1. Department of Chemistry, Northwestern University, Evanston, Illinois 60208, Department of Biological Chemistry, The University of Michigan, Ann Arbor, Michigan 48109-0606, Scripps Research Institute, MB-1, 10666 Torrey Pines Road, La Jolla, California 92037, Johnson Research Foundation and Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, and Department of Biology, University of California at San Diego, La Jolla, California 92093