New computational evidence for the catalytic mechanism of carbonic anhydrase

Some aspects of the catalytic mechanism of HCA have been investigated. Either a zinc-bound water or a zinc-bound hydroxide has been considered as a nucleophile attacking CO 2. No reaction path exists in the former case, while a transition state for the nucleophilic attack has been located in the latter (barrier of 7.6 kcal mol?1). This activation energy is determined by the breaking of the hydrogen-bond network that shields the zinc-bound hydroxide when the CO 2 molecule approaches the reaction center. No ambiguity exists about the mechanism for the internal rearrangement of the zinc–bicarbonate complex. The rotation pathway (Lindskog mechanism) proposed by many authors is too energy demanding since it causes the breaking of the hydrogen-bond network around the bicarbonate. The only possible rearrangement mechanism is a proton transfer (Lipscomb) that occurs in two steps (each step corresponding to a double proton transfer) and involves the Thr199 residue as a proton shuttle.