Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom. tammy@cryst.bioc.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom. tammy@cryst.bioc.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom. tammy@cryst.bioc.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom. tammy@cryst.bioc.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom. tammy@cryst.bioc.cam.ac.uk