1. Program in Structural Biology and Biophysics, Institute of Biotechnology, University of Helsinki, P.O. Box 65, FIN-00014, Helsinki, Finland, National Graduate School in Informational and Structural Biology, Department of Biochemistry, University of Turku, FIN-20014 Turku, Finland, A. N. Belozersky Institute of Physico-Chemical Biology and School of Chemistry, Moscow State University, Moscow 119899, Russia