1. Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, Graduate Program in Biochemistry and Structural Biology, State University of New York at Stony Brook, Stony Brook, New York 11794-5230, and Rudolf Virchow Center for Experimental Biomedicine and Institute of Structural Biology, Univeristy of Würzburg, Versbacher Str. 9, 97078 Würzburg, Germany