The soluble and membrane-bound transhydrogenases UdhA and PntAB have divergent functions in NADPH metabolism of Escherichia coli
The soluble and membrane-bound transhydrogenases UdhA and PntAB have divergent functions in NADPH metabolism of Escherichia coli
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DOI：
10.1074/jbc.m311657200
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发表时间：
2004-02-20
影响因子：
4.8
通讯作者：
Fischer, E
中科院分区：
文献类型：
--
作者：
Sauer, U;Canonaco, F;Fischer, E
戊糖磷酸途径和异柠檬酸脱氢酶通常被认为是合成代谢还原剂NADPH的主要来源。大肠杆菌是为数不多的微生物之一，它含有两种转氢酶同工型，其生理功能尚不清楚，可能直接将电子从NADH转移到NADP（+），反之亦然。利用确定的突变体和代谢通量分析，我们确定质子转运转氢酶PntAB是大肠杆菌NADPH的主要来源。杆菌在葡萄糖上的标准需氧分批生长期间，生物合成所需的35-45%的NADPH通过PntAB产生，而戊糖磷酸途径和异柠檬酸脱氢酶分别贡献35-45%和20- 25%。与此相反，能量非依赖性转氢酶UdhA，是必不可少的生长代谢条件下，过量的NADPH形成，即生长在乙酸盐或在磷酸葡萄糖异构酶突变体，通过戊糖磷酸途径分解代谢葡萄糖。因此，这两种亚型具有不同的生理功能：PntAB用NADH对NADP（+）进行能量依赖性还原，UdhA对NADPH进行再氧化。表达似乎受到细胞代谢的氧化还原状态的调节，因为增加或减少NADPH形成的遗传和环境操作分别下调pntA或udhA转录。两种转氢酶亚型提供了E.大肠杆菌的初级代谢具有非凡的灵活性，以科普不同的分解代谢和合成代谢的需求，这提出了两个一般性的问题：为什么只有少数细菌含有这两种异构体，以及其他生物体如何管理NADPH代谢？
Pentose phosphate pathway and isocitrate dehydrogenase are generally considered to be the major sources of the anabolic reductant NADPH. As one of very few microbes, Escherichia coli contains two transhydrogenase isoforms with unknown physiological function that could potentially transfer electrons directly from NADH to NADP(+) and vice versa. Using defined mutants and metabolic flux analysis, we identified the proton-translocating transhydrogenase PntAB as a major source of NADPH in E. coli. During standard aerobic batch growth on glucose, 35-45% of the NADPH that is required for biosynthesis was produced via PntAB, whereas pentose phosphate pathway and isocitrate dehydrogenase contributed 35-45% and 20-25%, respectively. The energy-independent transhydrogenase UdhA, in contrast, was essential for growth under metabolic conditions with excess NADPH formation, i.e. growth on acetate or in a phosphoglucose isomerase mutant that catabolized glucose through the pentose phosphate pathway. Thus, both isoforms have divergent physiological functions: energy-dependent reduction of NADP(+) with NADH by PntAB and reoxidation of NADPH by UdhA. Expression appeared to be modulated by the redox state of cellular metabolism, because genetic and environmental manipulations that increased or decreased NADPH formation down-regulated pntA or udhA transcription, respectively. The two transhydrogenase isoforms provide E. coli primary metabolism with an extraordinary flexibility to cope with varying catabolic and anabolic demands, which raises two general questions: why do only a few bacteria contain both isoforms, and how do other organisms manage NADPH metabolism?