Figure 1: Overview of oxygen-dependent anti-microbial systems in neutrophils/monocytes and the processing of myeloperoxidase. A. Myeloperoxidase (MPO) acts upon hydrogen peroxide to generate hypochlorus acid (HOCl) used to kill bacteria and fungi in the phagosome. The NADPH oxidase complex is pivotal in generating superoxide anion (O2•) from oxygen (O2), using NADPH generated by glutathione (GSH) pathway. Superoxide dismutase (SOD) then converts superoxide into hydrogen peroxide (H2O2), which is the substrate for MPO to create hypochlorus acid (HOCl). Both H2O2 and HOCl are directly involved in the oxygen-dependent anti-microbial systems (see text for details). B. Myeloperoxidase (MPO) undergoes a multistep process before reaching the phagosomes or being secreted. The prepro- MPO protein is translated as a single precursor. During the process of being folded, oligosaccharide modifications are added to the protein. The pre-pro-MPO has one mannose oligosaccharide on the pro-peptide, and 4 are placed on the a (large) subunit [33]. This pre-pro-MPO is then folded with the help of chaperones, which facilitate the insertion of a heme group. It is then cleaved to form the pro-MPO. The pro-MPO peptide acts to sort MPO for further processing, where it undergoes the formation of a disulfide bridge allowing the active mature dimer to be made (see text for details). Abbreviations: G6P: Glucose-6-Phosphate; GSH: glutathione; HOCl: Hypochlorus acid; MPO: Myeloperoxidase; NADPH: Nicotinamide Adenine Dinucleotide Phosphate. Panel A adapted from van der Veen [85]. Panel B adapted from: Olsson et al. [35].