catalase

Catalase is a widely distributed enzyme that catalyzes the decomposition of hydrogen peroxide, a reactive and potentially harmful byproduct of metabolism, into water and oxygen. The overall reaction is 2 H2O2 → 2 H2O + O2. Catalase operates at very high turnover rates and serves as a key antioxidant defense in aerobic organisms.

Structure and localization: Most catalases are heme-containing enzymes that function as tetramers, with four identical subunits of about 60 kDa each. The active site centers on a heme iron that cycles between oxidation states during turnover. In many eukaryotes, catalase localizes to peroxisomes, guided by targeting signals, while some species also have cytosolic pools.

Mechanism: The catalytic cycle involves oxidation-reduction steps of the heme prosthetic group. Hydrogen peroxide binds the ferric heme, generating reactive intermediates, and a second molecule of H2O2 is reduced to water, producing molecular oxygen. This two-step process explains how a single enzyme can rapidly detoxify substantial amounts of H2O2.

Biological role and distribution: Catalase is present in nearly all aerobic organisms, including bacteria, archaea, fungi, plants, and animals. It protects cells from oxidative damage by promptly removing hydrogen peroxide produced during metabolism and in responses to stress or inflammation. In humans, catalase is most abundant in organs with high metabolic rates, such as the liver and erythrocytes.

Regulation and properties: Catalase has an optimum pH near neutral and operates best at moderate temperatures. It can be inhibited by certain ligands that bind to the heme iron, such as cyanide and azide. Its activity can be modulated by cellular localization, availability of substrates, and environmental conditions.

Applications: In clinical and laboratory settings, catalase activity assays help study oxidative stress and diagnose related disorders. Industrial uses include removing residual hydrogen peroxide from solutions in food processing, textile and paper production, and in biosensors for hydrogen peroxide detection.