin IX) groups which allow them to transfer

electrons via reversible one-electron redox chemistry of the iron between ferrous and ferric

states. Cytochrome c functions in electron transport in respiration by shuttling electrons

between Complex III (cytochrome bc1 complex) and Complex IV (cytochrome c oxidase) in

mitochondria and bacteria. Cytochrome P450 enzymes act as monooxygenases that catalyze

regio- and stereospecific hydroxylations of organic substrates using reduced O2 and

performing important roles in oxidative drug metabolism, steroidogenesis, and fatty acid ω-

kidneys, stomach, pancreas, and lungs.

Superoxide dismutases are metalloenzymes that catalyze the disproportionation of the

from oxidative stress by removing superoxide. The classes of superoxide dismutases contain

different metal cofactors including manganese (Mn-SOD in mitochondria), iron (Fe-SOD in

chloroplasts and some bacteria), or copper-zinc (Cu/Zn-SOD in cytosol and extracellular

space). Each class has a characteristic eight-stranded Greek key beta-barrel fold which

sequesters and positions the metal cofactors for catalysis through histidine ligand

coordination. Their metal cofactors are important structural elements that enable distinct

redox chemistry during the dismutation reaction.

Nitrogenase enzymes contain the most complex metalloclusters known and are responsible

for biological nitrogen fixation, the conversion of atmospheric nitrogen to ammonia. They

catalyze the ATP-dependent reduction of N2 via a reducing substrate. The molybdenum-iron

(Mo-Fe) nitrogenase contains two metal clusters - the [8Fe-7S] P-cluster and [7Fe-9S-Mo-C-

homocitrate] FeMo-co factor, which are the active sites that coordinate substrate binding and

electron transfer during reduction. The vanadium-iron (V-Fe) nitrogenase contains similar

intracellular metal homeostasis. For example, copper chaperones like CCS deliver copper for

incorporation into copper/zinc superoxide dismutase or cytochrome c oxidase.

Metallochaperones utilize selective metal binding sites and protein-protein interactions to

Metallopeptidases contain catalytic zinc ions that activate water molecules for peptide bond

hydrolysis through coordination and polarization. Bacterial leukoproteases contain two zinc

ions in their active sites which act cooperatively during the catalytic mechanism. The

thermolysin family of neutral metallopeptidases utilizes a single zinc ion and two glutamate

residues to perform catalysis. Astacin family zinc metalloproteinases contain a single zinc

bound by three histidines. Their metal ion and coordinating side chains positions substrates in

activity and functionality. Let's take a deeper look at some molecular-level aspects of how

metal ions influence protein structure and function.

their structures precisely position ligands for metal binding. These "metal binding domains"

adopt characteristic folds that sequester the metal cofactor in the interior of the protein

structure. Examples are the four-helix bundle of cytochrome c, copper binding motifs like

cupredoxins, zinc finger domains, heme binding motifs in hemoproteins like myoglobin, and

iron-sulfur cluster binding motifs. Often non-covalent interactions between the metal binding

domain and other regions of the protein stabilize associations that maintains the precise

cofactor environment needed for activity.

Beyond just structural roles, metal ions undergo reversible redox reactions that allow them to

cycle between different oxidation states. This redox flexibility is often linked to their ability

to perform electron transfer reactions in electron transport, metabolic catalysis, and small

molecule activation/activation. For example, heme iron cycling between Fe2+ and Fe3+

couples oxygen binding to redox chemistry in hemoglobin. Other metalloproteins like

nitrogenases, ribonucleotide reductase, cytochromes, hydrogenases and methane

negatively charged carbon dioxide molecule for hydration. The Fe2+ in ribonucleotide

reductase and oxygenating cytochrome P450s weakly binds molecular oxygen through pi-

backbonding to activate it for substrate monooxygenation. Computational studies predict

development along reaction coordinates.

Nature has evolved clever strategies for manipulating metal ion coordination preferences and

ligand field stabilization energies through protein environments. This allows activation of

"reluctant" metals for function that otherwise would not be catalytically active. For example,

non-heme iron centers in enzymes like methane monooxygenase precisely coordinate Fe2+ to

perform oxygenase chemistry normally only associated with "harder" first row transition

in respiration by precisely tuning the Fe2+/Fe3+ couple to an optimized potential through

noncovalent interactions with surrounding residues. Similarly, the tetraheme cytochrome c3

from sulfate reducing bacteria sequentially reduces four hemes to shift redox potentials 100

redox potentials is vital for mediating electron transfer steps in biological energy conversions.

