Role of the [65-72] disulfide bond in oxidative folding of bovine pancreatic ribonuclease A

To assess the role of the 65-72] disulfide bond in the oxidative folding of RNase A, use has been made of C65S, C72S], a three-disulfide-containing mutant of RNase A which regenerates from its two-disulfide precursor in an oxidation and conformational folding-coupled rate-determining step. The distribution of disulfide bonds in the one-disulfide-containing ensemble of this mutant has been characterized. In general, the disulfide-bond distribution in its 1S ensemble agrees relatively well with the corresponding distribution in wt-RNase A and with distributions based on calculations of loop entropy, except for the absence of the 65-72] disulfide bond. There is no bias (over the entropic influence) for the three native disulfide bonds, 26-84], 40-95], and 58-110]. Previous oxidative folding results for wt-RNase A indicated the predominance of the des 40-95] intermediate over des 65-72] after the rate-determining step in the regeneration process. Considering that there is no preferential distribution of disulfides in the 1S ensemble of C65S, C72S], in contrast to the preferential population of the 65-72] disulfide bond in wt-RNase A, these results indicate a critical role for the 65-72] disulfide bond in the regeneration of wt-RNase A. Furthermore, analysis of the disulfide distribution of the 1S intermediates of C65S, C72S] compared to that of wt-RNase A lends support for a physicochemical basis for the previously observed slow folding rate of this mutant, compared to its analogue (des 65-72]) of wt-RNase A.