1. Department of Chemistry, University of York, Heslington, York, YO1 5DD, England, Department of Biological Structure, SM/20, University of Washington, Seattle, Washington 98195, and CEA, Département de Biologie Moléculaire et Structurale, Laboratoire Métalloprotéines, 17 avenue des Martyrs, 38054 Grenoble Cedex 9, France