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 a re devoid of catalase typically exhibit no adverse phenotypes during growth in aerobic media. We discovered that catalase mutants retain the ability t o rapidly scavenge H2O2 whether it is formed internally or provided exogeno usly. Analysis of candidate genes revealed that the residual activity is du e to alkyl hydroperoxide reductase (Ahp). Mutants that lack both Ahp and ca talase could not scavenge H2O2. These mutants excreted substantial amounts of H2O2, and they grew poorly in air. Ahp is kinetically a more efficient s cavenger of trace H2O2 than is catalase and therefore is likely to be the p rimary scavenger of endogenous H2O2. Accordingly, mutants that lack Ahp acc umulated sufficient hydrogen peroxide to induce the OxyR regulon, whereas t he OxyR regulon remained off in cataIase mutants. Catalase still has an imp ortant role in wild-type cells, because the activity of Ahp is saturated at a low (10(-5) M) concentration of H2O2. In contrast, cataIase has a high K -m, and it therefore becomes the predominant scavenger when H2O2 concentrat ions are high. This arrangement is reasonable because the cell cannot provi de enough NADH for Ahp to rapidly degrade large amounts of H2O2. In sum, E. coli does indeed generate substantial H2O2, but damage is averted by the s cavenging activity of Ahp.