The Heme-Containing N-Fragment (Residues 1–56) of Cytochrome c Is a Bis-histidine Functional System

The structural and redox properties of a heme-containing fragment (1–56 residues) of cytochrome c have been investigated by spectroscopic (circular dichroism, electronic absorption, and EPR) and voltammetric techniques. The results indicate that the N-fragment lacks ordered secondary structure and has two histidines axially bound to the heme-iron (the native His18 and a misligated His26 or His33). Despite the absence of ordered secondary structure, the peptide chain shields the heme group from solvent, as shown by (i) the pKa of protonation of the nonnative histidine ligand (5.18 ± 0.05), lower than that of the bis-histidine guanidine-unfolded cytochrome c (5.58 ± 0.05), and (ii) the redox potential, Eo = 0 ± 5 mV versus NHE, close to that of bis-histidine cytochrome c mutants but less negative than that of bis-histidine complexes of microperoxidase with short peptides. The electroactive N-fragment may be taken as a “minichrome c” model, with interesting potential for application to biosensor technology; further, the system provides useful information for a deeper understanding of cytochrome c folding and structural/functional organization.