Protoporphyrinogen oxidase (PPOX)

Protoporphyrinogen oxidase is an essential mitochondrial flavoprotein enzyme that catalyzes the six-electron oxidation of protoporphyrinogen IX into protoporphyrin IX—the penultimate step in heme biosynthesis. It is encoded by the PPOX gene on chromosome 1q22. The enzyme plays a critical role at a branch point between heme/bilin production and chlorophyll synthesis pathways across diverse organisms including animals, plants, fungi, cyanobacteria, and proteobacteria. Structurally, it contains FAD-binding domains typical for flavoenzymes and associates with mitochondrial membranes. Mutations impairing its function result in variegate porphyria—a dominantly inherited disorder characterized by photosensitivity due to accumulation of photoreactive intermediates. In agriculture, several classes of commercial herbicides act via potent inhibition of plant protox enzymes; these agents induce lethal oxidative damage through light-dependent lipid peroxidation following accumulation of phototoxic tetrapyrroles within plant cells.

Other names

Protoporphyrinogen IX oxidaseProtoxPPOPPOX

Competitive inhibition at the active site with substrate analogs or specific inhibitors such as diphenyl ether-type herbicides. These compounds block the oxidation of protoporphyrinogen IX, leading to accumulation of phototoxic intermediates that cause cell damage through oxidative stress when exposed to light and oxygen