Carbonic anhydrase proposed catalytic cycle

Scheme 22. Proposed catalytic cycle for carbonic anhydrase.

The results of site-directed mutagenesis analysis of zinc ligands of higher plant p-carbonic anhydrase and of P. purpureum carbonic anhydrase have confirmed that zinc is essential for catalysis. X-ray fine structure data indicated that a water molecule is hydrogen bonded to the zinc-ligated Asp-151 and Asp-405. The water molecule is not directly coordinated to the zinc atom. A possible catalytic mechanism of C02 hydration cycle (211) has been proposed as given in Scheme 10. 

All the above structural and kinetic information obtained under a variety of conditions with different metal ions can be used to propose a catalytic cycle for carbonic anhydrase (Figure 2.21). As shown by studies on the pH-dependent properties of native and metal-substituted CAs, both type-I and type-II proteins have two acidic groups, the zinc-coordinated water and a free histidine. At... 

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