Alpha-helix-to-random-coil transitions of two-chain, coiled coils: a theoretical model for the "pretransition" in cysteine-190-cross-linked tropomyosin

The thermal unfolding curve for alpha alpha tropomyosin in which the two chains are cross-linked at cysteine-190 shows two striking features that distinguish it from that of its counterpart for non-cross-linked molecules: a "pretransition" at 25-50 degrees C and a shift in the principal transition to higher temperature, but with the same steepness. Previously, the pretransition was explained by postulating that the cross-link produces local strains, yielding a pinched "bubble" of chain-separated random coil about C-190, whereas the rest of the coiled coil remains intact. Results from both enzymatic digestion kinetics and equilibrium calorimetric studies have been interpreted as consistent with the existence of such a bubble. To test this idea further, a theoretical model is devised whereby various physical features can be imposed and the resulting helix content and other properties calculated from the statistical mechanical theory of the helix-coil transition. Short-range interactions employed are the geometric mean values of those in alpha-tropomyosin. The helix-helix interaction free energy is also like that in alpha-tropomyosin, including its nonuniformity; i.e., it is made larger in the amino half of the molecule. Local strain is introduced by setting the helix-helix interaction to zero in a region about the cross-link. The results show that, alone, neither local strain nor nonuniformity serves to mimic the experiments. In concert, however, they reproduce all the main experimental features, if the strain is extensive (approximately 29 residues) and somewhat dissymmetric. Theoretical helix probability profiles, however, show that no bubble of unfolded chains forms about the cross-link. Instead, in the pretransition, residues unfold from the weakly interacting end (residue 284) in to, but not through, the cross-link at C-190. The theory also indicates that the augmented stability for the principal transition occurs largely as a result of loop entropy. The same strain and nonuniformity are then employed to explore the effects of other possible cross-link positions. The thermal curves are shown to depend markedly on cross-link location. The curves are discussed in terms of loop entropy, which has drastic, long-range effects. Under appropriate circumstances it can produce, in the coiled-coil model, a thermal transition that is essentially all or none.