NADH-dependent persulfide reductase (Npsr) has been proposed to facilitate dissimilatory sulfur respiration by reducing persulfide or sulfane sulfur-containing substrates to H₂S. The presence of this gene in the sulfate and thiosulfate-reducing Archaeoglobus fulgidus DSM 4304 and other hyperthermophilic Archaeoglobales appears anomalous, as A. fulgidus is unable to respire S⁰ and grow in the presence of elemental sulfur. To assess the role of Npsr in the sulfur metabolism of A. fulgidus DSM 4304, the Npsr from A. fulgidus was characterized. AfNpsr is specific for persulfide and polysulfide as substrates in the oxidative half-reaction, exhibiting on the order of 10⁴ M^-1 s^-1, which is similar to the kinetic parameters observed for hyperthermophilic CoA persulfide reductases. In contrast to the bacterial Npsr, AfNpsr exhibits low disulfide reductase activity with DTNB; however, similar to the bacterial enzymes, it does not show detectable activity with CoA-disulfide, oxidized glutathione, or cystine. The 3.1 Å X-ray structure of AfNpsr reveals access to the tightly bound catalytic CoA, and the active site Cys 42 is restricted by a flexible loop (residues 60-66) that is not seen in the bacterial homologs from Shewanella loihica PV-4 and Bacillus anthracis. Unlike the bacterial enzymes, AfNpsr exhibits NADH oxidase activity and also shows no detectable activity with NADPH. Models suggest steric and electrostatic repulsions of the NADPH 2-phosphate account for the strong preference for NADH. The presence of Npsr in the nonsulfur-reducing A. fulgidus suggests that the enzyme may offer some protection against S⁰ or serve in another metabolic role that has yet to be identified.

中文翻译：

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来自古生球藻的超嗜热 NADH 依赖性过硫化物还原酶的结构和动力学表征

NADH 依赖性过硫化物还原酶 (Npsr) 已被提议通过将含过硫化物或硫烷硫的底物还原为 H ₂ S来促进异化硫呼吸。该基因在硫酸盐和硫代硫酸盐还原古生菌 DSM 4304 和其他超嗜热古生菌中的存在出现异常，因为A. fulgidus无法呼吸 S ⁰并在元素硫存在下生长。为了评估 Npsr 在A. fulgidus DSM 4304硫代谢中的作用，对来自A. fulgidus的 Npsr进行了表征。AfNpsr 对作为氧化半反应底物的过硫化物和多硫化物具有特异性，表现出大约为 10 ⁴ M ^-1 s ^-1，这类似于对超嗜热 CoA 过硫化物还原酶观察到的动力学参数。与细菌 Npsr 相比，AfNpsr 与 DTNB 的二硫化物还原酶活性较低；然而，与细菌酶相似，它对 CoA-二硫化物、氧化型谷胱甘肽或胱氨酸没有显示出可检测的活性。AfNpsr 的 3.1 Å X 射线结构揭示了对紧密结合的催化 CoA 的访问，并且活性位点 Cys 42 受到柔性环（残基 60-66）的限制，这在Shewanella loihica PV-4的细菌同源物中没有看到和炭疽芽孢杆菌. 与细菌酶不同，AfNpsr 表现出 NADH 氧化酶活性，并且对 NADPH 也没有显示出可检测的活性。模型表明 NADPH 2 -磷酸盐的空间和静电排斥是对 NADH 的强烈偏好的原因。Npsr 在非硫还原A. fulgidus 中的存在表明该酶可能提供一些针对 S ^{0 的}保护或发挥另一种尚未确定的代谢作用。