1. Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94143-0446, Departments of Biochemistry and Chemistry and Rosenstiel Center for Basic Biomedical Research, Brandeis University, Waltham, Massachusetts 02154-9110, The Institute for Enzyme Research and Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, and Department of Biochemistry, University of Illinois, Urbana, Illinois 61801