Delicate control over metal binding properties is also crucial for regulating metalloprotein

activity. Metallochaperones maintain strict control over free metal ion concentrations by

selectively delivering specific metal cofactors like copper or iron to metalloprotein targets.

Synthesis of metal clusters also involves coordination of metal insertion by assembly factors

like Nif, Suf and ISC systems involved in iron-sulfur cluster biogenesis. Metal-sensing

underlie regulatory strategies, such as how copper sensing transcription factors AztA and

CsoR are activated or deactivated upon binding copper. Nature employs intricate mechanisms

to precisely govern transition metal homeostasis and trafficking via metalloregulatory

proteins and pathways.

There are some key universal considerations that govern metal binding to biomolecules. All

techniques have revealed nature innovatively generates new coordination preferences and

geometries versus inorganic complexes.

followed by characterization or activity assays. This is challenging due to protein structural

changes from disrupting native metal binding sites. Biochemical and spectroscopic

techniques are also complemented by computational bioinorganic studies that model

electronic structures, ligand interactions and redox properties to gain insight into reaction

mechanisms. Continued development of techniques that provide high resolution structures of

metal clusters and centers within intact metalloproteins will advance our understanding of

structure-function relationships in these complexes biological catalysts. Defining metallome

activity. Nature has evolved intricate and often unexpected strategies for constructing and

regulating metallocofactor environments. Continued investigation using complementary

biochemical, biophysical and computational approaches will further reveal metalloproteins'

Understanding metalloproteins' molecular design principles also stimulates efforts in

bioinspired catalyst design for important applications in energy, medicine and biotechnology.

Metalloproteins are proteins that contain metal ions. Metal ions play essential roles in the

structure, function and regulation of many metalloproteins. The specific metal ions bound and

their coordination geometry are important determinants of a metalloprotein's function.

Metalloproteins are ubiquitous and metal ion cofactors and clusters are present in nearly

Some important classes of metalloproteins and the roles of their metal ions include:

Hemoproteins such as hemoglobin contain iron protoporphyrin IX (heme) groups which

allow them to bind and transport oxygen and carbon dioxide via redox cycling between

ferrous and ferric iron states. The precise octahedral coordination geometry of ferrous iron

domains found in transcription factors where they mediate specific DNA binding. Variations

in residue sequence in different zinc finger domains allow them to recognize different DNA

sequences.

Ribonucleotide reductases catalyze the reduction of ribonucleotides to deoxyribonucleotides,

a key step in DNA synthesis and repair. They contain catalytic metal centers important for

substrate binding and redox activity including labeled tyrosyl radical cofactors. Class Ia

enzymes contain two subunits, one with a diferric tyrosyl radical cofactor critical for catalysis

and the other with a dinuclear manganese(III/IV) cluster or diiron(III/IV) cluster for long-

range electron transfer during the reaction.

hydroxylation.

Carbonic anhydrase contains a zinc ion tetrahedrally coordinated by three histidine residues

and a proton. Humans have at least 16 isozymes which differ in subcellular localization and

catalytic properties but all utilize a binuclear Zn cluster for catalysis. Carbonic anhydrase is

essential for pH and CO2 homeostasis and electrolyte secretion in many tissues including the

kidneys, stomach, pancreas, and lungs.

Superoxide dismutases are metalloenzymes that catalyze the disproportionation of the

superoxide radical anion into hydrogen peroxide and molecular oxygen. They protect cells

chloroplasts and some bacteria), or copper-zinc (Cu/Zn-SOD in cytosol and extracellular

space). Each class has a characteristic eight-stranded Greek key beta-barrel fold which

sequesters and positions the metal cofactors for catalysis through histidine ligand

coordination. Their metal cofactors are important structural elements that enable distinct

redox chemistry during the dismutation reaction.

molybdenum-deficient conditions. The electronic properties of the metal clusters underlie

nitrogenase's ability to break the very stable triple bond in dinitrogen gas.

specific recipient metalloproteins and prevent toxicity from free ions. They direct metalation

of recipient apoproteins, facilitate metal insertion or cluster assembly, and regulate

intracellular metal homeostasis. For example, copper chaperones like CCS deliver copper for

incorporation into copper/zinc superoxide dismutase or cytochrome c oxidase.

