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

Abstract

The trans → cis isomerization of Pro 93 was measured during refolding of bovine ribonuclease A. This isomerization is slow (τ = 500s) under marginally stable folding conditions of 2.0 M GdmCl, pH 6, at 10°C. However, it is strongly accelerated (τ = 100 s) in samples which, prior to isomerization, had been converted to a folding intermediate by a 15s refolding pulse under strongly native conditions (0.8 M ammonium sulfate, 0°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.