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Kinetics of hydrogen bond breakage in the process of unfolding of ribonuclease A measured by pulsed hydrogen exchange.

Abstract：

:A sensitive test for kinetic unfolding intermediates in ribonuclease A (EC 3.1.27.5) is performed under conditions where the enzyme unfolds slowly (10 degrees C, pH 8.0, 4.5 M guanidinium chloride). Exchange of peptide NH protons (2H-1H) is used to monitor structural opening of individual hydrogen bonds during unfolding, and kinetic models are developed for hydrogen exchange during the process of protein unfolding. The analysis indicates that the kinetic process of unfolding can be monitored by EX1 exchange (limited by the rate of opening) for ribonuclease A in these conditions. Of the 49 protons whose unfolding/exchange kinetics was measured, 47 have known hydrogen bond acceptor groups. To test whether exchange during unfolding follows the EX2 (base-catalyzed) or the EX1 (uncatalyzed) mechanism, unfolding/exchange was measured both at pH 8.0 and at pH 9.0. A few faster-exchanging protons were found that undergo exchange by both EX1 and EX2 processes, but the 43 slower-exchanging protons at pH 8 undergo exchange only by the EX1 mechanism, and they have closely similar rates. Thus, it is likely that all 49 protons undergo EX1 exchange at the same rate. The results indicate that a single rate-limiting step in unfolding breaks the entire network of peptide hydrogen bonds and causes the overall unfolding of ribonuclease A. The additional exchange observed for some protons that follows the EX2 mechanism probably results from equilibrium unfolding intermediates and will be discussed elsewhere.

journal_name

Proc Natl Acad Sci U S A

journal_title

Proceedings of the National Academy of Sciences of the United States of America

authors

Kiefhaber T,Baldwin RL

doi

10.1073/pnas.92.7.2657

subject

Has Abstract

pub_date

1995-03-28 00:00:00

pages

2657-61

issue

7

eissn

0027-8424

issn

1091-6490

journal_volume

92

pub_type

杂志文章

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