Department of Biochemistry, University of Cambridge, Cambridge, UK. res50@mole.bio.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge, UK. res50@mole.bio.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge, UK. res50@mole.bio.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge, UK. res50@mole.bio.cam.ac.uk|Department of Biochemistry, University of Cambridge, Cambridge, UK. res50@mole.bio.cam.ac.uk