1. Contribution from the Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461-1602, Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025, Department of Biochemistry, Stanford University, Stanford, California 94305-5307, Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461-1602, and Department of Chemistry, Stanford University, Stanford, California 94305-5080