1. Max F. Perutz Laboratories, University Departments at the Vienna Biocenter, Department of Medical Biochemistry, Medical University of Vienna, Dr. Bohr Gasse 9/3, A-1030 Vienna, Austria, Max F. Perutz Laboratories, Department of Chemistry, University of Vienna, Campus Vienna Biocenter 5/1, 1030 Vienna, Austria, and Center for Physiology and Pathophysiology, Institute for Medical Chemistry, Medical University of Vienna, Währingerstrasse 13, 1090 Vienna, Austria