Multivalency in lectins A crystallographic, modelling and light-scattering study involving peanut lectin and a bivalent ligand

Multivalency is believed to be important in the activity of lectins, although definitive structural studies on it have been few and far between. We have now studied the complexation of tetravalent peanut lectin with a synthetic compound containing two terminal lactose moieties, using a combination of crystallography, dynamic light scattering and modelling. Light scattering indicates the formation of an apparent dimeric species and also larger aggregates of the tetrameric lectin in the presence of the bivalent ligand. Crystals of presumably crosslinked lectin molecules could be obtained. They diffract poorly, but X-ray data from them are good enough to define the positions of the lectin molecules. Extensive modelling on possible crosslinking modes of protein molecules by the ligand indicated that systematic crosslinking could lead to crystalline arrays. The studies also provided a rationale for crosslinking in the observed crystal structure. The results obtained provide further insights into the general problem of multivalency in lectins. They indicate that crosslinking involving multivalent lectins and multivalent carbohydrates could lead to an ensemble of a finite number of distinct periodic arrays rather than a unique array.