Metallochaperones utilize selective metal binding sites and protein-protein interactions to

safely traffic metals to their proper destinations.

coagulation, degradation of extracellular matrix, and protein processing.

As illustrated above, metal ions perform a wide variety of critical structural, catalytic and

electronic properties. The specific metal ion identity and ligand coordination sphere it adopts

within the protein environment are important determinants of a metalloprotein's biological

activity and functionality. Let's take a deeper look at some molecular-level aspects of how

metal ions influence protein structure and function.

Metal ion cofactors often bind within well-defined domains or motifs in metalloproteins and

their structures precisely position ligands for metal binding. These "metal binding domains"

cupredoxins, zinc finger domains, heme binding motifs in hemoproteins like myoglobin, and

iron-sulfur cluster binding motifs. Often non-covalent interactions between the metal binding

domain and other regions of the protein stabilize associations that maintains the precise

cofactor environment needed for activity.

Beyond just structural roles, metal ions undergo reversible redox reactions that allow them to

Lewis acid-base interactions between metal ions and protein ligands are also fundamental to

the catalytic activity of many metalloenzymes. Metal ions polarize atom and group

positively charged Zn2+ ion in carbonic anhydrase's active site attracts and positions the

negatively charged carbon dioxide molecule for hydration. The Fe2+ in ribonucleotide

reductase and oxygenating cytochrome P450s weakly binds molecular oxygen through pi-

backbonding to activate it for substrate monooxygenation. Computational studies predict

metal ions lower transition state energies of catalyzed reactions through stabilizing charge

development along reaction coordinates.

Manipulation of metal coordination through protein interactions is a key design principle that

expands the catalytic repertoire of bioinorganic active sites.

transfer reactivity. For instance, cytochrome c exchanges electrons with other redox partners

in respiration by precisely tuning the Fe2+/Fe3+ couple to an optimized potential through

noncovalent interactions with surrounding residues. Similarly, the tetraheme cytochrome c3

from sulfate reducing bacteria sequentially reduces four hemes to shift redox potentials 100

mV per heme to efficiently transfer electrons down an energy gradient. Control of metal ion

redox potentials is vital for mediating electron transfer steps in biological energy conversions.

selectively delivering specific metal cofactors like copper or iron to metalloprotein targets.

Synthesis of metal clusters also involves coordination of metal insertion by assembly factors

like Nif, Suf and ISC systems involved in iron-sulfur cluster biogenesis. Metal-sensing

transcription factors sense intracellular metal levels and tightly modulate expression of

import, efflux and storage proteins. Metal-dependent protein conformational changes also

underlie regulatory strategies, such as how copper sensing transcription factors AztA and

nucleophilic and polarizable. The stronger the donating ability of a ligand, the more likely its

ligand substituent effect will stabilize higher formal oxidation states of transition metal

cations. Spectroscopic techniques like electron paramagnetic resonance (EPR) and X-ray

environments, oxidation states and spin states of metal ions in metalloproteins. These

techniques have revealed nature innovatively generates new coordination preferences and

geometries versus inorganic complexes.

Elucidating metalloprotein functions also often requires reconstitution of metal centers

followed by characterization or activity assays. This is challenging due to protein structural

changes from disrupting native metal binding sites. Biochemical and spectroscopic

In conclusion, this discussion illustrates how metal ions inserted into proteins serve as

indispensable cofactors for diverse biological functions from oxygen transport to metabolic

and redox properties it assumes integrated within the protein matrix dictate a metalloprotein's

activity. Nature has evolved intricate and often unexpected strategies for constructing and

regulating metallocofactor environments. Continued investigation using complementary

biochemical, biophysical and computational approaches will further reveal metalloproteins'

structure-function mysteries and expand upon the remarkable catalytic diversity they exhibit.

Understanding metalloproteins' molecular design principles also stimulates efforts in

bioinspired catalyst design for important applications in energy, medicine and biotechnology.