1. Department of Biochemistry and Molecular Biology, University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60649, and Department of Physiology and Biophysics, Weill Cornell Medical College, 1300 York Avenue, New York, New York 10065