ИСТИНА

Cytochrome bd is a prokaryotic respiratory quinol:O2 oxidoreductase, phylogenetically unrelated to the extensively studied heme–copper oxidases (HCOs). Relevant to patho-physiology, members of the bd-family were shown to promote virulence in some pathogenic bacteria, which makes these enzymes of interest also as potential drug targets. Beyond contributing to energy conservation, cytochrome bd accomplishes other physiological functions, being apparently implicated in the response of the bacterial cell to a number of stress conditions. Cytochrome bd-I oxidase from Escherichia coli was suggested to enhance bacterial tolerance to nitrosative stress conditions, due to its high NO dissociation rate. More recently, we observed that the enzyme, as-prepared or in turnover with O2, is endowed with catalase activity, rapidly decomposing H2O2 with formation of approximately half a mole of O2 per mole of H2O2. Such activity vanishes upon cytochrome bd reduction, does not compete with the O2-reductase activity, is inhibited by cyanide (Ki ~ 2.5 µM) and azide, but is insensitive to NO, CO, antimycin-A and N-ethylmaleimide. The activity, possibly associated with heme-b595, was interestingly observed also in catalase-deficient E. coli cells following cytochrome bd over-expression, suggesting a protective role against oxidative stress in vivo. All together, this information led us to speculate that the preferential (over HCOs) expression of cytochrome bd in pathogenic bacteria may represent a strategy to evade the host immune attack based on production of NO and reactive oxygen species.