Cobalamin (Cbl) is an essential cofactor for methionine synthase (MS) and methylmalonyl-CoA mutase (MUT), but it must first undergo chemical processing for utilization in animals. In humans, this processing comprises β-axial ligand cleavage and Cbl reduction and is performed by the enzyme MMACHC (HsCblC). Although the functionality of CblC is well-understood in higher order organisms, little is known about the evolutionary origin of these enzymes and the reactivity of CblCs in lower-order organisms with unique environmental and cellular conditions. Therefore, we investigated the CblC of Trichoplax adhaerens (TaCblC), a marine organism considered to be one of the earliest evolutionarily diverging and simplest living animals. The TaCblC sequence contained conserved residues important for Cbl processing in higher-order organisms. The predicted structure of TaCblC closely resembled known CblC structures and had features consistent with Cbl and co-substrate binding capabilities. Recombinantly expressed TaCblC could bind and process several Cbl analogues using glutathione or NADH as co-substrates, similarly to previously characterized CblCs, but with variable rates and dependencies on the presence of oxygen. Notably, TaCblC dealkylates methylcobalamin at a rate ca. 2-times higher than HsCblC, although this comes with a lower ratio of product to glutathione oxidation, suggesting higher unproductive electron transfer in the TaCblC system. This reflects differences in cellular conditions of the more ancient homologue, which lives in low oxygen levels and an environment of low Cbl biovailability (∼2 pM in sea water).

中文翻译：

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Trichoplax adhaerens 的钴胺素加工酶。


钴胺素 （Cbl） 是蛋氨酸合酶 （MS） 和甲基丙二酰辅酶 A 变位酶 （MUT） 的重要辅因子，但它必须首先经过化学加工才能在动物中使用。在人类中，这种加工包括 β 轴配体切割和 Cbl 还原，由 MMACHC 酶 （HsCblC） 进行。尽管 CblC 的功能在高阶生物体中已广为人知，但人们对这些酶的进化起源以及 CblC 在具有独特环境和细胞条件的低阶生物体中的反应性知之甚少。因此，我们研究了 Trichoplax adhaerens （TaCblC） 的 CblC，这是一种海洋生物，被认为是最早进化分化和最简单的现生动物之一。TaCblC 序列包含对高阶生物体中的 Cbl 加工很重要的保守残基。预测的 TaCblC 结构与已知的 CblC 结构非常相似，并且具有与 Cbl 和共底物结合能力一致的特征。重组表达的 TaCblC 可以使用谷胱甘肽或 NADH 作为共底物结合和加工几种 Cbl 类似物，类似于先前表征的 CblC，但速率可变，并且依赖于氧的存在。值得注意的是，TaCblC 以比 HsCblC 高约 2 倍的速率处理甲基钴胺素，尽管这与产物与谷胱甘肽氧化的比率较低，表明 TaCblC 系统中的非生产性电子转移更高。这反映了更古老的同系物的细胞条件的差异，它生活在低氧水平和低 Cbl 生物利用性（海水中 ∼2 pM）的环境中。