CRYSTAL-STRUCTURE OF CONCANAVALIN-B AT 1.65 ANGSTROM RESOLUTION - AN INACTIVATED CHITINASE FROM SEEDS OF CANAVALIA-ENSIFORMIS

Seeds of Canavalia ensiformis (jack bean) contain besides large amount s of canavalin and concanavalin A, a protein with a molecular mass of 33,800 which has been named concanavalin B. Although concanavalin B sh ares about 40% sequence identity with plant chitinases belonging to gl ycosyl hydrolase family 18, no chitinase activity could be detected fo r this protein. To resolve this incongruity concanavalin B was crystal lised and its three-dimensional structure determined at 1.65 Angstrom (1 Angstrom = 0.1 nm) resolution. The structure consists of a single d omain with a (beta/alpha)(8) topology A 30 amino acid residue long loo p occurs between the second beta-strand of the barrel and the second a lpha-helix. This extended loop is unusual for the (beta/alpha)(8) topo logy but appears in a similar conformation in the structures of the se ed protein narbonin and several chitinases as well. Two non-proline ci s-peptide bonds are present in the structure of concanavalin B: Ser34- Phe, and Trp265-Asn. This structural feature is rarely observed in pro teins, but could also be identified in the three-dimensional structure s of family 18 chitinases and narbonin in coincident positions. In the chitinases the aromatic residues of the non-proline cis-peptides have been proposed to have a function in the binding of the substrate. The region in concanavalin B, where in chitinases the active site is loca ted, shows two significant differences. First, the catalytic glutamic acid is a glutamine in concanavalin B. Second, although part of the su bstrate binding cleft of the chitinases is present in concanavalin B, it is much shorter. From this we conclude that concanavalin B and fami ly 18 chitinases are closely related, but that concanavalin B has lost its enzymatic function. It still may act as a carbohydrate binding pr otein, however. (C) 1995 Academic Press Limited