1. University of Gent, Department of Biochemistry, Physiology, and Microbiology, Laboratory of Protein Biochemistry and Protein Engineering, K. L. Ledeganckstraat 35, B-9000 Gent, Belgium, and University of Gent, Department of Organic Chemistry, Synthesis and Bio-organic Chemistry, Krijgslaan 281, S.4, B-9000 Gent, Belgium