Is the Entner-Doudoroff pathway an overlooked glycolytic route in cyanobacteria and did cyanobacteria transfer its key enzyme via endosymbiotic gene transfer to plants?

葡萄糖的氧化是生命所有领域能量代谢的中心。它为细胞提供ATP，还原当量（NADH， NADPH）和生物合成的前体。众所周知，蓝藻和植物通过embp - meyerhoff - parnass（通常称为糖酵解）或氧化戊糖磷酸（OPP）途径分解葡萄糖。我们中断了蓝细菌Synechocystis sp. PCC 6803的EMP和OPP途径，并惊讶地发现该突变体仍然能够在光照下增强其对葡萄糖的生长。EMP和OPP途径是真核生物中最普遍的糖酵解途径，而原核生物在糖酵解途径上表现出令人印象深刻的多样性。最常见的是EMP、OPP和enterner - doudoroff （ED）通路。因此，我们在聚囊藻基因组中寻找ED通路的关键酶，即Edd （6p -葡萄糖酸脱水酶）和Eda （kdpg -醛缩酶），并找到了两者的候选基因。当敲除EMP、OPP和ED通路的关键酶以及OPP通路的旁路反应时，敲除突变体不能再促进其在葡萄糖上的生长。这表明ED途径可能在聚囊虫中起作用。13C葡萄糖标记实验证实，葡萄糖在光线下通过EMP和ED途径代谢。对EMP途径（即Pfk）和ED途径（即Eda）关键酶的研究显示，96%的蓝藻具有Eda， 58%的蓝藻同时具有Pfk和Eda， 4%的蓝藻单独具有Pfk，而显著的是，38%的蓝藻单独具有Eda。这表明ED途径可能是蓝藻中一个被忽视的具有生理意义的糖酵解途径。值得注意的是，人们还发现Eda在植物中广泛存在。系统发育分析表明，植物通过蓝藻内共生基因转移获得了ED通路的关键酶。本文讨论了entert - doudoroff途径作为糖酵解途径在蓝藻和植物中的生理意义。

The oxidation of glucose is central for energy metabolism throughout all domains of life. It supplies cells with ATP, reduction equivalents (NADH, NADPH) and precursors for biosynthesis. Cyanobacteria and plants are known to break down glucose either via the Embden-Meyerhoff-Parnass (EMP, often referred to as glycolysis) or the oxidative pentose phosphate (OPP) pathway. We interrupted both the EMP and OPP pathway in the cyanobacterium Synechocystis sp. PCC 6803 and found to our surprise that this mutant was still able to enhance its growth on glucose in the light. Whereas the EMP and OPP pathway are the most prevalent glycolytic routes in eukaryotes, prokaryotes display an impressive diversity in glycolytic pathways. The most prevalent are EMP, OPP and the Entner-Doudoroff (ED) pathway. We therefore searched the genome of Synechocystis for the key enzymes of the ED pathway, namely Edd (6P-gluconate dehydratase) and Eda (KDPG-aldolase) and found candidate genes for both. A knockout mutant in which the key enzymes of EMP, OPP and ED pathway and a bypass reaction of the OPP pathway were knocked out, could no longer enhance its growth on glucose. This suggests that the ED pathway might operate in Synechocystis. Labelling experiments with 13C glucose confirmed that glucose is metabolized both via the EMP and ED pathway in the light. A search for the key enzymes of both EMP (namely Pfk) and ED (namely Eda) pathway revealed that 96% of all sequenced cyanobacteria hold Eda, 58% hold both Pfk and Eda, 4% hold Pfk alone and remarkably 38% hold Eda alone. This indicates that the ED pathway might be a glycolytic route of physiological significance in cyanobacteria that has been overlooked. Remarkably Eda was also found to be widespread in plants. Phylogenetic analyses revealed that plants received this key enzyme of the ED pathway via endosymbiotic gene transfer from cyanobacteria. The physiological significance of the Enter-Doudoroff pathway as a glycolytic route in both cyanobacteria and plants is discussed.