Practice question

Because peroxisomal oxidases produce stoichiometric hydrogen peroxide during oxidation of fatty acids, urate, D-amino acids and polyamines, cells require robust detoxification to prevent oxidative damage to proteins, lipids and DNA. Catalase is the signature antioxidant enzyme residing in peroxisomal matrix, often forming electron-dense crystalline core visible by electron microscopy in rat liver. It is a 240 kDa heme-containing homotetramer that disproportionates hydrogen peroxide into water and molecular oxygen with extremely high turnover number near ten million molecules per second, one of fastest enzymes known, operating without additional cofactors and via compound I ferryloxo heme intermediate. Two molecules of H2O2 are consumed per catalytic cycle, protecting unsaturated ether lipids and peroxisomal proteins and limiting leakage to cytosol where glutathione peroxidase would be overwhelmed. Catalase is imported via noncanonical PTS1 recognized by PEX5 despite lacking classic SKL because of extended binding interface. Its activity complements cytosolic glutathione peroxidase, peroxiredoxins and mitochondrial superoxide dismutase in antioxidant network. Genetic catalase deficiency causes acatalasemia in Japan with mild phenotype due to redundancy, but peroxisome biogenesis failure impairs plasmalogen synthesis and causes severe neurologic disease.