1. Contribution from the Centre de Recherches sur les Macromolécules Végétales, CNRS, BP 53, 38041 Grenoble Cedex 9, France, Structural Biology Laboratory, Department of Chemistry, University of York, Heslington, York, Y010 5DD, UK, and Novo-Nordisk A/S, Novo allé, DK-2880 Bagvaerd, Denmark