Structure of Glycinin and Its Significance in Thermal Polymerization of Glycinin Molecule

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lls globulin (referred to as glycinin), one of the major components of the soybean storage protein, is a multisubunit protein consisting of six subunits in which the acidic and basic polypeptides are linked by disulfide bridges in 1:1 ratio. The glycinin exhibits thermal gelling ability, i.e., polymerization of glycinin molecules. In this study, the correlation between the gelling ability and the protein structure of glycinin was investigated. (1) The constituent subunits of glycinin, ASI,ASII,ASIII, and ASIV, exhibited each characteristic contribution to interactions of glycinin molecules in the course of polymerization. (2) The nucleotide and deduced amino acid sequences of ASI (A_<la>B_x) subunit from G. max var. Shirotsurunoko were determined. The amino acid sequence was compared with those from var. CX635-1-1-1 and Bonminori. The comparison indicated that a considerable degree of homology exists among those subunits, and that homologous regions, variable regions and cysteine residues are located in specific regions of the polypeptide sequence similarly in all the subunits. (3) The interactions of glycinin molecules in the course of polymerization seemed to occur at specific regions of the surface of glycinin molecule through hydrophobic interactions and disulfide exchange reactions. The variable regions and free cysteine residues in polypeptide sequence may be involved in the hydrophobic interaction and disulfide exchange reaction, respectively.