Scheibe Renate
Pflanzenphysiologie, FB Biologie/Chemie, Universität Osnabrück, D-49069 Osnabrück, Germany.
Physiol Plant. 2004 Jan;120(1):21-26. doi: 10.1111/j.0031-9317.2004.0222.x.
In green parts of the plant, during illumination ATP and NAD(P)H act as energy sources that are generated mainly in photosynthesis and respiration, whereas in darkness, glycolysis, respiration and the oxidative pentose-phosphate pathway (OPP) generate the required energy forms. In non-green parts, sugar oxidation in glycolysis, respiration and OPP are the only means of producing energy. For energy-consuming reactions, the delivery of NADPH, NADH, reduced ferredoxin and ATP has to take place at the required rates and in the specific compartments, since the pool sizes of these energy carriers are rather limited and, in general, they are not directly transported across biomembranes. Indirect transport of reducing equivalents can be achieved by malateoxaloacetate shuttles, involving malate dehydrogenase (MDH) for the interconversion. Isoenzymes of MDH are present in each cellular compartment. Chloroplasts contain the redox-controlled NADP-MDH that is only active in the light. In addition, a plastid NAD-MDH that is permanently active and is present in all plastid types has been found. Export of excess NAD(P)H through the malate valves will allow for the continued production of ATP (1) in photosynthesis, and (2) in oxidative phosphorylation. In the latter case, the coupled production of NADH is catalysed by the bispecific NAD(P)-GAPDH (GapAB) in chloroplasts that is active with NAD even in darkness, or by the specific plastid NAD-GAPDH (GapCp) in non-green tissues. When plants are subjected to conditions such as high light, high CO(2), NH(4) (+) nutrition, cold stress, which require changed activities of the enzymes of the malate valves, changed expression levels of the MDH isoforms can be observed. In nodules, the induction of a nodule-specific plastid NAD-MDH indicates the changed requirements for energy supply during N(2) fixation. Furthermore, the induction of glucose 6-phosphate dehydrogenase isoforms by ammonium and of ferredoxin and ferredoxin-NADP reductase by nitrate has been described. All these findings are in line with the assumption that a changed redox state caused by metabolic variability leads to the induction of enzymes involved in redox poise.
在植物的绿色部分，光照期间ATP和NAD(P)H作为主要在光合作用和呼吸作用中产生的能量来源，而在黑暗中，糖酵解、呼吸作用和氧化戊糖磷酸途径（OPP）产生所需的能量形式。在非绿色部分，糖酵解、呼吸作用和OPP中的糖氧化是产生能量的唯一方式。对于耗能反应，NADPH、NADH、还原型铁氧还蛋白和ATP必须以所需的速率并在特定的区室中进行传递，因为这些能量载体的库容量相当有限，而且一般来说，它们不能直接跨生物膜运输。还原当量的间接运输可以通过苹果酸-草酰乙酸穿梭来实现，这涉及苹果酸脱氢酶（MDH）进行相互转化。MDH的同工酶存在于每个细胞区室中。叶绿体含有受氧化还原控制的NADP-MDH，其仅在光照下有活性。此外，还发现了一种在所有质体类型中都永久有活性的质体NAD-MDH。通过苹果酸阀输出过量的NAD(P)H将允许（1）在光合作用中以及（2）在氧化磷酸化中持续产生ATP。在后一种情况下，叶绿体中的双特异性NAD(P)-GAPDH（GapAB）催化NADH的偶联产生，GapAB即使在黑暗中也对NAD有活性，或者非绿色组织中的特定质体NAD-GAPDH（GapCp）催化NADH的偶联产生。当植物受到高光、高CO₂、NH₄⁺营养、冷胁迫等条件影响时，这些条件需要改变苹果酸阀中酶的活性，此时可以观察到MDH同工型的表达水平发生变化。在根瘤中，一种根瘤特异性质体NAD-MDH的诱导表明在固氮过程中能量供应的需求发生了变化。此外，还描述了铵对葡萄糖6-磷酸脱氢酶同工型的诱导以及硝酸盐对铁氧还蛋白和铁氧还蛋白-NADP还原酶的诱导。所有这些发现都符合这样一种假设，即代谢变异性引起的氧化还原状态变化导致参与氧化还原平衡的酶的诱导。
