Human catalase is a tetrameric enzyme composed of four identical subunits, each containing a heme group coordinated by tyrosine at the active site. This iron-dependent oxidoreductase catalyzes the rapid decomposition of hydrogen peroxide (2H2O2→2H2O+O2). Beyond its primary antioxidant role, catalase works with superoxide dismutase and glutathione peroxidase to neutralize reactive oxygen species (ROS), protecting cellular components from oxidative damage.

Genetic mutations in the CAT gene cause acatalasemia, characterized by severe catalase deficiency (<10% activity) linked to oral ulcerations, gangrene, and heightened susceptibility to type 2 diabetes due to pancreatic β-cell damage. Polymorphisms in catalase (e.g., -262 C/T promoter variant) correlate with oxidative stress-related pathologies, including hypertension, Alzheimer's disease, and vitiligo. Elevated catalase activity occurs in atherosclerosis and certain cancers as a compensatory response to chronic oxidative stress.