1. Department of Chemical and Pharmaceutical Biology, Groningen Research Institute of Pharmacy, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands
2. Structural Biology Group, Groningen Institute of Biomolecular Sciences and Biotechnology, University of Groningen, Nijenborgh 7, 9747 AG Groningen, The Netherlands
3. Prozomix Ltd., Station Court, Haltwhistle, Northumberland NE49 9HN, United Kingdom
4. Molecular Enzymology Group, Groningen Institute of Biomolecular Sciences and Biotechnology, University of Groningen, Nijenborgh 7, 9747 AG Groningen, The Netherlands