Intact disulfide bonds decelerate the folding of ribonuclease T1

Disulfide bonds in a folding protein chain are equivalent to prematurely formed native-like tertiary interactions. We investigated whether the mechanism of protein folding is changed by the presence of disulfide bonds. As a model we used the S54G/P55N-variant of ribonuclease T1, a protein with two disulfide bonds and a single cis proline (Pro39), and we measured both the direct and the proline-limited folding reactions before and after breaking of the disulfide bonds. The folding kinetics were compared under refolding conditions, in the regions of the urea-induced unfolding transitions of the two forms, and under unfolding conditions. The kinetics in the transition regions were analyzed on the basis of a three-species mechanism and all microscopic rate constants of folding and of prolyl isomerization could be determined as a function of the urea concentration from the measured rates and amplitudes. These kinetic analyses indicated that the disulfide bonds can be rather unfavorable for the folding of S54G/P55N-ribonuclease T1. Under strongly native conditions they retard the rate-limiting trans-->cis isomerization of Pro39 because they allow the rapid formation of partially ordered structure prior to the proline-limited refolding reaction. Under unfolding conditions the isomerization of Pro39 is not affected. The direct unfolding and refolding reactions in the transition region of polypeptide chains with correct prolyl isomers are also decelerated when the disulfide bonds are present. These changes in the folding kinetics are possibly related to the decrease in chain flexibility that is caused by the disulfide bonds. A high flexibility is probably important throughout folding, and in the case of ribonuclease T1 a premature locking of tertiary contacts by intact disulfide bonds can interfere unfavorably with both the direct and the proline-limited folding reactions.