Asp274 and His346 are essential for heme binding and catalytic function of human indoleamine 2,3-dioxygenase
Asp274 and His346 are essential for heme binding and catalytic function of human indoleamine 2,3-dioxygenase
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DOI：
10.1074/jbc.m301700200
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发表时间：
2003-08-08
影响因子：
4.8
通讯作者：
Walker, MJ
中科院分区：
文献类型：
--
作者：
Littlejohn, TK;Takikawa, O;Walker, MJ
色氨酸是人体中含量最低的必需氨基酸。吲哚胺2，3-二氧代酶（IDO）是一种胞质血红素蛋白，其与肝酶色氨酸2，3-双加氧酶一起催化色氨酸代谢的主要途径（犬尿氨酸途径）中的第一和限速步骤。IDO的生理作用尚未完全理解，但引起了极大的兴趣，因为IDO广泛分布于人体组织中，可以通过细胞因子如干扰素-γ上调，从而可以调节色氨酸的水平，色氨酸对细胞生长至关重要。为了鉴定哪些氨基酸残基在IDO中的底物或血红素结合中是重要的，进行IDO基因中保守残基的定点诱变。因为已经提出组氨酸残基可能是IDO中的近端血红素配体，所以进行了三个高度保守的组氨酸His（16）、His（303）和His（346）的丙氨酸突变。其中，只有His（346）被证明是血红素结合所必需的，表明该组氨酸残基可能是近端配体，并表明His（303）和His（16）都不是近端配体。Asp（274）的定点突变也损害了IDO结合血红素的能力。这一观察结果表明，Asp（274）可能直接作为远端配体与血红素配位，或者在维持血红素口袋的构象中是必不可少的。
L-Tryptophan is the least abundant essential amino acid in humans. Indoleamine 2,3-dioxgyenase (IDO) is a cytosolic heme protein which, together with the hepatic enzyme tryptophan 2,3-dioxygenase, catalyzes the first and rate-limiting step in the major pathway of tryptophan metabolism, the kynurenine pathway. The physiological role of IDO is not fully understood but is of great interest, because IDO is widely distributed in human tissues, can be up-regulated via cytokines such as interferon-gamma, and can thereby modulate the levels of tryptophan, which is vital for cell growth. To identify which amino acid residues are important in substrate or heme binding in IDO, site-directed mutagenesis of conserved residues in the IDO gene was undertaken. Because it had been proposed that a histidine residue might be the proximal heme ligand in IDO, mutation to alanine of the three highly conserved histidines His(16), His(303), and His(346) was conducted. Of these, only His(346) was shown to be essential for heme binding, indicating that this histidine residue may be the proximal ligand and suggesting that neither His(303) nor His(16) act as the proximal ligand. Site-directed mutagenesis of Asp(274) also compromised the ability of IDO to bind heme. This observation indicates that Asp(274) may coordinate to heme directly as the distal ligand or is essential in maintaining the conformation of the heme pocket.