Zinc-containing Biomolecules
Zinc proteins are common, but the absence of any accessible oxidation states apart from Zn (II) means that they cannot act as redox proteins. One of the problems in working with Zn (II) is that it is, as a d10 system, spectroscopically 'silent' which limits the type of studies that can be used to probe its chemistry in biomolecules. However several zinc proteins are relatively low molecular weight species and these were characterized by crystal structure analysis some decades ago.
Carbonic anhydrase is an enzyme that catalyses the carbonic acid/carbon dioxide hydra- tion/dehydration reaction, and the hydrolysis of certain esters. Tetrahedral zinc(II) is present at the active site. The mechanism has been fully elucidated. Carboxypeptidase is a fairly small enzyme (MW~34 600) that hydrolyses (cleaves) the terminal peptide (amide) bond of a peptide chain specifically. It is selective for terminal peptides where the terminal amino acid has an aromatic or branched aliphatic substituent of L absolute configuration. Zinc(II) sits in a protein pocket, bound in a distorted square pyramidal geometry to two histidine imidazole groups, a chelated glutamic acid carboxylate and a water molecule in its resting state. Didentate coordination of the carboxylate in the resting state of the latter shown in Figure 8.10 reverts to monodentate coordination (and distorted tetrahedral geometry for the zinc ion) in the active state. The ligand environment in carbonic anhydrase differs somewhat, involving just histidine imidazole groups and one water group in a tetrahedral environment (Figure 8.10).
Figure 8.10 A simple representation of the tetrahedral and distorted square pyramidal ligand environments for zinc(II) in carbonic anhydrase II and carboxypeptidase A respectively; both feature a coordinated water group. A simplified mechanism for the CO2/HCO3− process catalysed by carbonic anhydrase is also shown, at right.
The mechanism depicted for carbonic anhydrase (Figure 8.10 right) is an example of a biological reaction involving a coordinated hydroxide nucleophile, reminiscent of examples discussed in Chapter 6. The cyclic nature of the mechanism indicates the enzyme is able to be reused, as required in a catalytic process.
