Electrochemistry of cytochrome c(4) from Pseudomonas stutzeri

The cyclic voltammetry of the dipolar, overall negatively charged bacterial di-heme protein cytochrome c(4) from Pseudomonas stutzeri is composite and shows nontraditional features. Close to reversible voltammetry, with individual peaks corresponding to electron exchange of each heme, is found at high ionic strength (0.1 M phosphate, pH = 7.5) using gold electrodes modified by any of the promoters 4,4'-bipyridyl disulfide (4,4'-bpySS), 2,2'-dithiobisethaneamine (cystamine), and 3-pyridinylmethylenehydrazine carbothiamide. These are otherwise known to promote electrochemistry of proteins with positive, negative, and either positive or negative overall charges, respectively. This observation is indicative of weak surface orientational selectivity of cyt c(4) at high ionic strength. In contrast, the voltammograms at low ionic strength (0.01 M phosphate, pH 7.5) point toward orientational selectivity in accord with the expected charge compatibility of the promoters with given domains of the protein. Numerical analysis of the voltammograms provide first macroscopic midpoint potentials and interfacial electron-transfer (ET) rate constants of each heme. The midpoint potentials at high buffer concentration are close to values previously determined from ET kinetics in homogeneous solution. At low ionic strength where orientational selectivity at 4,4'-bpySS- and cystamine-modified electrodes is likely, intramolecular ET between the heme groups is, secondly, a feature of the overall interfacial kinetics. The intramolecular rate constants can be determined from the voltammetric peak shapes, giving 40 s(-1) for ET from the high- to the low-potential heme and 1600 s(-1) for the reverse reaction. These values hold interesting implications in relation to the electronic structure and ET patterns in homogeneous solution.