Alkyl hydroperoxide reductase is the primary scavenger of endogenous hydrogen peroxide in Escherichia coli

Hydrogen peroxide is generated during aerobic metabolism and is capable of damaging critical biomolecules. However, mutants of Escherichia coli that are devoid of catalase typically exhibit no adverse phenotypes during growth in aerobic media. We discovered that catalase mutants retain the ability to rapidly scavenge H₂O₂ whether it is formed internally or provided exogenously. Analysis of candidate genes revealed that the residual activity is due to alkyl hydroperoxide reductase (Ahp). Mutants that lack both Ahp and catalase could not scavenge H₂O₂. These mutants excreted substantial amounts of H₂O₂, and they grew poorly in air. Ahp is kinetically a more efficient scavenger of trace H₂O₂ than is catalase and therefore is likely to be the primary scavenger of endogenous H₂O₂. Accordingly, mutants that lack Ahp accumulated sufficient hydrogen peroxide to induce the OxyR regulon, whereas the OxyR regulon remained off in catalase mutants. Catalase still has an important role in wild-type cells, because the activity of Ahp is saturated at a low (10^-5 M) concentration of H₂O₂. In contrast, catalase has a high K_m, and it therefore becomes the predominant scavenger when H₂O₂ concentrations are high. This arrangement is reasonable because the cell cannot provide enough NADH for Ahp to rapidly degrade large amounts of H₂O₂. In sum, E. coli does indeed generate substantial H₂PO₂, but damage is averted by the scavenging activity of Ahp.