Practice question

Hydrogen peroxide is unavoidable byproduct of flavoprotein oxidases such as NADH oxidase and autoxidation of menaquinones generating reactive oxygen stress capable of hydroxyl radical formation via Fenton reaction damaging iron-sulfur clusters and DNA. Aerobic bacteria depend on extremely efficient enzymatic detoxification. Catalase is tetrameric heme enzyme containing heme b active site where first H2O2 oxidizes ferric heme to Compound I ferryl oxo porphyrin radical, second H2O2 reduces Compound I releasing water and dioxygen. Catalytic rate is among highest known about ten million molecules per second per active site, essentially diffusion limited. Genes encoding catalases katG catalase-peroxidase and katE are induced via OxyR peroxide sensor recognizing hydrogen peroxide and general stress sigmaS RpoS during stationary phase. DNA polymerase replicates chromosome, RNA helicase unwinds RNA secondary structures, topoisomerase decatenates chromosomes, none scavenge peroxide, leaving catalase together with peroxidases like alkyl hydroperoxide reductase AhpCF as primary peroxide defense system distinguishing them from central information processing enzymes and explaining positive bubble test for catalase-positive organisms.