1. Laboratory of Structural and Mechanistic Enzymoloy, Department of Biochemistry, Queen Mary & Westwield College, University of London, Mile End Road, London, E1 4NS, U.K.
2. Protein Structure Research Group, Department of Chemistry, University of York, Heslington, York, YO1 5DD, U.K.
3. Department of Biological Sciences, University of Salford, Salford, M5 4JW, U.K.