Glucose degradation pathways are central for energy and carbon metabolism throughout all domains of life. They provide ATP, NAD(P)H, and biosynthetic precursors for amino acids, nucleotides, and fatty acids. It is general knowledge that cyanobacteria and plants oxidize carbohydrates via glycolysis [the Embden-Meyerhof-Parnas (EMP) pathway] and the oxidative pentose phosphate (OPP) pathway. However, we found that both possess a third, previously overlooked pathway of glucose breakdown: the Entner-Doudoroff (ED) pathway. Its key enzyme, 2-keto-3-deoxygluconate-6-phosphate (KDPG) aldolase, is widespread in cyanobacteria, moss, fern, algae, and plants and is even more common among cyanobacteria than phosphofructokinase (PFK), the key enzyme of the EMP pathway. Active KDPG aldolases from the cyanobacterium Synechocystis and the plant barley (Hordeum vulgare) were biochemically characterized in vitro. KDPG, a metabolite unique to the ED pathway, was detected in both in vivo, indicating an active ED pathway. Phylogenetic analyses revealed that photosynthetic eukaryotes acquired KDPG aldolase from the cyanobacterial ancestors of plastids via endosymbiotic gene transfer. Several Synechocystis mutants in which key enzymes of all three glucose degradation pathways were knocked out indicate that the ED pathway is physiologically significant, especially under mixotrophic conditions (light and glucose) and under autotrophic conditions in a day/ night cycle, which is probably the most common condition encountered in nature. The ED pathway has lower protein costs and ATP yields than the EMP pathway, in line with the observation that oxygenic photosynthesizers are nutrient-limited, rather than ATP-limited. Furthermore, the ED pathway does not generate futile cycles in organisms that fix CO 2 via the Calvin-Benson cycle. T he breakdown of glucose is central for energy and biosynthetic metabolism throughout all domains of life. The Embden-Meyerhof-Parnas (EMP) pathway (glycolysis) and the oxidative pentose phosphate (OPP) pathway are the backbones of eukaryotic carbon and energy metabolism (1, 2). They generate ATP, NAD(P)H, and biosynthetic precursors for amino acids, nucleotides, and fatty acids. Prokaryotes, in contrast, exhibit a broad diversity in sugar oxidation pathways (3-5). These routes differ in ATP yield, in the enzymes and cofactors involved, and in the chemical intermediates of the pathways. The most common glycolytic routes in prokaryotes are the EMP, ED, and OPP pathways (Fig. 1). The key enzyme unique to the ED pathway is 2-keto-3-deoxygluconate-6-phosphate (KDPG) aldolase (Eda), whereas phosphofructokinase (PFK) is unique to the EMP pathway in the catabolic direction (3, 6). KDPG as a metabolite is exclusively found in the ED pathway (Fig. 1). The first two steps of the OPP pathway are catalyzed by glucose 6-phosphate-dehydrogenase (Zwf) and 6-phosphogluconate dehydrogenase (Gnd). As the pentose phosphate pathway can either run in its oxidative mode (OPP pathway) to oxidize carbohydrates or in its reductive mode ...
Background: Cyanobacterial hydrogenases are claimed to produce hydrogen via NAD(P)H, which contradicts thermodynamic considerations; the physiological function of these hydrogenases is unresolved. Results: Flavodoxin/ferredoxin reduce cyanobacterial hydrogenases, which are essential under mixotrophic, nitrate-limiting conditions. Conclusion: Cyanobacterial bidirectional hydrogenases are electron sinks for reduced flavodoxin/ferredoxin. Significance: This study provides a basis for a target-oriented enhancement of hydrogen production and explains the aquatic distribution of cyanobacterial hydrogenases.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.