1. Department of Chemistry, University of British Columbia, Vancouver, BC, Canada, V6T 1Z1, Structural Biology Laboratory, Department of Chemistry, University of York, Heslington, York Y010 5YW, U.K., Molecular Pharmacology and Biological Chemistry, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, and Microbial Biochemistry and Genetics Unit, NIDCR, National Institutes of Health, Bethesda, Maryland 20892