A physiologically significant role in nitrite reduction of the CcoP subunit of the cytochrome oxidase cbb3 from Neisseria gonorrhoeae

The CcoP subunit of cytochrome oxidase cbb(3) of Neisseria gonorrhoeae is predicted to include a C-terminal extension in which there is a C-A-A-C-H- motif typical of heme attachment sites in c-type cytochromes. Substitutions of key cysteine and histidine residues of this motif resulted in mutants that grew normally in oxygen-sufficient cultures and reduced oxygen at the same rate as the parent strain. In contrast, after oxygen-limited growth in the presence of nitrite, rates of nitrite reduction were significantly lower than those of the parent, consistent with a role for this third heme-binding domain in electron transfer to the nitrite reductase, AniA, located in the outer membrane. As the mutants were still able to reduce nitrite at approximately 65% of the rate of the parent, there are multiple pathways in the gonococcus for electron transfer to AniA. On the basis of sequence similarity between the C-terminal extension of CcoP and cytochrome c(5), it is proposed that cytochrome c(5) might also transfer electrons across the periplasm from the cytochrome bc(1) complex in the cytoplasmic membrane to AniA in the outer membrane. This is the first example of a cytochrome oxidase component that plays a physiologically significant role in nitrite reduction.