P450

P450 refers to the cytochrome P450 superfamily of heme-containing enzymes that catalyze oxidation reactions in biology. These monooxygenases use NADPH as an electron donor and molecular oxygen to insert one atom of oxygen into substrates while reducing the other oxygen atom to water. The name P450 arises from the characteristic absorbance peak at 450 nanometers of the reduced carbon monoxide-bound form of the enzyme.

The enzyme family is highly diverse. In humans there are about 57 functional CYP genes with numerous pseudogenes, organized into families and subfamilies (for example CYP3A, CYP2D, CYP2C). Each gene encodes a heme-containing protein whose active site determines substrate specificity. The shorthand CYP denotes cytochrome P450, followed by a family number, subfamily letter, and gene number (e.g., CYP3A4).

Functions include metabolism of xenobiotics (drugs, pollutants) and endogenous compounds (steroids, fatty acids). Reactions include hydroxylation, epoxidation, N- and O-dealkylation, and desaturation. The most clinically relevant enzymes include CYP3A4, which metabolizes a large fraction of drugs; CYP2D6, CYP2C9, and others. Genetic polymorphisms and environmental factors influence activity, leading to interindividual variability and potential drug interactions.

Catalytic cycle steps involve substrate binding, electron transfer from NADPH via cytochrome P450 reductase, oxygen activation, substrate oxidation, and product release. P450 enzymes are distributed widely, especially in liver and gut, and play essential roles in pharmacology, toxicology, and steroid biosynthesis. Evolutionarily, the P450 superfamily is large and diverse across life forms.