Gram+ bacteria are capable of complete denitrification just like Gram− (Gram-negative) bacteria. However, Gram+ (Gram-positive) bacteria have a very small periplasmic-like space. This leads to the question of whether those enzymes and electron carriers involved in denitrification, which are normally located in the periplasmic space in Gram− bacteria, are located in the periplasmic-like space in Gram+ bacteria or have been modified as membrane-bound proteins. Using Bacillus azotoformans as a Gram+ bacterial model, our study demonstrates that anaerobic denitrification is catalysed by four membrane-bound enzymes and that the electron carriers are membrane-bound c-type cytochromes and menaquinol. NADH dehydrogenase is coupled with the denitrification pathway providing menaquinol. In addition, the cytochrome b6f complex forms part of the denitrification pathway, oxidizing menaquinol and reducing at least three different membrane-bound c-type cytochromes. We determined that the NO reductase, qCuANOR (where NOR stands for nitric oxide reductase), can accept electrons from two donors, a specific cytochrome c551 and menaquinol. Similarly, nitrite reductase, a copper enzyme, and nitrous oxide reductase may be bifunctional enzymes. Regarding the bifunctionality of qCuANOR, we propose that the menaquinol-linked pathway is involved in the detoxification of NO.

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