Effect of preformed correct tertiary interactions on rapid two-state tendamistat folding: evidence for hairpins as initiation sites for beta-sheet formation.

The role of preformed correct side chain interactions, such as disulfide bonds, on protein folding kinetics is still not well understood. We investigated the effect of disulfide bond replacements on folding and stability of the small beta-sheet protein tendamistat. Tendamistat folds very fast (tau = 10 ms at pH 7 in water) and without detectable intermediates, which facilitates molecular interpretation of the kinetic data. Tendamistat contains two disulfide bonds, one between cysteines 11 and 27, which connects the ends of a beta-hairpin, and a second one between cysteines 45 and 73, which brings together the two outer strands of a three-stranded beta-sheet. Two single-disulfide variants of the protein were prepared by site-directed mutagenesis (tendamistat C11A/C27S and tendamistat C45A/C73A), and the effects on stability and on folding were monitored. Replacement of either disulfide bond leads to a large decrease in protein stability (DeltaDeltaG0 = 6.0 kcal/mol for the C11A/C27S variant and 5.1 kcal/mol for the C45A/C73A variant). This effect is caused both by entropic stabilization of the unfolded state and by enthalpic destabilization of the native structure. Kinetic experiments show that the main effect of fixed side chain contacts is on the unfolding rate. For both single-disulfide variants, unfolding is strongly accelerated (4250 times in the C11A/C27S variant and 250 times in the C45A/C73A variant) whereas the refolding rate constants are only slightly decreased. The activation parameters show that the observed small effect on the refolding reaction in the C11A/C27S variant is a consequence of large and compensating changes in the entropy and enthalpy of activation. Structural interpretation of the kinetic data suggests that formation of the beta-hairpin stabilized by the C11-C27 disulfide bond forms in the rate-limiting step of the refolding process. The interactions between the outer strands of the beta-sheet connected by the C45-C73 disulfide bond, in contrast, are made late in refolding. These results support the idea that beta-hairpins are initiation sites for beta-sheet formation and that additional strands are added late in the folding process.