A common folding mechanism in the cytochrome c family

Of the globular proteins, cytochrome c (cyt c) has been used extensively as a model system for folding studies. Here we analyse the folding pathway of different cyt c proteins from prokaryotes and eukaryotes, and attempt to single out general correlations between structural determinants and folding mechanisms. Recent studies provide evidence that the folding pathway of several cyt c proteins involves the formation of a partially structured intermediate. Using state-of-the-art kinetic analysis on published data, we show that such a folding intermediate is an obligatory on-pathway species that might represent either a defined local minimum in the reaction coordinate or an unstable high-energy state. Available data also indicate that some essential structural features of the folding intermediate and transition states are highly conserved across this protein family. Thus, cyt c proteins share a consensus folding mechanism in spite of large differences in physico-chemical properties and thermodynamic stability. This novel outlook on the folding of cyt c can shed light on much published data and might offer a general scheme by which to plan new experiments.