STRUCTURAL-ANALYSIS OF 2 CRYSTAL FORMS OF LENTIL LECTIN AT 1.8-ANGSTROM RESOLUTION

The structures of two crystal forms of lentil lectin are determined an d refined at high resolution. Orthorhombic lentil lectin is refined at 1.80 Angstrom resolution to an R-factor of 0.184 and monoclinic lenti l lectin at 1.75 Angstrom resolution to an R-factor of 0.175. These tw o structures are compared to each other and to the other available leg ume lectin structures, The monosaccharide binding pocket of each lecti n monomer contains a tightly bound phosphate ion, This phosphate makes hydrogen bonding contacts with Asp-81 beta, Gly-99 beta, and Asn-125 beta, three residues that are highly conserved in most of the known le gume lectin sequences and essential for monosaccharide recognition in all legume lectin crystal structures described thus far. A detailed an alysis of the composition and properties of the hydrophobic contact ne twork and hydrophobic nuclei in lentil lectin is presented. Contact ma p calculations reveal that dense clusters of nonpolar as well as polar side chains play a major role in secondary structure packing, This is illustrated by a large cluster of 24 mainly hydrophobic amino acids t hat is responsible for the majority of packing interactions between th e two beta-sheets, Another series of four smaller and less hydrophobic clusters is found to mediate the packing of a number ofloop structure s upon the front sheet. A very dense, but not very conserved cluster i s found to stabilize the transition metal binding site. The highly con served and invariant nonpolar residues are distributed asymmetrically over the protein. (C) 1994 Wiley-Liss, Inc.