A native tertiary interaction stabilizes the A state of cytochrome c

Certain kinetic intermediates in protein folding are similar to the molten globule, or A state, an equilibrium state of many proteins that is populated under high salt and low pH conditions. Many A states are nearly as compact as native proteins and have native-like secondary structure, but the extent to which nonlocal interactions stabilize the A state is unclear. In this study, thermal denaturation, monitored by circular dichroism, was used to determine the free energy of denaturation of the A state (delta GA<-->D) for Saccharomyces cerevisiae iso-1-ferricytochrome c. Specifically, we examined the wild-type protein, seven variants with amino acid substitutions at the interface between the N- and C-terminal helices, and two variants with mutations at a position close to, but not involved in, the interface. A plot of delta GA<-->D versus delta GN<-->D (the free energy of denaturation of the native state) has a slope near unity, showing that the evolutionarily conserved helix-helix interaction stabilizes the A state to the same degree that it stabilizes the native state.