Kinetic intermediates in the formation of the cytochrome c molten globule.

The relationship between molten globules and transient intermediates in protein folding has been explored by equilibrium and kinetic analysis of the compact acid-denatured A-state of cytochrome c. The chloride-induced formation of the A-state is a complex reaction with structural intermediates resembling those found under native refolding conditions, including a rapidly formed compact state and a subsequent intermediate with interacting N- and C-terminal helices. Together with mutational evidence for specific helix-helix packing interactions, this shows that the A-state is a stable analogue of a late folding intermediate. The L94A mutation blocks all folding steps after the initial collapse and its equilibrium state resembles early kinetic intermediates.