Question

You start thinking about the mechanism of carbonic anhydrase. Describe what role the metal(mercury) plays in the reaction and the other type of catalysis present in carbonic anhydrase.

Mercury was found to be an inhibitor in a previous question.

Answer 1

Carbonic anhydrase catalyze the conversion of CO2 and H2O to carbonic acid followed by formation of bicarbonate ion and proton.

CO₂ + H₂O $\rightleftharpoons$ H₂CO₃$\rightleftharpoons$ H⁺ + HCO₃^-

In physiological system this reaction helps to maintain the buffer system in our body. If we look upon the structure we would come to now that carbonic anhydrase contain Zn++ ion which helps in metal ion catalysis strategy.

In absence of metal ion the catalysis is slow but when Zn++ ion present in active site it helps in binding with substrate and lower the pKa of the active site as a result nucleophilic attack on CO2 speeds up and by the reaction with OH- ion CO2 produces HCO3-. So Zn plays an important role here.

But as we know enzymes are very specific to its action and substrate, if it binds to mercury the action of carbonic anhydrase inhibits. Carbonic anhydrase mistakes mercury and zinc as both are bivalent cations it binds to mercury if present and as a result the metal ion catalysis stops.