Tests of the simple model of Lin and Brandts for the folding kinetics of ribonuclease A

Titelangaben

Schmid, Franz X. ; Buonocore, Michael H. ; Baldwin, Robert L.:
Tests of the simple model of Lin and Brandts for the folding kinetics of ribonuclease A.
In: Biochemistry. Bd. 23 (1984) Heft 15 . - S. 3389-3394.
ISSN 1520-4995
DOI: https://doi.org/10.1021/bi00310a002

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Abstract

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 in equilibrium 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 in equilibrium Y reaction in unfolded ribonuclease are the same as those of tyrosine-detected folding. We find, however, that the kinetics of the X in equilibrium 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 in equilibrium Y reaction in unfolded ribonuclease are characteristic of proline isomerization.