NADPH-dependent imine reductases (IREDs) are enzymes capable of enantioselectively reducing imines to chiral secondary amines, which represent important building blocks in the chemical and pharmaceutical industry. Since their discovery in 2011, many previously unknown IREDs have been identified, biochemically and structurally characterized and categorized into families. However, the catalytic mechanism and guiding principles for substrate specificity and stereoselectivity remain disputed. Herein, we describe the crystal structure of S-IRED-Ms from Mycobacterium smegmatis together with its cofactor NADPH. S-IRED-Ms belongs to the S-enantioselective superfamily 3 (SFam3) and is the first IRED from SFam3 to be structurally described. The data presented provide further evidence for the overall high degree of structural conservation between different IREDs of various superfamilies. We discuss the role of Asp170 in catalysis and the importance of hydrophobic amino acids in the active site for stereospecificity. Moreover, a separate entrance to the active site, potentially functioning according to a gatekeeping mechanism regulating access and, therefore, substrate specificity is described.

中文翻译：

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来自耻垢分枝杆菌的S-对映选择性亚胺还原酶的结构表征。

NADPH依赖性亚胺还原酶（IREDs）是能够将亚胺对映选择性还原为手性仲胺的酶，手性仲胺是化学和制药行业的重要组成部分。自2011年被发现以来，已经鉴定了许多以前未知的IRED，对其进行了生化和结构表征，并归类为各个科。然而，底物特异性和立体选择性的催化机理和指导原则仍存在争议。在本文中，我们描述的晶体结构小号-IRED-女士从耻垢分枝杆菌与其辅因子NADPH在一起。S -IRED- M属于S-对映选择性超家族3（SFam3），并且是从结构上描述的第一个来自SFam3的IRED。所提供的数据为各种超家族的不同IRED之间的总体高度结构保守性提供了进一步的证据。我们讨论了Asp170在催化中的作用以及疏水性氨基酸在立体位点活性位点中的重要性。此外，还描述了一个单独的进入活性部位的入口，该入口可能根据调节进入的门禁机制起作用，因此可能会影响底物特异性。