1. Institute of Biophysics and X-ray Structure Research of the Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria, Institute of Physiology, University of Graz, Harrachgasse 21, A-8010 Graz, Austria, and Institute of Medical Biochemistry, University of Graz, Harrachgasse 21, A-8010 Graz, Austria