Cytochrome c 553 from Desulfovibrio vulgaris (Hildenborough)

An electrochemical study of the periplasmic cytochrome c 553 of Desulfovibrio vulgaris (Hildenborough) is presented. The dependence of the midpoint potential on temperature and pH was studied with cyclic voltammetry. The voltammograms obtained were reversible and revealed that this cytochrome showed fast electron transfer on a bare glassy carbon electrode. The midpoint potential at pH 7.0 and 25°C was found to be 62 mV versus the normal hydrogen electrode. It was observed that the temperature dependence was discontinuous with a transition temperature at 32°C. The standard reaction entropy at the growth temperature of the organism (37°C) was calculated to be ΔS°′=–234 J mol −1 K −1 . The pH dependence of the midpoint potential could be described with one p K of the oxidized form with a value of 10.6. The second‐order rate constant for electron transfer between cytochrome c 553 and the Fe‐hydrogenase from D. vulgaris (H) was also determined with cyclic voltammetry. The equivalent rate constant for cytochrome c 3 and hydrogenase was measured for comparison. The second‐order rate constants are 2×10 7 M −1 s −1 for cytochrome c 553 and 2×10 8 M −1 s −1 for cytochrome c 3 . The kinetic parameters of the hydrogenase for both cytochromes were determined using the spectrophotometric hydrogen consumption assay. With cytochrome c 553 this resulted in a K m of 46 μM and a maximum turnover number of 7.1×10 2 s −1 in the H 2 consumption assay. The values with cytochrome c 3 were 17 μM and 6.4×10 2 s −1 , respectively. The importance of the different kinetic parameters for contrasting models proposed to describe the function of the Fe‐hydrogenase are discussed.