MODIFICATION OF A METAL-LIGAND IN CARBONIC-ANHYDRASE - CRYSTAL-STRUCTURE OF HIS(94)-]GLU HUMAN ISOZYME-II

One of the zinc ligands in human carbonic anhydrase II, His(94), has b een replaced with glutamic acid by site-directed mutagenesis. The muta tion leads to a less stable zinc binding site and to significant non-l ocal perturbations of the protein structure. The crystals are composed of a mixture of holo- and apoenzyme, and the side chain of Glu(94) ha s two conformations. In the holoenzyme, Glu(94) coordinates to the met al ion and is hydrogen bonded to Gln(92). In the apo form, Glu(94) is hydrogen bonded to Asn(67). The mutation has resulted in a 500-fold de crease of the catalyzed rate of CO2 hydration (k(cat)/K-m).