1. 1Department of Biochemistry, University College London, Gower Street, London WC1E 6BT, U.K.
2. 2Department of Biochemistry, University College London, Gower Street, London WC1E 6BT, U.K.
3. 3Department of Biochemistry, University College London, Gower Street, London WC1E 6BT, U.K.