1. Laboratory of Structural and Mechanistic Enzymology, Department of Biochemistry, Queen Mary and Westfield College, University of London, London E1 4NS, U.K., Department of Biological Sciences, University of Salford, Salford M5 4JW, U.K., and Protein Structure Research Group, Department of Chemistry, University of York, Heslington, York Y01 5DD, U.K.