1. Center for Structural Biochemistry, Department of Biosciences at Novum, Karolinska Institutet, 14157 Huddinge, Sweden, Dipartimento di Scienze Biochimiche ‘A Rossi Fanelli', Università ‘La Sapienza', Piazzale A. Moro 5, 00185 Rome, Italy, and Faculty of Natural Sciences, Södertörns Högskola, 141 89 Huddinge, Sweden