Zinc

Zinc is perhaps the most ubiquitous and versatile of all metal cofactors. More than 300 enzymes have a zinc cofactor. Table 4 lists some of the important zinc enzymes. Zinc-binding proteins that engage DNA, the so-called zinc finger proteins, attest to the versatility of zinc in biological systems. Approximately 3% of the genome of mammals codes for zinc finger protein. As a cofactor, zinc can perform both structural and catalytic functions. In carbonic anhydrase, for example, zinc enters into a coordinate bond with the CO2 substrate (Figure 5). In carboxypeptidase, zinc takes an active part in the cleavage of the peptide bond (Figure 6). Multisubu-nit enzymes such as aspartate transcarbamylase use zinc to coordinate the positions of the catalytic and regulatory subunits, a structural role. Cu2,Zn2 superoxide dismutase requires zinc to position the copper atom in the channel accessed by the substrate HO'2, another structural role. In zinc finger proteins, Zn2+ contributes to the stability of the loop structure that contacts the major and minor grooves of DNA. These examples illustrate why zinc is an important companion to enzymes and proteins.

Reactivity Zinc is considered a bland metal because it behaves as a divalent cation with no special geometric preference. It is perhaps this blandness that allows zinc to adapt to so many different enzyme environments. Zinc exists in one

Table 4 Important zinc enzymes

Enzyme

Source

Function

Zn/ protein

Alcohol

Liver

Alcohol

4

dehydrogenase

metabolism

Alkaline

Placenta

Unknown

4

phosphatase

Carbonic

Erythrocyte

CO2 hydration

1

anhydrase

Carboxypeptidase

Pancreas

Protein catabolism

1

Glutamate

Liver

Glutamate

2-6

dehydrogenase

synthesis

Leucine

Intestine

Peptide

4-6

aminopeptidase

catabolism

His'

H2CO3 H+

His'

HCQ:

ST'"-His His

^CO2

His'

His'

Figure 5 Zinc in carbonic anhydrase. Zinc in the enzyme 'activates' a water molecule (1) creating a better nucleophile to attack the CO2 (2). Once formed (3) the hydrated CO2 as HCO3 is displaced from the enzyme via a second water molecule (4) regenerating the active enzyme.

valence state, Zn2+, and hence has no redox properties. The Zn2+ ion is configured as a 3d10' which denotes a filled 3d orbital. For that reason, zinc complexes lack color and zinc itself behaves mostly as a cation. Zn2+ is a good electron acceptor (Lewis acid) that can enter into a coordinate bonding arrangement that polarizes groups to which it binds. This property allows zinc to increase the susceptibility of a chemical bond to attack. For example, Zn2+ polarizes water:

This makes the water behave more like a hydroxide ion and be more effective in attacking the CO2 to form HCO3 in the reaction catalyzed by carbonic anhydrase. Another example is the use of zinc to polarize the ester or amide bonds thus promoting nucleophilic attack of water on the bond as in reactions catalyzed by carboxypeptidase and aminopeptidase.

Figure 6 Zinc in carboxypeptidase. In carboxypeptidase, the zinc atom forms a binary complex with groups on the C-terminal end of the protein. Arrow shows bond that will be cleaved with water. Only the C-terminus residue is released from the protein.

Figure 6 Zinc in carboxypeptidase. In carboxypeptidase, the zinc atom forms a binary complex with groups on the C-terminal end of the protein. Arrow shows bond that will be cleaved with water. Only the C-terminus residue is released from the protein.

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