Department of Biochemistry, University of Cambridge, Cambridge CB2 1GA, UK. swanand@cryst.bioc.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge CB2 1GA, UK. swanand@cryst.bioc.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge CB2 1GA, UK. swanand@cryst.bioc.cam.ac.uk