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. Author manuscript; available in PMC: 2013 Dec 1.
Published in final edited form as: Mol Microbiol. 2012 Oct 24;86(6):1376–1392. doi: 10.1111/mmi.12063

Fig. 10. A model for MK- and haem-associated superoxide production.

Fig. 10

A. Under standard growth conditions, a certain level of biosynthesized haem exists within S. aureus cytochromes in order to support aerobic respiration. During the process of respiration, a portion of the cellular MK or DMK pool – labeled as (D)MK for the purposes of this figure – can lose a single electron, resulting in the formation of the highly reactive semiquinone (SQ) species. SQs can react with molecular oxygen to generate superoxide, but the levels of SQs and superoxide formed during typical respiration are kept in check by various cellular factors. B. Under conditions of haem excess, haem molecules accumulate in the cell membrane within an unknown haem reservoir. Upon interaction with atmospheric oxygen, haem can autoxidize into haemin, producing superoxide in the process. C. In the presence of excess haem, cellular pools of reduced MK continuously redox cycle haemin to regenerate haem. This process likely results in the formation of SQs, which react with oxygen to produce superoxide.