1. Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459, Centre d'Ingeniere des Proteines et Laboratoire d'Enzymologie, Université de Liège, B-4000 Sart Tilman, Liège, Belgium, Department of Biochemistry, Medical University of South Carolina, Charleston, South Carolina 29425, and Department of Pharmacology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7365