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

葡萄糖的氧化是贯穿生命所有领域的能量代谢的中心。它为细胞提供ATP，还原当量（NADH，NADPH）和生物合成的前体。已知蓝细菌和植物通过Embden-Meyerhoff-Parnass（EMP，通常称为糖酵解）或氧化戊糖磷酸（pentose phosphate）途径分解葡萄糖。我们中断了蓝细菌集胞藻PCC 6803中的EMP和E2途径，并惊讶地发现，该突变体仍然能够在光照下增强其在葡萄糖上的生长。而EMP和β-内酰胺酶途径是真核生物中最普遍的糖酵解途径，原核生物在糖酵解途径中显示出令人印象深刻的多样性。最常见的是EMP、EMPs和Entner-Doudoroff（艾德）途径。因此，我们在集胞藻的基因组中寻找艾德途径的关键酶，即Edd（6 β-葡萄糖酸脱氢酶）和Eda（KDPG-aldolase），并找到了两者的候选基因。敲除EMP途径的关键酶、EMP 3和艾德途径的旁路反应的敲除突变体不能再增强其在葡萄糖上的生长。这表明艾德途径可能在集胞藻中起作用。用13 C葡萄糖的标记实验证实，葡萄糖在光照下通过EMP和艾德途径代谢。对EMP（Pfk）和艾德（Eda）两条途径的关键酶进行了搜索，发现96%的蓝藻同时具有Eda，58%的蓝藻同时具有Pfk和Eda，4%的蓝藻仅具有Pfk，而38%的蓝藻仅具有Eda。这表明，艾德途径可能是一个糖酵解途径的生理意义，蓝藻已被忽视。值得注意的是，埃达也被发现广泛存在于植物中。系统发育分析表明，植物通过内共生基因转移从蓝藻中获得了该艾德途径的关键酶。讨论了Enter-Doudoroff途径作为蓝藻和植物糖酵解途径的生理意义。

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.