1. Department of Biochemistry and Biophysics, Göteborg University, Medicinaregatan 9C, P.O. Box 462, SE-405 30 Göteborg, Sweden, Department of Biochemistry, The Arrhenius Laboratories for Natural Sciences, Stockholm University, SE-106 91 Stockholm, Sweden, and Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Rua da Quinta Grande, 6, Apartado 127, 2780 Oeiras, Portugal