1. Structural Biology and Biochemistry, Research Institute, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8, Canada, Department of Biochemistry, Faculty of Medicine, Medical Sciences Building, University of Toronto, Toronto, Ontario M5S 1A8, Canada, and Centre for Biological Calorimetry, Biotechnology Laboratory, University of British Columbia, 237-6174 University Boulevard, Vancouver, British Columbia V6T 1Z3, Canada