Institute of Molecular Systems Biology, ETH Zürich, Zürich, Switzerland.
Institute of Molecular Systems Biology, ETH Zürich, Zürich, Switzerland
mBio. 2021 Apr 6;12(2):e03438-20. doi: 10.1128/mBio.03438-20.
The redox cofactor NADPH is required as a reducing equivalent in about 100 anabolic reactions throughout metabolism. To ensure fitness under all conditions, the demand is fulfilled by a few dehydrogenases in central carbon metabolism that reduce NADP with electrons derived from the catabolism of nutrients. In the case of growing on glucose, quantitative flux analyses indicate that NADPH production largely exceeds biosynthetic needs, suggesting a hitherto unknown mechanism for NADPH balancing. We investigated the role of the four malic enzymes present in that could bring about a metabolic cycle for transhydrogenation of NADPH into NADH. Using quantitative C metabolic flux analysis, we found that isoform YtsJ alone contributes to NADPH balancing and demonstrated relevant NADPH-oxidizing activity by YtsJ To our surprise, we discovered that depending on NADPH, YtsJ switches activity from a pyruvate-producing malic enzyme to a lactate-generating malolactic enzyme. This switch in activity allows YtsJ to adaptively compensate for cellular NADPH over- and underproduction upon demand. Finally, NADPH-dependent bifunctional activity was also detected in the YtsJ homolog in MaeB. Overall, our study extends the known redox cofactor balancing mechanisms by providing first-time evidence that the type of catalyzed reaction by an enzyme depends on metabolite abundance. A new mechanism for NADPH balancing was discovered in It pivots on the bifunctional enzyme YtsJ, which is known to catalyze NADP-dependent malate decarboxylation. We found that in the presence of excessive NADPH, the same enzyme switches to malolactic activity and creates a transhydrogenation cycle that ultimately converts NADPH to NADH. This provides a regulated mechanism to immediately adjust NADPH/NADP in response to instantaneous needs.
氧化还原辅助因子 NADPH 作为还原当量，在代谢过程中的大约 100 种合成反应中都需要用到。为了确保在所有条件下的适应性，中央碳代谢中的少数几种脱氢酶通过利用营养物质分解代谢产生的电子将 NADP 还原来满足需求。在以葡萄糖为唯一碳源的情况下，定量通量分析表明 NADPH 的产生量大大超过生物合成的需求，这表明存在一种未知的 NADPH 平衡机制。我们研究了存在于 中的四种苹果酸酶的作用，它们可能带来 NADPH 向 NADH 的代谢循环。通过定量 C 代谢通量分析，我们发现单独的同工型 YtsJ 就有助于 NADPH 的平衡，并证明了 YtsJ 的相关 NADPH 氧化活性。令我们惊讶的是，我们发现 YtsJ 可以根据 NADPH 切换其活性，从产生丙酮酸的苹果酸酶转换为生成乳酸的苹果酸酶。这种活性转换使 YtsJ 能够根据需要自适应地补偿细胞 NADPH 的过度产生和不足。最后，在 中的 YtsJ 同源物 MaeB 中也检测到了 NADPH 依赖性的双功能活性。总的来说，我们的研究通过提供首次证据表明，酶催化的反应类型取决于代谢物的丰度，从而扩展了已知的氧化还原辅助因子平衡机制。在 中发现了一种新的 NADPH 平衡机制，其关键在于双功能酶 YtsJ，该酶已知能催化 NADP 依赖的苹果酸脱羧反应。我们发现，在存在过量 NADPH 的情况下，同一种酶会切换到苹果酸酶的活性，并创建一个转氢酶循环，最终将 NADPH 转化为 NADH。这提供了一种受调控的机制，可根据即时需求立即调节 NADPH/NADP。
