Biochemical Properties of Indoleamine 2,3-dioxygenase: From Structure to Optimized Design of Inhibitors
Abstract:The enzyme indoleamine 2,3-dioxygenase (IDO, EC 184.108.40.206) belongs to the family of heme-containing oxidoreductases and catalyzes the first and rate-limiting step in the kynurenine pathway, the major pathway of tryptophan metabolism. IDO is folded into one large and one small distinct α-helical domains, with the heme prosthetic ring positioned between them. The enzyme, through the oxidative properties of the Fe3+ atom present at the centre of the heme ring, catalyses the oxidative cleavage of the pyrrole ring of L-Trp to generate N-formyl-kynurenine. The active IDO conformer exists only in the presence of reducing cofactors (such as cytochrome b5), requiring the single electron reduction of ferric-to-ferrous iron (Fe3+ Fe2+), which facilitates binding of L-Trp and O2 to the enzyme active site. IDO, through production of kynurenine and other downstream metabolites, can regulate immune responses, suppressing effector T-cell function and favouring the differentiation of regulatory T cells. Local expression of the enzyme during inflammation is another self-protection mechanism, which limits antigen-specific immune responses, especially in some organs, as the central nervous system. The detailed knowledge of the structural and functional properties of IDO, was a fundamental step to design and develop new molecules for the pharmacological inhibition of IDO activity in several clinical settings.
Keywords: IDO; Immune tolerance; Indoleamine 2,3-dioxygenase; N-formyl-kynurenine; X-ray structure of oxygenases; enzyme; helical domains; heme-containing oxidoreductases; kynurenine; tryptophan metabolism
Document Type: Research Article
Publication date: May 1, 2011
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