THE CRYSTAL-STRUCTURE OF THE COMPLEXES OF CONCANAVALIN-A WITH 4'-NITROPHENYL-ALPHA-D-MANNOPYRANOSIDE AND 4'-NITROPHENYL-ALPHA-D-GLUCOPYRANOSIDE

Concanavalin A (Con A) is the best-known plant lectin and has importan t in vitro biological activities arising from its specific saccharide- binding ability. Its exact biological role still remains unknown, The complexes of Con A with 4'-nitrophenyl-alpha-D-mannopyranoside (alpha- PNM) and 4'-nitrophenyl-or-D-glucopyranoside (alpha-PNG) have been cry stallized in space group P2(1)2(1)2 with cell dimensions a = 135.19 An gstrom, b = 155.38 Angstrom, c = 71.25 and a = 134.66 Angstrom, b = 15 5.67 Angstrom and c = 71.42 Angstrom, respectively. X-ray diffraction intensities to 2.75 Angstrom for the alpha-PNM and to 3.0 Angstrom res olution for the alpha-PNG complex have been collected, The structures of the complexes were solved by molecular replacement and refined by s imulated annealing methods to crystallographic R-factor values of 0.18 5/0.186 and free-R-factor values of 0.260/0.274, respectively, In both structures, the asymmetric unit contains four molecules arranged as a tetramer, with approximate 222 symmetry, A saccharide molecule is bou nd in the sugar-binding site near the surface of each monomer. The non sugar (aglycon) portion of the compounds used helps to identify the ex act orientation of the saccharide in the sugar-binding pocket and is i nvolved in major interactions between tetramers. The hydrogen bonding network. in the region of the binding site has been analyzed, and only minor differences with the previously reported Con A-methyl-alpha-n-m annopyranoside complex structure have been observed. Structural differ ences that may contribute to the slight preference of the lectin for m annosides over glucosides are discussed. Calculations indicate a negat ive electrostatic surface potential for the saccharide binding site of Con A, which may be important for its biological activity. It is also shown in detail how a particular class of hydrophobic ligands interac t with one of the three so-called characteristic hydrophobic sites of the lectins. (C) 1996 Academic Press, Inc.