Meta-Biol

• Pathways

In the phagosome, hydrogen peroxide (H2O2) is formed from the dismutation of superoxide that occurs either spontaneously or via reactions catalyzed by myeloperoxidase (MPO). Even though H2O2 is generated from superoxide at a high rate, it stabilizes in the low micromolar range (Winterbourn CC 2008; Winterbourn CC & Kettle AJ 2013). H2O2 is efficiently consumed by MPO to generate HOCl. If chloride is limited, MPO functions more as a catalyst for removal of superoxide and H2O2. Although H2O2 can permeate bacteria, it is unlikely to be directly bactericidal at the concentrations achieved in the phagosome (Winterbourn CC & Kettle AJ 2013). H2O2 reacts rapidly with heme proteins, thus MPO is likely to be its main target within phagosomes (Paumann-Page M et al. 2013; Winterbourn CC 2013). H2O2 can oxidize thiol proteins (Paulsen CE & Carroll KS 2013; Winterbourn CC 2013). The initial oxidation product of the cysteine (Cys) residue is sulfenic acid (Cys‐SOH) (Wall SB et al. 2012; Paulsen CE & Carroll KS 2013; Trujillo M et al 2016). The Cys‐SOH is highly reactive, its stability is influenced by neighboring cysteine residues (Cys‐SH), which can generate a more stable disulfide bond. The formation of disulfide bridges, either between the same or different polypeptide chains, is important for protein structure and folding, and is often involved in the regulation of protein function. Alternatively, sulfenic acids can be overoxidized to form irreversible sulfinic acid (Cys‐SO2H) or sulfonic acid (Cys‐SO3H) (Wall SB et al. 2012; Paulsen CE & Carroll KS 2013; Trujillo M et al 2016).