Human PAICS is a bifunctional enzyme that is involved in the de novo purine biosynthesis, catalyzing the conversion of aminoimidazole ribonucleotide (AIR) into N-succinylcarboxamide-5-aminoimidazole ribonucleotide (SAICAR). It comprises two distinct active sites, AIR carboxylase (AIRc) where the AIR is initially converted to carboxyaminoimidazole ribonucleotide (CAIR) by reaction with CO₂ and SAICAR synthetase (SAICARs) in which CAIR then reacts with an aspartate to form SAICAR, in an ATP-dependent reaction. Human PAICS is a promising target for the treatment of various types of cancer, and it is therefore of high interest to develop a detailed understanding of its reaction mechanism. In the present work, density functional theory calculations are employed to investigate the PAICS reaction mechanism. Starting from the available crystal structures, two large models of the AIRc and SAICARs active sites are built and different mechanistic proposals for the carboxylation and phosphorylation–condensation mechanisms are examined. For the carboxylation reaction, it is demonstrated that it takes place in a two-step mechanism, involving a C–C bond formation followed by a deprotonation of the formed tetrahedral intermediate (known as isoCAIR) assisted by an active site histidine residue. For the phosphorylation–condensation reaction, it is shown that the phosphorylation of CAIR takes place before the condensation reaction with the aspartate. It is further demonstrated that the three active site magnesium ions are involved in binding the substrates and stabilizing the transition states and intermediates of the reaction. The calculated barriers are in good agreement with available experimental data.

中文翻译：

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用量子化学计算阐明人类 PAICS 的反应机制

人类 PAICS 是一种双功能酶，参与嘌呤从头生物合成，催化氨基咪唑核糖核苷酸 (AIR) 转化为N-琥珀酰羧酰胺-5-氨基咪唑核糖核苷酸 (SAICAR)。_{它包含两个不同的活性位点，AIR 羧化酶 (AIRc)，其中 AIR 通过与 CO 2}反应最初转化为羧基氨基咪唑核糖核苷酸 (CAIR)和 SAICAR 合成酶 (SAICARs)，其中 CAIR 然后与天冬氨酸反应形成 SAICAR，这是一种 ATP 依赖性反应。人类 PAICS 是治疗各种癌症的有希望的靶点，因此对其反应机制的详细了解具有很高的兴趣。在目前的工作中，采用密度泛函理论计算来研究 PAICS 反应机理。从可用的晶体结构开始，建立了 AIRc 和 SAICARs 活性位点的两个大型模型，并研究了羧化和磷酸化缩合机制的不同机制建议。对于羧化反应，证明它以两步机制发生，涉及形成 C-C 键，然后在活性位点组氨酸残基的辅助下对形成的四面体中间体（称为 isoCAIR）进行去质子化。对于磷酸化-缩合反应，表明 CAIR 的磷酸化发生在与天冬氨酸的缩合反应之前。进一步证明，三个活性位点镁离子参与结合底物并稳定反应的过渡态和中间体。计算出的障碍与可用的实验数据非常吻合。进一步证明，三个活性位点镁离子参与结合底物并稳定反应的过渡态和中间体。计算出的障碍与可用的实验数据非常吻合。进一步证明，三个活性位点镁离子参与结合底物并稳定反应的过渡态和中间体。计算出的障碍与可用的实验数据非常吻合。