1. Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 København Ø, Denmark, Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-3900, Department of Protein Chemistry, Institute of Molecular Biology, University of Copenhagen, Øster Farimagsgade 2A, DK-1353 København K, Denmark, and School of Biological Sciences, University of Sussex, Brighton BN1 9QG, United Kingdom