Theoretical study of carbonic anhydrase‐catalyzed hydration of co2: A brief review

Abstract

Quantum mechanical calculations have been used to study the reaction mechanism of human carbonic anhydrase‐catalyzed hydration of CO₂. This reaction is responsible for fast metabolism of CO₂ in the human body. For each of the reaction steps, possible catalytic effects of active site residues are examined. The pertinent results are as follows. (1) For CO₂ binding, the experimentally observed 2.5 cm⁻¹ frequency shift of the asymmetic stretching frequency between measurements taken in the aqueous solution and in the enzyme is reproduced in our theoretical calculations. Our results suggest that CO₂ binds to the zinc ion within the hydrophobic pocket. (2) No energy barrier is found for the nucleophilic attack from Zn²⁺−bound OH⁻ to C of CO₂ to form Zn²⁺−bound HCO₃⁻. (3) For the internal proton transfer within zinc‐bound HCO₃⁻, the barrier of 35.6 kcal/mol for the direct internal proton transfer is reduced to 3.5 and 1.4 kcal/mol, respectively, when one or two water molecules are included for proton relay. (4) Displacement of Zn²⁺−bound HCO₃⁻ by H₂O is facilitated by the presence of the negatively charged Glu 106‐Thr 199 chain and by the association and the subsequent ionization of a fifth water ligand. (5) For the intramolecular proton transfer between Zn²⁺‐bound H₂O and His 64, the Zn²⁺ ion lowers the pK_a of Zn²⁺−bound water and repels the proton. His 64, or a similar proton receptor with a larger proton affinity than H₂O, functions as proton receiver; and the active site water molecules visualized by x‐ray crystallography are important for the proton relay function. In summary, it is demonstrated that in order to achieve effective catalysis, a sequence of precisely coordinated catalytic events among all participating catalytic elements in the enzyme's active site is essential.