Nicotinamide adenine dinucleotide (NAD⁺) is a common cofactor in enzyme-catalyzed reactions that involve hydride transfers. In contrast, urocanase and urocanase-like enzymes use NAD⁺ for covalent electrophilic catalysis. Deciphering avenues by which this unusual catalytic strategy has diversified by evolution may point to approaches for the design of novel enzymes. In this report, we describe the S-methyl thiourocanate hydratase (S-Me-TUC) from Variovorax sp. RA8 as a novel member of this small family of NAD⁺-dependent hydratases. This enzyme catalyzes the 1,4-addition of water to S-methyl thiourocanate as the second step in the catabolism of S-methyl ergothioneine. The crystal structure of this enzyme in complex with the cofactor and a product analogue identifies critical sequence motifs that explain the narrow and nonoverlapping substrate scopes of S-methyl thiourocanate-, urocanate-, thiourocanate-, and Nτ-methyl urocanate-specific hydratases. The discovery of a S-methyl ergothioneine catabolic pathway also suggests that S-methylation or alkylation may be a significant activity in the biology of ergothioneine.

中文翻译：

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S-甲基硫脲酸水合酶的结构和底物特异性

烟酰胺腺嘌呤二核苷酸 (NAD ⁺ ) 是涉及氢化物转移的酶催化反应中的常见辅因子。相反，尿卡酶和类尿卡酶使用 NAD ⁺进行共价亲电催化。破译这种不寻常的催化策略通过进化而多样化的途径可能会为新型酶的设计提供方法。在本报告中，我们描述了来自Variovorax sp. 的S-甲基硫代尿刊酸水合酶 ( S -Me-TUC) 。^{RA8 是 NAD +}依赖性水合酶小家族的新成员。该酶催化水 1,4-加成至S-甲基硫代尿嘧啶，作为S-甲基麦角硫因分解代谢的第二步。该酶与辅因子和产物类似物复合的晶体结构确定了关键序列基序，这些基序解释了S-甲基硫代尿刊酸、尿刊酸、硫代尿刊酸和N τ-甲基尿刊酸特异性水合酶的狭窄且不重叠的底物范围。S-甲基麦角硫因分解代谢途径的发现也表明S-甲基化或烷基化可能是麦角硫因生物学中的重要活性。