Carbonic anhydrase zinc-containing model systems

The manifold important zinc-containing biological systems have spawned a multitude of model complexes,thereby greatly enriching knowledge of the coordination chemistry of zinc (see Sections 8 and 9). Types of model complexes include those with amino acid ligands to model zinc-peptide interactions, NNN-donor ligands to model the active site of carbonic anhydrase or carboxypeptidases, NO-donor Schiff bases to model Zn2 sites, and and polynuclear... 

The system illustrated by (272) forms the basis of a model for the zinc-containing metalloenzyme, carbonic anhydrase (Tabushi Kuroda, 1984). It contains Zn(n) bound to imidazole groups at the end of a hydrophobic pocket, as well as basic (amine) groups which are favourably placed to interact with a substrate carbon dioxide molecule. These are both features for the natural enzyme whose function is to catalyze the reversible hydration of carbon dioxide. The synthetic system is able to mimic the action of the enzyme (although side reactions also occur). Nevertheless, the formation of bicarbonate is still many orders of magnitude slower than occurs for the enzyme. 

We have already seen a number of models for the zinc(II) containing enzymes such as carbonic anhydrase in Section 11.3.2. Zinc is an essential component in biochemistry, and forms part of the active site of more then 100 enzymes, of which hydrolases (such as alkaline phosphatase and carboxypeptidase A), transferases (e.g. DNA and RNA polymerase), oxidoreductases (e.g. alcohol dehydrogenase and superoxide dismutase) and lysases (carbonic anhydrase) are the most common. In addition, the non-enzyme zinc finger proteins have an important regulatory function. In many of these systems, the non-redox-active Zn2+ ion is present as a Fewis acidic centre at which substrates are coordinated, polarised and hence activated. Other roles of zinc include acting as a template and playing a structural or regulatory role. 

---