1. Laboratoire d'Enzymologie and Centre d'Ingénierie des Protéines, Institut de Chimie B6, Université de Liège, Sart-Tilman, B-4000 Liège, Belgium, Department of Biochemistry and Genetics, University of Newcastle upon Tyne, NE2 4HH, United Kingdom, Department of Biochemistry and Molecular Biology, Jožef Stefan Institute, SLO-1111 Ljubljana, Slovenia, and Laboratorium für Biochemie, Universität Bayreuth, D-8580 Bayreuth, Germany