Proline isomerization during refolding of ribonuclease A is accelerated by the presence of folding intermediates.

The trans----cis isomerization of Pro 93 was measured during refolding of bovine ribonuclease A. This isomerization is slow (tau = 500 s) under marginally stable folding conditions of 2.0 M GdmCl, pH 6, at 10 degrees C. However, it is strongly accelerated (tau = 100 s) in samples which, prior to isomerization, had been converted to a folding intermediate by a 15 s refolding pulse under strongly native conditions (0.8 M ammonium sulfate, 0 degree C). The results demonstrate that extensive folding is possible before Pro 93 isomerizes to its native cis state and that the presence of structural folding intermediates leads to a marked increase in the rate of subsequent proline isomerization.