Active-site sulfhydryl chemistry plays a major role in the misfolding of urea-denatured rhodanese

Unfolded bovine rhodanese, a sulfurtransferase, does not regain full activity upon refolding due to the formation of aggregates and disulfide-linked misfolded states unless a large excess of reductant such as 200 mM beta -ME and 5 mg/ml detergent are present [Tandon and Horowitz (1990), J. Biol. Chem. 265, 5967]. Even then, refolding is incomplete. We have studied the unfolding and refolding of three rhodanese forms whose crystal structures are known: ES, containing the transferred sulfur as a persulfide; E, without the transferred sulfur, and carboxymethylated rhodanese (CMR), in which the active site was blocked by chemical modification. The X-ray structures of ES, E, and CMR are virtually the same, but their tertiary structures in solution differ somewhat as revealed by near-UV CD. Among these three, CMR is the only form of rhodanese that folds reversibly, requiring 1 mM DTT. A minimum three-state folding model of CMR (N<->I<->U) followed by fluorescence at 363 nm, (N<->I) by fluorescence at 318 nm, and CD (I<->U) is consistent with the presence of a thermodynamically stable molten globule intermediate in 5-6 M urea. We conclude that the active-site sulfhydryl group in the persulfide form is very reactive; therefore, its modification leads to the successful refolding of urea-denatured rhodanese even in the absence of a large excess of reductant and detergent. The requirement for DTT for complete reversibility of CMR suggests that oxidation among the three non-active-site SH groups can represent a minor trap for refolding through species that can be easily reduced.