In the seeds of the legume plants, a class of sugar-binding proteins with high structural and sequential identity is found, generally called the legume lectins. The seeds of the common bean (Phaseolus vulgaris) contain, besides two such lectins, a lectin-like defense protein called arcelin, in which one sugar binding loop is absent. Here we report the crystal structure of arcelin-5 (Arc5), one of the electrophoretic variants of arcelin, solved at a resolution of 2.7 A. The R factor of the refined structure is 20.6%, and the free R factor is 27.1%. The main difference between Arc5 and the legume lectins is the absence of the metal binding loop. The bound metals are necessary for the sugar binding capabilities of the legume lectins and stabilize an Ala-Asp cis-peptide bond. Surprisingly, despite the absence of the metal binding site in Arc5, this cis-peptide bond found in all legume lectin structures is still present, although the Asp residue has been replaced by a Tyr residue. Despite the high identity between the different legume lectin sequences, they show a broad range of quaternary structures. The structures of three different dimers and three different tetramers have been solved. Arc5 crystallized as a monomer, bringing the number of known quaternary structures to seven.