How Escherichia coli Tolerates Profuse Hydrogen Peroxide Formation by a Catabolic Pathway

Abstract

ABSTRACT When Escherichia coli grows on conventional substrates, it continuously generates 10 to 15 μM/s intracellular H 2 O 2 through the accidental autoxidation of redox enzymes. Dosimetric analyses indicate that scavenging enzymes barely keep this H 2 O 2 below toxic levels. Therefore, it seemed potentially problematic that E. coli can synthesize a catabolic phenylethylamine oxidase that stoichiometrically generates H 2 O 2 . This study was undertaken to understand how E. coli tolerates the oxidative stress that must ensue. Measurements indicated that phenylethylamine-fed cells generate H 2 O 2 at 30 times the rate of glucose-fed cells. Two tolerance mechanisms were identified. First, in enclosed laboratory cultures, growth on phenylethylamine triggered induction of the OxyR H 2 O 2 stress response. Null mutants ( ΔoxyR ) that could not induce that response were unable to grow. This is the first demonstration that OxyR plays a role in protecting cells against endogenous H 2 O 2 . The critical element of the OxyR response was the induction of H 2 O 2 scavenging enzymes, since mutants that lacked NADH peroxidase (Ahp) grew poorly, and those that additionally lacked catalase did not grow at all. Other OxyR-controlled genes were expendable. Second, phenylethylamine oxidase is an unusual catabolic enzyme in that it is localized in the periplasm. Calculations showed that when cells grow in an open environment, virtually all of the oxidase-generated H 2 O 2 will diffuse across the outer membrane and be lost to the external world, rather than enter the cytoplasm where H 2 O 2 -sensitive enzymes are located. In this respect, the periplasmic compartmentalization of phenylethylamine oxidase serves the same purpose as the peroxisomal compartmentalization of oxidases in eukaryotic cells.