The 2.2 angstrom resolution structure of the O(H) blood-group-specific lectin I from Ulex europaeus

The tertiary and quaternary structure of the lectin I from Ulex eurpaeus (U E-I) has been determined to 2.2 Angstrom resolution. UE-I is a dimeric meta lloglycoprotein that binds the II-type 2 human blood group determinant [alp ha -Fuc alpha (1 --> 2)-beta -D-Gal beta (1 --> 4)-beta -D-GlcNAc alpha-]. Nine changes from the published amino acid sequence were necessary to accou nt for the electron density. The quaternary structural organization of UE-I is that of the most commonly occurring legume lectin dimer. The tertiary s tructure of the monomeric subunits is similar to that in the conventional l ectin subunit; however, some structural differences are noted. These differ ences include a four-stranded anti-parallel "S " sheet in UE-I versus the f ive-stranded S sheet in other lectin monomers. The Ala residue of the Ala-A sp cis-peptide bond present in the carbohydrate-binding site of the convent ional lectin monomer is replaced with a Thr in the UE-I structure. Also, a novel disulfide bridge linking Cys115 and Cys150 is present. There are two metallic ions, one calcium and the other manganese, per subunit. hr-linked oligosaccharides are at residues 23 and 111 of each subunit. One molecule o f R-2-methyl-2,4-pentanediol (R-MPD) is present in a shallow depression on the surface of each subunit. In order to examine the binding of the II-type ;! blood group determinant by UE-I, its beta -methyl glycoside (H-type 2-OM e) was docked into the binding site of R-MPD. The epitope previously identi fied for H-type 2-OMe by chemical mapping proved, with only minor adjustmen t of amino acid residues, to be complementary to the shallow cavity occupie d by R-MPD in the structure. Several key interactions have been proposed be tween the Ii-type 2-OMe and UE-I. (C) 2000 Academic Press.