Download AKUBOH OLIVIA 13/SCI03/001 BCH 413 METALLOENZYMES

AKUBOH OLIVIA
13/SCI03/001
BCH 413
METALLOENZYMES
Metalloenzymes, which comprise approximately one-third of the known enzymes, require
stoichiometric quantities of metal ions as cofactors, typically transition metal ions, for their
catalytic activities. The roles of metal ions in enzyme active sites (asides from structure
maintenance) include electron transfer, oxygen atom transfer, formation of coordinated
hydroxide, electrophilic catalysis, as well as substrate binding. Metalloenzymes catalyze
numerous reactions of physiological importance, including mitochondrial O2 reduction, peptide
bond cleavage, hydrocarbon hydroxylation, destruction of O2− and H2O2, and hydration of CO2.
Numerous examples of enzymes containing metals include iron, copper, zinc, magnesium,
cadmium, magnesium, cobalt, nickel, vanadium, molybdenum and so on. Metalloenzymes play
key roles in many processes central to human physiology, including the biosynthesis of DNA
and certain amino acids, steroid metabolism, destruction of superoxide and hydrogen peroxide,
biosynthesis of leukotrienes and prostaglandins, carbon dioxide hydration, neurotransmitter
metabolism, digestion, collagen biosynthesis, and, of course, respiration.
Na+, K+, Ca2+, and Mg2+ are considered macro minerals, or bulk elements; high concentrations of
these ions are needed for osmotic homeostasis, neuromuscular transmission, and
biomineralization (e.g. bone formation). Other essential metals are present in trace quantities in
humans and most other organisms. Even the most prominent biologically active transition metals
(iron, zinc, and copper) are trace elements.
The selection of metal ions for incorporation into metalloenzymes is strongly influenced by
bioavailability – a given element must be abundant in the environment and must be present in an
extractable form. A striking exception to this generalization involves the nearly universal
requirement for iron. Organisms have evolved selective uptake mechanisms for this element, the
most abundant transition metal in the earth’s crust, which forms insoluble ferric hydroxides in
the presence of O2. The incorporation of metal ions into metalloenzymes is also influenced by
other, chemically oriented, parameters such as ionic radius, charge, preferred coordination
geometry, ligand substitution and redox kinetics, aqueous solution chemistry, and
thermodynamic stability.
Incorporation of a metal ion, a posttranslational biosynthetic event, requires that the folding of
the polypeptide chain permit several side chains to congregate to form an appropriate metalbinding site. The primary metal-binding amino acid side chains are imidazole (His), carboxylate
(Asp and Glu), thiol (Cys), thioether (Met), and hydroxyl (Ser, Thr, and Tyr). Less frequently,
indole (Trp), guanidinium (Arg), and amide (Asn and Gln) groups are used. Backbone carbonyl
groups can also participate in metal binding. The side chain functional groups must usually be
deprotonated in order for a donor atom (O, N, or S) to form a metal–ligand bond. Some metal
ions coordinate to their binding sites in apo-metalloenzymes as simple aqua ions. For example,
Zn2+ binds to Apo-carbonic anhydrase in a multi-dentate ligand reaction: the metal ion sheds
coordinated water molecules as it binds to the active site of the Apo enzyme, which could be
viewed as an elaborate chelating agent. However, redox-active metal ions (e.g. Cu, Fe, Mn, Mo)
pose special problems by virtue of their reactivity with O2 − and H2O2. For example, it has
become clear during the last decade that pools of soluble copper ions are not used in the
physiological activation of eukaryotic copper enzymes, such as Cu, Zn superoxide dismutase and
cytochrome c oxidase. Copper ions are known Fenton reagents (OH• generators), and their
sequestration is a straightforward way of circumventing unwanted redox reactions. Intracellular
copper delivery is mediated by ‘‘chaperone’’ proteins that form complexes with their apoenzyme
targets prior to metal-ion exchange. Emerging research results indicate that chaperones likely
exist for other redox-active metals as well. The biosynthesis of some metalloenzymes of
physiological importance requires the prior synthesis of a specialized organometallic complex.
Such species include vitamin B12 (a cobalt corrin), hemes, and molybdenum cofactors.
Molybdoenzymes contain unusual metal-containing cofactors that have been shown to be
traceable to the participation of accessory biosynthetic genes. The FeMo cofactor (FeMoCo) of
nitrogenase also illustrates a more complex phenomenon – the use of metal clusters as cofactors
in enzymes. Such aggregates typically utilize sulfide or oxide (hydroxide) ions as bridges
between metal centers. Sometimes (e.g. the iron–sulfur cluster of aconitase) the cluster
assembles in a stepwise fashion after apoenzyme biosynthesis. In other cases (e.g. the
nitrogenase FeMo cofactor), accessory proteins, ‘‘molecular scaffolds’’, are needed for complete
cluster assembly; the cluster is subsequently transferred to the apoenzyme to complete the
biosynthesis of the mature metalloenzyme.
There are two principal types of metal dependent enzyme. Metalloenzymes contain at least one
tightly bound metal ion, at the active site, that is required for activity. Metal-activated enzymes,
on the other hand, generally lose catalytic activity during purification because their affinity for
the required metal is rather low. Mg2+-, K+-, and (most) Ca2+-dependent enzymes are metalactivated. Na+-dependency has yet to be unequivocally demonstrated for an enzyme. Examples
of metalloenzymes are known for each class of enzyme designated by the International Union of
Biochemistry and are as follows: oxidoreductases, transferases, hydrolases, lyases, isomerases,
and ligases. The following properties of metal ions make them well suited as catalysts for these
types of reactions:
1. Metal ions bind at least three (usually four or more) ligands, thereby promoting the
organization of protein structure.
2. With the notable exception of zinc, all of the other known transition metal cofactors can
potentially exist in two or more oxidation states; metalloenzyme catalysis of oxidation-reduction
(redox) reactions is thus quite common.
3. Metal-ion cofactors are electrophilic and can serve as effective Lewis acids for binding and
activating substrates. Two types of metal-ion reactivity are of fundamental importance in
considering the wide range of activities displayed by metalloenzymes: changes in oxidation state
and changes in bound ligands. Numerous examples of electron-transfer metalloproteins (blue
copper proteins, iron–sulfur proteins, cytochromes) are known; the metal coordination spheres of
these electron carriers do not change during electron transfer. Metal ions possessing static
coordination spheres could alternatively play purely structural roles, as do Ca2+ in calmodulin,
horseradish peroxidase, or stromelysin. Certain metals (e.g. Zn2+ in carboxypeptidase A) cannot
undergo redox reactions within the constraints imposed by nature; instead, acid–base catalysis is
the raison d’ˆetre for the metal in the enzyme. Lastly, both the metal-ion oxidation state and
coordination sphere could change during enzymatic turnover. Regardless, it is clear that metal
ions that directly participate in enzymatic catalysis must possess dynamic coordination
environments.