Hydrogen exchange in a bacterial cytochrome c: a fingerprint of the cytochrome c fold.

The hydrogen exchange rates of backbone amides in a minimal (71 amino acid long) monoheme cytochrome c were determined as a function of pH in the absence and in the presence of guanidinium chloride. These data permitted the identification of units undergoing the opening reaction that precedes hydrogen exchange through a common mechanism. The opening units broadly correlate with the secondary structure elements of the protein. It is found that, despite the significant difference in primary sequence, the distribution of the opening units within the three-dimensional structure of the cytochrome studied here closely resembles that determined in mitochondrial c-type cytochromes. It is proposed that the observed distribution represents a fingerprint of the cytochrome c fold and has a role in directing the folding/unfolding of the protein.