Binding of Medium Chain Fatty Acid Anions to Soybean β-Conglycinin

Abstract

The binding isotherms of aliphatic medium-chain carboxylate anions (octanoate and decanoate) to soy β-conglycinin have been measured by equilibrium dialysis at 30°C in a Tris-HC1 buffer, pH8.0, containing 10mM 2-mercaptoethanol. It has an intrinsic binding constant of 663M^-1 for decanoate at 30°C. The number of binding sites on the native β-conglycinin was estimated to be 3. Decanoate binding was independent on the ionic strength and temperature of the incubation medium. Decanoate binding was decreased when reductant was removed from the medium. The native tertiary structure with reduced sulfhydryl group was required maximal binding of decanoate to β-conglycinin. A decrease in decanoate binding occurred when pH of the medium was raised from 8.0 to 10.0, indicating that the presence of positively charged protein sites were required for the binding. The binding affinity was enhanced by an increase in chain length of the ligand, suggesting that the major binding energy is derived from hydrophobic interactions between non-polar tail of fatty acid and hydrophobic regions of protein.