Is a "proton wire" concerted or stepwise? A model study of proton transfer in carbonic anhydrase

The energetics of proton transfer reactions in carbonic anhydrase (CA) have been studied with an active site model. Specifically, proton transfer from a zinc-bound water molecule to a histidine residue mediated by a numbers of water molecules was investigated. With two or three bridging water molecules, the proton transfers are fully or nearly fully concerted and only one saddle point exists. With an additional water molecule that forms a ring bridge, an intermediate is formed in which one of the water molecules exists as a hydronium ion. In contrast to previous calculations in which either a low-level of theory was employed or a stepwise mechanism was assumed, the energetics obtained from the current work are approximately consistent with the experimental estimates. In all of the scenarios, the motion of more than one proton is involved in the transition state, which is in agreement with the experimental observation that the reaction rates in H2O/D2O mixture have an exponential dependence on the fraction of D2O in the solvent. For three (W3) or four waters (W4), the proton transfer to the "His 64" model is hardly involved in the transition state, suggesting that the orientation of the proton acceptor is less important than for only two waters (W2). Thus, the W3 and W4 results are consistent with the experimental observation that many kinetic properties of the H64A mutant of CA in well-buffered imidazole solution are similar to the wild type. The barrier height increases, and the barrier frequency (and therefore, the contribution of tunneling) decreases as the number of bridging water molecules increases. Overall, these investigations demonstrate that the proton transfer reaction in CA is sensitive to the nature and structure of the water bridge, which would be influenced by the dynamics of the water molecules and amino acids in the active site of the protein.