1. School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK
2. Department of Mathematical Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK
3. Laboratoire de Biochimie Théorique, CNRS UPR9080, Institut de Biologie-Physico-Chimique, 13 rue Pierre et Marie Curie, 75005 Paris, France