Protein-radical enzymes use a free radical located on an intrinsic amino acid residue as a cofactor. The amino acid involved can be a tyrosine (ribonucleotide reductase, photosystem II, prostaglandin H synthase), a modified tyrosine (amine oxidase, galactose oxidase), a tryptophan (cytochrome c peroxidase), a modified tryptophan (methylamine dehydrogenase) or a glycine (ribonucleotide reductase, pyruvate formate lyase). The mechanistic role of these radicals appears to be that of a one-electron gate, allowing the separation of single reducing equivalents in time and space.