A Conserved Mechanism for Sulfonucleotide Reduction

Sulfonucleotide reductases are a diverse family of enzymes that catalyze the first committed step of reductive sulfur assimilation. In this reaction, activated sulfate in the context of adenosine-5′-phosphosulfate (APS) or 3′-phosphoadenosine 5′-phosphosulfate (PAPS) is converted to sulfite with reducing equivalents from thioredoxin. The sulfite generated in this reaction is utilized in bacteria and plants for the eventual production of essential biomolecules such as cysteine and coenzyme A. Humans do not possess a homologous metabolic pathway, and thus, these enzymes represent attractive targets for therapeutic intervention. Here we studied the mechanism of sulfonucleotide reduction by APS reductase from the human pathogen Mycobacterium tuberculosis, using a combination of mass spectrometry and biochemical approaches. The results support the hypothesis of a two-step mechanism in which the sulfonucleotide first undergoes rapid nucleophilic attack to form an enzyme-thiosulfonate (E-Cys-S-SO3−) intermediate. Sulfite is then released in a thioredoxin-dependent manner. Other sulfonucleotide reductases from structurally divergent subclasses appear to use the same mechanism, suggesting that this family of enzymes has evolved from a common ancestor.

磺酰核苷酸还原酶（Sulfonucleotide reductases）是一类多样的酶家族，催化还原性硫同化途径中的首个不可逆关键步骤。在该反应中，结合于腺苷-5'-磷酸硫酸（APS）或3'-磷酸腺苷-5'-磷酸硫酸（PAPS）的活化硫酸盐，会在硫氧还蛋白提供的还原当量作用下转化为亚硫酸盐。该反应生成的亚硫酸盐可在细菌与植物体内被用于最终合成半胱氨酸、辅酶A等必需生物分子。人类不具备与之同源的代谢途径，因此这类酶可作为治疗干预的理想靶点。本研究针对人类病原体结核分枝杆菌（Mycobacterium tuberculosis）来源的APS还原酶所介导的磺酰核苷酸还原机制，结合质谱法与生化手段展开了探究。研究结果支持两步反应机制假说：磺酰核苷酸首先经历快速亲核攻击，形成酶-硫代磺酸盐（E-Cys-S-SO3−）中间体。随后亚硫酸盐以硫氧还蛋白依赖的方式被释放。其他来自结构差异显著亚类的磺酰核苷酸还原酶似乎也采用相同的作用机制，这表明该酶家族起源于一个共同的祖先。