Oxidation-reduction properties of several low potential iron-sulfur proteins and of methylviologen

Abstract

Apparent oxidation-reduction potentials at pH 7.0 and 25.degree. C were determined using the H2-hydrogenase system with ferredoxins of the following sources: Clostridium pasteurianum, -403 mV; C. tartarivorum, -424 mV; C. acidiurici, -434 mV; Peptococcus aerogenes, -427 mV; Chromatium vinosum, -482 mV (pH 8.0); Bacillus polymyxa, ferrodoxin I, -377 mV, and ferrodoxin II, -422 mV; and spinach, -428 mV. The pH dependence of these values was variable, ranging from -2 to -24 mV pH unit increase for different ferredoxins. Over the buffer concentrations range between 0.05 and 0.2 M, the potentials did not vary significantly. The number of electrons transferred during reduction (as determined by integrations of EPR spectra and by dithionite titration) is 2 for the first 5 proteins, while potentiometric data for all the cases fit a Nernst equation for which n = 1. The normal redox potential value for the redox indicator methylviologen at pH 7.4 was -460 mV according to both the H2-hydrogenase system and cyclic voltammetry, significantly different from the value reported at higher pH''s. The presence of C. pasteurianum ferrodoxin probably shift the electrical potential constant value of methylviologen to even more negative values. An analysis source of error inherent with potential determinations with H2 and hydrogenase is presented. The electronic and EPR spectra of P. aerogenes ferredoxin are given. The determination of ferredoxin and other low-potential proteins potentials with the H2-hydrogenase system apparently affords certain experimental advantages over alternative methods currently employed with these and similar substances.