1. Contribution from the Department of Medical Genetics, Department of Biochemistry, Department of Chemistry, University of Toronto, Toronto, ON, Canada, M5S 1A8, Structural Biology and Biochemistry Program, Hospital for Sick Children, Toronto, ON, Canada, M5G 1X8, Department of Biology, York University, Toronto, ON, Canada, M3J 1P3, and Protein Engineering Network Centres of Excellence