Cereal Chem 53:513 - 524.  |  VIEW ARTICLE
Binding of Phytic Acid to Glycinin.

K. Okubo, D. V. Myers, and G. A. Iacobucci. Copyright 1976 by the American Association of Cereal Chemists, Inc. 

The binding of phytic acid, the hexaorthophosphate ester of myo-inositol, to glycinin, the major globulin of the soybean, was investigated over a broad range of pH. Above the isoelectric point of glycinin, pH 4.9, no binding could be detected by gel filtration through Sephadex G-75 at pH values of 6,8, and 10. Between pH 5.0 and 2.5, binding that resulted in insoluble complexes occurred. The extent of binding was found to increase with decreasing pH, from a value of zero at the isoelectric point to a maximum value of 424 equivalents of phytate per mole glycinin dimer (360,000 daltons). The latter value is in good agreement with the 411 cationic groups calculated from prior knowledge of the 12 amino-terminal groups and the experimentally determined number of basic amino acid residues of the glycinin dimer. At pH 2.5, glycinin is fully dissociated into 2S subunits; at pH 4.5-5.0, the protein is presumably fully associated in the 11S form. At these pH values, the binding sites were judged to be fully accessible on the surface and to reflect the net positive charge of the protein. However, between ph 4.0 and 3.0, where glycinin exists in partially dissociated states, some of the binding sites were found to be hindered. Clearly, the state of quaternary structure is a determinant of the binding, and electrostatic interaction of carboxylate groups with the cationic binding sites is proposed to explain this hindrance. Calcium ions promoted dissociation of phytate- glycinin complexes at pH 3, as measured by dissolution of the insoluble complexes and by gel filtration through Sephadex G-75. The dissociation can be explained in terms of competition between Ca2+ and the cationic sites of the protein for the phosphate groups of phytate. At pH 3.0, a 105-fold equivalent excess of calcium with respect to the protein cationic groups was necessary to completely dissociate the complex.