The role of metal substitution in the promiscuity of natural and artificial carbonic anhydrases

Quantum mechanical cluster model and quantum mechanical/molecular mechanical (QM/MM) calculations have been performed on series of natural and artificial carbonic anhydrases. Insight is gained into the promiscuous nature of the enzymatic activity upon metal-ion substitution (Zn-, Co-, Cd-CA hydrating CO2), substrate substitution (Zn-CA hydrating carbodiiimide) and reaction substitution (Rh-CA hydrogenating CO2 to formic acid). Substituting Zn for Co slightly increases the efficiency while Cu decreases the activity of the enzyme. The Cd variant, which has two cysteine residues in its first coordination shell, chooses a different reaction mechanism from the native Zn enzyme, Cd preferring a bidentate mode of binding bicarbonate. Carbodiimide acts as a competitive inhibitor. Rh-CA is predicted to be an effective catalyst for the hydrogenation of CO2, aided by two explicit water molecules that lower the activation barrier of the rate-limiting step, the release of the product.