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Hetero‐α‐helical, two‐chain, coiled coils: αβ hybrid tropomysin
Author(s) -
Holtzer Marilyn Emerson,
Breiner Teresa,
Holtzer Alfred
Publication year - 1984
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360231003
Subject(s) - chemistry , dissociation (chemistry) , population , molecule , non equilibrium thermodynamics , kinetics , recombination , thermodynamic equilibrium , thermodynamics , chemical physics , crystallography , organic chemistry , biochemistry , physics , demography , quantum mechanics , sociology , gene
The nature of the interhelix interaction in two‐chain, α‐helical, coiled coils is studied by experiments on the formation of hydrid molecules in which one helical chain is an α‐tropomyosin and the other a β‐tropomoysin. By means of a recently developed assay, the population of heterohelical (i.e., αβ hybrid) molecules relative to their homohelical (αα and ββ) parent species is determined under a variety of conditions, both equilibrium and nonequilibrium. It is found that mixed intact αα and ββ molecules do not form hybrid species in detectible amounts even after incubation at room temperature (or below) for periods of over one week. That the lack of αβ species in this “native‐exchange” system is a result of a kinetic barrier is evident from experiments involving a thermal denaturation–renaturation cycle in which the largely dissociated, unfolded chains at higher temperature are annealed to benign temperatures over a period of 6 h, thus assuring an equilibrium population of two‐chain species. In the resulting equilibrium state, the αβ population is one‐half the total, indicating that recombination is random. Furthermore, this same (equilibrium) state is reached if the separated, mostly unfolded chains are renatured by a rapid (∼ 40 s) quench to benign temperatures. Some implications of these results for the thermodynamics of interhelix interation, for kinetics of chain dissociation and recombination, and for in vivo genesis of two‐chain coiled coils are discussed.