Partially formed native tertiary interactions in the A-state of cytochrome c.

Considerable insight into protein structure, stability, and folding has been obtained from studies of non-native states. We have studied the extent of native tertiary contacts in one such molecule, the A-state of yeast iso-1-ferricytochrome c. Previously, we showed that the interface between the N and C-terminal helices is completely formed in the A-state. Here, we focus on interactions essential for forming the heme pocket of eukaryotic cytochromes c. To determine the extent of these interactions, we used saturation mutagenesis at the evolutionarily invariant residue leucine 68, and measured the free energy of denaturation for the native states and the A-states of functional variants. We show that, unlike the interaction between the terminal helices, the native interactions between the 60s helix and the rest of the protein are not completely formed in the A-state.