The reactions of the oxidase and reductases of Paracoccus denitrificans with cytochromes c.

Electron transport in the Paracoccus denitrificans respiratory chain system is considerably more rapid when it includes the membrane-bound cytochrome c552 than with either soluble Paracoccus c550 or bovine cytochrome c; a pool function for cytochrome c is not necessary. Low concentrations of Paracoccus or bovine cytochrome c stimulate the oxidase activity. This observation could explain the multiphasic Scatchard plots which are obtained. A negatively charged area on the "back side" of Paracoccus c which is not present in mitochondrial c could be a control mechanism for Paracoccus reactions. Paracoccus oxidase and reductase reactions with bovine c show the same properties as mammalian systems; and this is true of Paracoccus oxidase reactions with its own soluble cytochrome c if added polycation masks the negatively charged area. Evidence for different oxidase and reductase reaction sites on cytochrome c include: (1) stimulation of the oxidase but not reductase by a polycation; (2) differences in the inhibition of the oxidase and reductases by monoclonal antibodies to Paracoccus cytochrome c; and (3) reaction of another bacterial cytochrome c with Paracoccus reductases but not oxidase. Rapid electron transport occurs in cytochrome c-less mutants of Paracoccus, suggesting that the reactions result from collision of diffusing complexes.