The membrane-bound high-molecular-mass cytochromes c from Desulfovibrio gigas and desulfovibrio vulgaris Hildenborough; EPR and Mossbauer studies

The high-molecular-mass cytochromes c (Hmcs) from the sulfate-reducing bacteria Desulfovibrio gigas and Desulfovibrio vulgaris (Hildenborough) were found to be strongly bound to the cytoplasmic membrane. After detergent solubilization they were shown to be water soluble and to be similar to those previously isolated from the soluble fractions in terms of N-terminal sequence, molecular mass, UV-visible and EPR spectroscopies. In D. gigas, higher amounts of Hmc can be obtained from the membranes than from the soluble fraction. This enabled further characterization of both cytochromes. The apparent heme reduction potentials of both Hmcs, determined at pH 7.5 through visible and EPR redox titrations, span a large range of redox potentials, approximately between 0 and -280mV, and can be roughly divided into three groups: four to five hemes have E⁰s of -30 mV to -100mV, three to four hemes have E⁰s around 170 mV, and seven to eight hemes have a lower E⁰ of -250 to -280 mV. Several of these redox potentials are strongly pH dependent. Mossbauer studies of oxidized and reduced D. vulgaris Hmc show that this protein contains two high-spin hemes in both oxidation states. The rate of reduction of both Hmcs with the periplasmic hydrogenases from the corresponding organisms is extremely slow.