Combined kinetic and thermodynamic analysis of α-helical membrane protein unfolding

被引:103
作者
Curnow, Paul [1 ]
Booth, Paula J. [1 ]
机构
[1] Univ Bristol, Sch Med Sci, Dept Biochem, Bristol BS8 1TD, Avon, England
关键词
bacteriorhodopsin; folding; free energy; transition state;
D O I
10.1073/pnas.0705067104
中图分类号
O [数理科学和化学]; P [天文学、地球科学]; Q [生物科学]; N [自然科学总论];
学科分类号
07 ; 0710 ; 09 ;
摘要
The analytical toolkit developed for investigations into water-soluble protein folding has yet to be applied in earnest to membrane proteins. A major problem is the difficulty in collecting kinetic data, which are crucial to understanding any reaction. Here, we combine kinetic and thermodynamic studies of the reversible unfolding of an a-helical membrane protein to provide a definitive value for the reaction free energy and a means to probe the transition state. Our analyses show that the major unfolding step in the SDS-induced denaturation of bacteriorhodopsin involves a reduction in a-helical structure and proceeds with a large free-energy change; both our equilibrium and kinetic measurements predict that the free energy of unfolding in the absence of denaturant is +20 kcal-mol-1, with an associated m-value of 25 kcal-mol-1. The rate of unfolding in the absence of denaturant, kuH20, is surprisingly very slow (approximate to 10(-15) s(-1)). The kinetics also give information on the transition state for this major unfolding step, with a value for beta (m(f)/[m(f) + m(u)]) of approximate to 0.1, indicating that the transition state is close to the unfolded state. We thus present a basis for mapping the structural and energetic properties of membrane protein folding by mutagenesis and classical kinetics.
引用
收藏
页码:18970 / 18975
页数:6
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