Carbohydrate binding specificity of pea lectin studied by NMR spectroscopyand molecular dynamics simulations

The conformational derails of Man(alpha 1,6)Man(alpha)OMe are investigated through NMR spectroscopy in conjunction with molecular modeling. The lowest energy structure (M1) in the adiabatic energy map calculated with a dielec tric constant of 50 has glycosidic dihedral angles of phi = - 60 degrees, p si = 180 degrees and omega = 180 degrees. The other low energy structure (M 2) has glycosidic dihedral angles of phi = - 60 degrees, psi = 180 degrees and omega = - 60 degrees. Molecular dynamics simulations and NMR experiment s prove that Man(alpha 1,6)Man(alpha)OMe in the free form exists with confo rmational averaging of M1 and M2 conformers predominantly. Molecular dynami cs simulations of the pea lectin-carbohydrate complex with explicit water m olecules starting from the X-ray crystallographic structure of pea lectin s how that the protein-carbohydrate interaction centers mainly on the hydroge n bonds and van der Waals interactions between protein and carbohydrate. Fr om the molecular dynamics simulation, it is found that the M1 structure can bind to pea lectin better than the M2 structure. The origin of this select ivity is the water- mediated hydrogen bond interactions between the remote mannose and the binding site of pea lectin as well as the direct hydrogen b ond interaction between the terminal mannose and pea lectin. Extensive netw orks of interactions in the carbohydrate binding site and the metal binding site are important in maintaining the carbohydrate binding properties of p ea lectin. Especially, the predominant factors of mannose binding specifici ty of pea lectin are the hydrogen bond interactions between the 4th hydroxy l groups of the terminal sugar ring and the side chains of Asp-81 and Asn-1 25 in the carbohydrate binding site, and the additional interactions betwee n these side chains of Asp-81 and Asn-125 and the calcium ion in the metal binding site of pea lectin. (C) 1999 Elsevier Science B.V. All rights reser ved.