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 res idues) of cytochrome c have been investigated by spectroscopic (circular di chroism, electronic absorption, and EPR) and voltammetric techniques. The r esults indicate that the N-fragment lacks ordered secondary structure and h as two histidines axially bound to the heme-iron (the native His18 and a mi sligated His26 or His33). Despite the absence of ordered secondary structur e, the peptide chain shields the heme group from solvent, as shown by (i) t he pK(a) 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, E-o = 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 elec troactive N-fragment may be taken as a "minichrome c" model, with interesti ng potential for application to biosensor technology; further, the system p rovides useful information far a deeper understanding of cytochrome c foldi ng and structural/functional organization. (C) 2000 Academic Press.