Conformational characteristics of legume 7S globulins as revealed by circular dichroic, derivative u.v. absorption and fluorescence techniques.

The 7S globulin storage proteins, phaseolin, vicilin, and beta-conglycinin of, respectively, dry bean, field pea, and soybean, are highly homologous, have similar predicted protein structures, and yet exhibit considerable differences in their susceptibility to various proteinases [Nielsen, S.S., Deshpande, S.S., Hermodson, M.A. & Scott, M.P. (1988) J. Agric. Food Chem. 36, 896-902]. These differences in their proteolytic behavior were studied in relation to their solution conformational states. The secondary structures of these three proteins determined by far u.v. circular dichroism were characterized by predominantly beta-sheet and beta-turn parameters. However, characterization of tertiary and quaternary structures using second derivative u.v. absorption spectroscopy, surface hydrophobicity using cis-parinaric acid as hydrophobic probe, and fluorescence quenching studies of intrinsic Trp fluorescence using an ionic (iodide) and a neutral (acrylamide) quencher indicated sharp differences in the conformation of these proteins. About 9.6 and 10.2 out of 13 and 15 tyrosyls/subunit of phaseolin and beta-conglycinin, respectively, were exposed to polar solvent, while the surface hydrophobicity varied beta-conglycinin greater than vicilin greater than phaseolin. The Trp residues in phaseolin were not accessible to iodide, while half those of vicilin and beta-conglycinin were quenched. The order of Trp accessibility to acrylamide was vicilin greater than beta-conglycinin greater than phaseolin. The relative compactness of these three proteins based on these studies was related to the observed differences in their susceptibility to various proteinases.