STUDY OF THE CONFORMATIONAL STABILITY OF 7S-GLOBULIN FROM FRENCH BEANS (PHASEOLIN) USING HIGH-SENSITIVITY DIFFERENTIAL SCANNING MICROCALORIMETRY

The change of the conformational stability and quaternary structure of the 7S globulin from french beans (phaseolin) has been investigated in the pH range 2.0-11.0 using the high-sensitivity differential scanning microcalorimetry technique. It has been established that each polypeptide chain of phaseolin consists of two thermodynamically unequivalent cooperative domains. The number and type of the side-chain hydrogen bonds which participate in the stabilization of the folded structure of each domain have been determined. The more stable domain contains six side-chain hydrogen bonds: four of the carboxylate-tyrosyl type and two of the carboxylate-histidyl type. The less stable domain contains four side-chain hydrogen bonds: two of the carboxylate-tyrosyl type and two of the carboxylate-histidyl type. All these side-chain hydrogen bonds appear to be localized within the hydrophobic interior of the domains. It has been found that the 3S form of phaseolin that is a product of the complete phaseolin dissociation at extreme pH values does not undergo any cooperative transition at heating. Consequently, this form probably has a conformation of 'molten globule' type.