1. Department of Bio-Organic Chemistry, Bijvoet Center, Utrecht University, 3584 CH Utrecht, The Netherlands, Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, 9751 NN Haren, The Netherlands, and Centre for Carbohydrate Bioprocessing, TNO - University of Groningen, 9750 AA Haren, The Netherlands