零价硫是同化亚硫酸盐还原酶的产物，也是半胱氨酸合酶的底物

摘要(中文译文)

同化型硫酸盐还原过程包括硫酸盐还原为硫化氢（HS），随后HS被同化用于半胱氨酸合成。然而，近期研究表明，异化型硫酸盐还原过程中同时产生HS和零价硫（S），这引出了S是否也参与同化型硫酸盐还原的问题。在本研究中，系统发育分析将同化型亚硫酸盐还原酶分为五个簇，我们重新检验了来自两个主要簇的四个代表性酶。这些酶从亚硫酸盐中同时产生S和HS。然而，S是直接产物，而HS是S经化学还原衍生的产物。半胱氨酸合成酶的系统发育分析将其分为六个簇，我们重新检验了来自两个主要簇的六个代表性酶。这些酶同时利用谷胱甘肽过硫化物（GSSH，一种常见的含S原子的细胞内化合物）和HS作为底物合成半胱氨酸，但更偏好使用GSSH。这些发现表明，在无机硫向有机硫转化的过程中，关键中间体是S而非HS。硫酸盐还原是全球硫循环中的重要环节。同化型硫酸盐还原包括硫酸盐还原为硫化氢（HS），随后HS被同化用于半胱氨酸合成。这一概念建立于20世纪50年代，至今未被质疑。在本研究中，我们重新检验了四种代表性同化型亚硫酸盐还原酶（aSiRs），发现这些酶直接将亚硫酸盐还原为零价硫（S）。所产生的一小部分S通过不依赖aSiR的化学反应被还原为HS。此外，我们重新检验了六种代表性半胱氨酸合成酶，发现这些酶利用S衍生物（硫代硫酸盐和谷胱甘肽过硫化物）作为底物合成半胱氨酸。因此，S而非HS，才是同化型硫酸盐还原过程的关键中间体。

Abstract

The process of the assimilatory sulfate reduction consists of sulfate reduction to hydrogen sulfide (HS) and then HS assimilation for cysteine synthesis. However, recent studies indicate that both HS and zero-valent sulfur (S) are produced during dissimilatory sulfate reduction, which leads to the question of whether S is also involved in assimilatory sulfate reduction. In this study, phylogenetic analysis grouped assimilatory sulfite reductases into five clusters, and we re-examined four representatives from two major clusters. These enzymes produced both S and HS from sulfite. However, S was a direct product, while HS was a derivative of S via chemical reduction. Phylogenetic analysis of cysteine synthases grouped them into six clusters, and we re-examined six representatives from two major clusters. These enzymes used both glutathione persulfide (GSSH, a common intracellular compound containing a S atom) and HS as substrates to synthesize cysteine, but preferred to use GSSH. These findings indicate that S, rather than HS, is the key intermediate during the process of inorganic sulfur conversion to organic sulfur. Sulfate reduction is an important link in the global sulfur cycle. Assimilatory sulfate reduction consists of sulfate reduction to hydrogen sulfide (HS) and then HS assimilation for cysteine synthesis. This concept was founded in the 1950s and has not been challenged until today. In this study, we re-examined four representative assimilatory sulfite reductases (aSiRs) and found that these enzymes directly reduced sulfite to zero-valent sulfur (S). A small portion of produced S was reduced to HS via aSiR-independent chemical reactions. Further, we re-examined six representative cysteine synthases and found that these enzymes used S derivative compounds (thiosulfate and glutathione persulfide) as substrates to synthesize cysteine. Thus, S is the key intermediate of assimilatory sulfate reduction process, rather than HS.

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