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

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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.