cytochrome peroxidase is a respiratory oxidase that enables the use of hydrogen peroxide as a terminal electron acceptor.

Microbial cytochrome peroxidases (Ccp) have been studied for 75 years, but their physiological roles are unclear. Ccps are located in the periplasms of bacteria and the mitochondrial intermembrane spaces of fungi. In this study, Ccp is demonstrated to be a significant degrader of hydrogen peroxide in anoxic Intriguingly, transcription requires both the presence of HO and the absence of O Experiments show that Ccp lacks enough activity to shield the cytoplasm from exogenous HO However, it receives electrons from the quinone pool, and its flux rate approximates flow to other anaerobic electron acceptors. Indeed, Ccp enabled to grow on a nonfermentable carbon source when HO was supplied. behaved similarly. This role rationalizes repression in oxic environments. We speculate that micromolar HO is created both biologically and abiotically at natural oxic/anoxic interfaces. The OxyR response appears to exploit this HO as a terminal oxidant while simultaneously defending the cell against its toxicity.