Modification of a metal ligand in carbonic anhydrase: crystal structure of His94-->Glu human isozyme II.

One of the zinc ligands in human carbonic anhydrase II, His94, has been replaced with glutamic acid by site-directed mutagenesis. The mutation leads to a less stable zinc binding site and to significant non-local perturbations of the protein structure. The crystals are composed of a mixture of holo- and apoenzyme, and the side chain of Glu94 has two conformations. In the holoenzyme, Glu94 coordinates to the metal ion and is hydrogen bonded to Gln92. In the apo form, Glu94 is hydrogen bonded to Asn67. The mutation has resulted in a 500-fold decrease of the catalyzed rate of CO2 hydration (kcat/Km).