When faced with increased osmolarity in the environment, many bacterial cells accumulate the compatible solute ectoine and its derivative 5-hydroxyectoine. Both compounds are not only potent osmostress protectants, but also serve as effective chemical chaperones stabilizing protein functionality. Ectoines are energy-rich nitrogen and carbon sources that have an ecological impact that shapes microbial communities. Although the biochemistry of ectoine and 5-hydroxyectoine biosynthesis is well understood, our understanding of their catabolism is only rudimentary. Here, we combined biochemical and structural approaches to unravel the core of ectoine and 5-hydroxy-ectoine catabolisms. We show that a conserved enzyme bimodule consisting of the EutD ectoine/5-hydroxyectoine hydrolase and the EutE deacetylase degrades both ectoines. We determined the high-resolution crystal structures of both enzymes, derived from the salt-tolerant bacteria Ruegeria pomeroyi and Halomonas elongata. These structures, either in their apo-forms or in forms capturing substrates or intermediates, provided detailed insights into the catalytic cores of the EutD and EutE enzymes. The combined biochemical and structural results indicate that the EutD homodimer opens the pyrimidine ring of ectoine through an unusual covalent intermediate, N-α-2 acetyl-l-2,4-diaminobutyrate (α-ADABA). We found that α-ADABA is then deacetylated by the zinc-dependent EutE monomer into diaminobutyric acid (DABA), which is further catabolized to l-aspartate. We observed that the EutD–EutE bimodule synthesizes exclusively the α-, but not the γ-isomers of ADABA or hydroxy–ADABA. Of note, α-ADABA is known to induce the MocR/GabR-type repressor EnuR, which controls the expression of many ectoine catabolic genes clusters. We conclude that hydroxy–α-ADABA might serve a similar function.

中文翻译：

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细菌水解酶-脱乙酰酶复合物降解微生物应激保护剂和化学伴侣四氢嘧啶和羟基四氢嘧啶。

当环境中的渗透压增加时，许多细菌细胞会积累相容的溶质四氢嘧啶及其衍生物 5-羟基四氢嘧啶。这两种化合物不仅是有效的渗透压保护剂，而且还可以作为稳定蛋白质功能的有效化学伴侣。四氢甲基嘧啶是富含能量的氮和碳源，具有塑造微生物群落的生态影响。尽管四氢嘧啶和 5-羟基四氢嘧啶生物合成的生物化学已广为人知，但我们对其分解代谢的了解还只是初步的。在这里，我们结合了生化和结构方法来揭示四氢嘧啶和 5-羟基-四氢嘧啶分解代谢的核心。我们发现，由EutD四氢嘧啶/5-羟基四氢嘧啶水解酶和EutE脱乙酰酶组成的保守酶双模块可降解两种四氢嘧啶。我们确定了这两种酶的高分辨率晶体结构，这两种酶源自耐盐细菌 Ruegeria pomeroyi 和 Halomonas elongata。这些结构，无论是其脱辅基形式还是捕获底物或中间体的形式，都为EutD和EutE酶的催化核心提供了详细的见解。综合生化和结构结果表明，EutD 同二聚体通过一种不寻常的共价中间体 N-α-2 乙酰基-1-2,4-二氨基丁酸酯 (α-ADABA) 打开四氢嘧啶的嘧啶环。我们发现α-ADABA随后被锌依赖性EutE单体脱乙酰化为二氨基丁酸(DABA)，其进一步分解代谢为L-天冬氨酸。我们观察到EutD-EutE双模块只合成ADABA或羟基-ADABA的α-异构体，而不合成γ-异构体。值得注意的是，α-ADABA 已知可诱导 MocR/GabR 型阻遏物 EnuR，后者控制许多四氢嘧啶分解代谢基因簇的表达。我们得出结论，羟基-α-ADABA 可能具有类似的功能。