## H₂O₂ Neutralization System **Key Point:** Catalase and glutathione peroxidase (GPx) are the two primary enzymatic systems that convert hydrogen peroxide (H₂O₂) to water and oxygen. H₂O₂ is generated as a byproduct of SOD activity and various oxidative reactions. ### Comparison of H₂O₂-Neutralizing Enzymes | Enzyme | Cofactor | Location | Reaction | Km for H₂O₂ | |--------|----------|----------|----------|-------------| | **Catalase** | Heme (Fe) | Peroxisomes | 2 H₂O₂ → 2 H₂O + O₂ | ~1 mM (high) | | **Glutathione Peroxidase** | Selenium (selenocysteine) | Cytoplasm, mitochondria | H₂O₂ + 2 GSH → 2 H₂O + GSSG | ~1 μM (low) | ### Functional Differences **Catalase:** - Highly efficient at HIGH H₂O₂ concentrations - Concentrated in peroxisomes (site of fatty acid oxidation and H₂O₂ generation) - Particularly important in liver and red blood cells - Does not require cofactors beyond heme **Glutathione Peroxidase:** - More efficient at LOW H₂O₂ concentrations (physiologic levels) - Distributed throughout cytoplasm and mitochondria - Requires reduced glutathione (GSH) as electron donor - Requires selenium for selenocysteine synthesis - More important for basal antioxidant defense **High-Yield:** Selenium deficiency → impaired GPx synthesis → increased susceptibility to oxidative stress (Keshan disease in China). **Clinical Pearl:** In catalase deficiency (acatalasemia), glutathione peroxidase compensates and prevents severe pathology in most tissues, but oral tissues may show granulomatous inflammation due to H₂O₂ accumulation. **Mnemonic:** "**CAT**alase in **CAT**astrophic peroxide levels; **GP**x for **G**eneral **P**rotection" 
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