O

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 anhy-drase. 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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