The NrfH Cytochrome c Quinol Dehydrogenase

Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are heme-containing enzymes from a small family of homologous enzymes. Despite both catalyzing the dioxygenation of l-tryptophan, the first step in the kynurenine pathway, the sequence similarity between TDO and IDO is low. Alignment of sequences across this family of enzymes is only possible on the basis of their structures. Human IDO shows activity toward a wider range of substrates than TDO, and is found throughout the body, while TDO is limited to the liver and epidermis in mammals. To date, no functional prokaryotic IDO has been identified, while TDO is found in many bacteria. TDO and IDO have been implicated in a number of human physiological conditions, including suppression of T-cell proliferation and the immune escape of cancers, making them attractive targets for drug discovery. 3D Structure Keywords: kynurenine pathway; l-tryptophan; dioxygen; crystallography; immune response; heme