Water in the active cavity of copper/zinc superoxide dismutase. A water 1H-nuclear-magnetic-relaxation-dispersion study.

Water 1H-nuclear-magnetic-relaxation-dispersion (NMRD) measurements of solutions of several copper/zinc superoxide dismutase isoenzymes as well as mutants of the human isoenzyme have been performed in order to monitor the presence of exchangeable water at the copper(II) center. The results have been compared with other spectroscopic features of the various derivatives and with their catalytic efficiency. The decrease in the amount of water in the first coordination sphere, detected through NMRD, parallels, in most cases, the increase in the tetragonal nature of the copper ion. On the other hand, the catalytic activity seems unrelated to the presence of water. Most strikingly, the Ile137 mutant of the human isoenzyme, approximately equal to 90% active, has no water in the copper coordination sphere; this is taken as evidence that the electron transfer is not a water-mediated process. The variation in the pH dependence of NMRD data between the wild-type enzyme and the human Ile137 mutant has been found to parallel the variation in the pH dependence of activity.