Multiple forms of cytochrome b in the electron transport system of Propionibacterium shermanii.

Membrane particles from Propionibacterium shermanii contain cytochromes b and d as the major cytochrome components. Potentiometric titration of the membranes indicate two distinct groups of b-type cytochromes differing in their midpoint potentials by at least 100 mV. Low temperature spectra of redox-poised membranes show that the high potential group consists of two components with approximate lambda max and midpoint potentials as follows: cytochrome b562-563 (+120 mV); cytochrome bHP556-557 (+90 mV). Resolution of the low potential group of b-type cytochromes is less clear cut but there appear to be two further components with different lambda max values but very similar midpoint potentials: cytochrome bLP556-557 (-20 mV) and cytochrome b553-554 (-20 mV). Cytochrome d630 titrates as a single species with an approximate midpoint potential of +140 mV. The low potential pair of b-type cytochromes have midpoint potentials sufficiently low to permit their functioning as components of the anaerobic electron transport path to fumarate while the high potential pair of b-type cytochromes and cytochrome d probably function on an aerobic branch of the electron transport pathway. The characteristics of the aerobic and anaerobic steady-state spectra are largely consistent with these suggestions.