X-ray Crystallographic Studies of Alanine-65 Variants of Carbonic Anhydrase II Reveal the Structural Basis of Compromised Proton Transfer in Catalysis

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Abstract
The three-dimensional structures of A65F, A65L, A65H, A65T, A65S, and A65G human carbonic anhydrase II (CAII) variants have been solved by X-ray crystallographic methods to probe the importance of residue 65 and the structural implications of its evolutionary drift in the greater family of carbonic anhydrase isozymes. Structure−activity relationships in this series of CAII variants are correlated with those established for other carbonic anhydrase isozymes. We conclude that a bulky side chain at position 65 hinders the formation of an effective solvent bridge between zinc-bound water and H64 and thereby hinders solvent-mediated proton transfer between these two groups [Jackman, J. E., Merz, K. M., Jr., & Fierke, C. A. (1996) Biochemistry 35, 16421−16428]. Despite the introduction of a polar hydroxyl group at this position, smaller side chains such as serine or threonine substituted for A65 do not perturb the formation of a solvent bridge between H64 and zinc-bound solvent. Thus, the evolution of residue 65 size is one factor affecting the trajectory of catalytic proton transfer.