(Top) Three-dimensional structure of a peanut agglutinin (PNA) monomer showing the four loops involved in sugar binding: loops A, B, C, and D. The bound sugar (lactose) is shown as a “ball-and-stick” model. Calcium and manganese ions are required for ligand binding. (Bottom) Sequence alignment of loops A–D in other legume lectins. The size of binding-site loop D and monosaccharide specificity show an explicit correlation. Monosaccharide specificity and number of gaps are indicated at the right. Key residues are highlighted in blue and highly conserved residues have been indicated with an asterisk. (Adapted, with permission of Elsevier, from Vijayan M, Chandra N. 1999. Curr Opin Struct Biol 9: 707–714; and the PBD deposited structure 1V6I.) EcorL, Erythrina corallodendron lectin; DBL, Dolichos biflorus lectin; E-PHA, erythroagglutining phytohemagglutinin from Phaseolus vulgaris; L-PHA, leukoagglutinating phytohemagglutinin from P. vulgaris; SBA, soybean agglutinin Glycine max; PNA, peanut agglutinin Arachis hypogaea; LTA, Lotus tetragonolobus agglutinin; UEA-I, Ulex europaeus agglutinin-I; UEA-II, U. europaeus agglutinin-II; LAA-I, Laburnum alpinum agglutinin-I; LSL, Lonchocarpus sericeus lectin; ConA, concanavalin A Canavalia ensiformis; LenL, lentil lectin Lens culinaris; PSL, pea lectin Pisum sativum; Favin, Vicia faba lectin.