Unexpected pH-Dependent Conformation of His-64, the Proton Shuttle of Carbonic Anhydrase II

The pH-dependent structural variations of human carbonic anhydrase II (CAII) have been studied by X-ray crystallographic methods at 2.3 Å resolution. The overall structure of CAII at pH 6.5 or 5.7 is quite similar to that determined at pH 8.5 (Eriksson, A. E.; Jones, T. A.; Liljas, A. Proteins: Struct. Fund. Gen. 1988, 4, 274–282). However, an important structural change is observed for His-64, the catalytic proton shuttle, at pH 5.7: its side chain rotates away from the active site by 64° about Ç- This alternate conformation of His-64 is interpretable with low occupancy in enzyme structures at higher pH values, although in some cases the corresponding electron density may be interpretable as a partially ordered solvent molecule. Given the unexpected mobility revealed for His-64, it is intriguing that significant conformational changes may accompany the function of the proton shuttle in catalysis.