7.4.1　Structure of Indoleamine 2,3-dioxygenase

IDO is folded into two distinct domains (small and large).The large domain is an all-helical domain and is comprised of 13α-helices and two 310 helices.Four long helices (G, I, Q, and S) in the large domain run parallel to the heme plane and interact with the neighboring helix by hydrophobic interactions.Helix Q provides an endogenous ligand (H346 imidazole) for the heme iron at the fifth coordination position (proximal side) (Figure 7.4B).The heme-binding pocket is created mainly by these four helices and other helices (K-L and N).The side chains of helices K-L and N also contribute to heme-protein interactions and connect the two domains.The small domain and a long loop (residues 250-267) connecting the two domains above the sixth-coordination site of the heme (distal side) cover the top of the heme pocket.The small domain is comprised of six α-helices, two short β-sheets, and three 3 helices [18].(https://www.daowen.com)