Cereal Chem 61:490 - 495. | VIEW
Effect of pH on the Binding of Calcium Ions by Soybean Proteins.
R. D. Kroll. Copyright 1984 by the American Association of Cereal Chemists, Inc.
A calcium ion-selective electrode to measure free Ca2+ activity was used to study the binding of calcium ions to soy protein as a function of pH. pH strongly affects the extent of Ca2+ binding because hydrogen ions compete with calcium ions for the same binding sites on the protein molecule. These binding sites were identified as being associated with the side-chain carboxyl groups of the aspartic and glutamic acid residues and with the imidazole group of the histidine residues. The number of Ca2+ binding sites was found to be 43 per 10 5g of low-phytate protein. The affinity of the binding sites for calcium ions was shown to increase as pH increased over the pH range of 4-9, since the binding constant increased with increasing pH. Significant calcium ion binding did not occur much below pH 3. Between pH 3 and 7, a small change in pH results in a large change in the amount of Ca 2+ bound. Above pH 7, no further change occurs in calcium ion binding. Thus a critical pH range was found that corresponds to the ionization (deprotonation) of the side-chain carboxyl and imidazole groups. On the basis of the linear Scatchard plots, which were obtained for the binding data, these binding sites appear to be equivalent and independent. That is, all sites have the same intrinsic association constant and are noninteracting.