Role of disulfide bonds upon the structural stability of an amaranth globulin

Analysis of globulin-P, the polymerized amaranth globulin, gave a low amount of free sulfhydryls (10.2 +/- 0.5 micromol/g) from which 7 +/- 1 micromol/g was buried inside the molecule. In addition, its disulfide content was high (51 +/- 1 micromol/g) and similar to soybean 11S globulin content. The more exposed disulfide bridges were found to be stabilizing polymers, whereas the less reactive bridges were either linking P(20) and P(30) polypeptides or forming intrachain linkages. It was found that the buried bonds participate in the stabilization of folded polypeptides and the quaternary structure of the globulin. In turn, the dissociation of polymers and disruption of the quaternary structure by the action of 2-mercaptoethanol reverted upon removal of the reducing agent. This demonstrates that the polymerized state and the quaternary structure of the molecules are most favorable from the thermodynamic point of view. The similar content of SH and SS in globulin-P and globulin-S found in this laboratory suggests that the differences between these proteins may be ascribed to other compositional differences.