1. Department of Biological and Nutritional Sciences, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K., Laboratory of Molecular Enzymology, The Babraham Institute, Babraham, Cambridge CB2 4AT, U.K., and Department of Molecular Biology and Biotechnology, Krebs Institute, University of Sheffield, Sheffield S10 2TN, U.K.