1. Institute of Biochemistry, Department of Enzymology, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany, Max Planck Research Unit for Enzymology of Protein Folding, Weinbergweg 22, 06120 Halle/Saale, Germany, and Max Planck Institute of Biochemistry, Am Klopferspitz 18a, 82152 Martinsried, Germany