FIG. 5.

Production of hydrogen peroxide from proline metabolism in E. coli. Oxidative proline metabolism by PutA leads to reduction of membrane-bound ubiquinone (Q) and formation of NADH, supplying electrons into the electron transport chain via NADH dehydrogenase (NDH-1). Hydrogen peroxide is generated as a by-product during respiratory chain activity to levels sufficient enough to activate the transcriptional regulator OxyR (-S-S-). OxyR subsequently upregulates expression of antioxidant genes such as thioredoxin (trxC), glutaredoxin (grxA), and catalase (katG).