L.-N. Lin and J. F. Brandts recently proposed a simple model for the folding kinetics of ribonuclease A in which folding intermediates are not detectable. We have tested the basic assumption of the simple model for the major unfolded species, which is produced by a slow isomerization (the "X ⇌ Y reaction" according to Lin and Brandts) after unfolding. The simple model assumes that in refolding the slow Y → X reaction must occur before any folding can take place. We have measured the Y → X reaction during folding. Tyrosine-detected folding occurs before the Y → X reaction; the difference in rate between the Y → X reaction and folding monitored by tyrosine absorbance becomes large when the stabilizing salt 0.56 M (NH4)2SO4 is added. The simple model predicts that the kinetic properties of the X ⇌ Y reaction in unfolded ribonuclease are the same as those of tyrosine-detected folding. We find, however, that the kinetics of the X ⇌ Y reaction in unfolded ribonuclease are independent of urea concentration, whereas the rate of tyrosine-detected folding decreases almost 100-fold between 0.3 and 5 M urea, as reported by Lin and Brandts. We point out that the kinetic properties of the X ⇌ Y reaction in unfolded ribonuclease are characteristic of proline isomerization.
|Original language||English (US)|
|Number of pages||6|
|State||Published - 1984|
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