Helix-Coil Transitions of a-Helical, Two-Chain, Coiled Coils

A theory of the helix-coil transition for in-register, two-chain, a-helical, coiled coils such as tropomyosin and paramyosin is developed. The treatment differs from those formulated previously for DNAor collagen-like double helices; in the present treatment, isolated single chains and each of the two strands in the dimer may be partially helical. We calculate the fraction of helix in the two-chain, coiled coil as a function of Zimm-Bragg cooperativity (u) and helix stability (s) parameters appropriate to single chains and of a new parameter w that takes account of the enhanced helix stability in a paired chain vis-&vis an isolated chain. The importance of the quasi-repeating heptet (observed in the amino acid sequence of rabbit tropomyosin) in stabilizing the helix conformation in a two-chain, coiled coil is accounted for via a coarse-graining approximation. Singly cross-linked homopolypeptide chains and both singly cross-linked and non-cross-linked chains with the rabbit a-tropomyosin sequence are treated in detail and estimates of the stability per helical residue are given. For rabbit tropomyosin, the helix probability profile is also calculated, and the possible location of a reported extrastable region is identified. In the case of non-cross-linked molecules, general expressions for two experimentally accessible quantities, the degree of chain association and the overall helix content, are derived. Application of the treatment to tropomyosin predicta that the principal thermal transition involves simultaneous dissociation and denaturation and that any thermal transition at higher temperature must be due to melting of extrastable region(s) in isolated, single, partially a-helical chains.