Central Carbon Metabolism

The central carbohydrate metabolism in cyanobacteria and plants includes photosynthesis, CO₂-fixation, the breakdown of carbohydrates via glycolytic routes, the TCA cycle and the respiratory chain. The entire life on our planet is substantially driven by photosynthesis and CO₂ fixation of photoautotrophs. These processes provide other organisms with energy, carbohydrates and further metabolic products. In the absence of light, cyanobacteria and plants tap their own carbohydrate reservoirs, in order to sustain their own metabolism. The shift between day and night thus requires a shift from photoautotrophic to heterotrophic metabolism. During the day, carbohydrates are synthesized and at night they are broken down again. Besides, hybrid forms exist. Cyanobacteria are able to take up carbohydrates from their environment and green plant cells can likewise accept additional carbohydrates from other cells that are metabolized in parallel to CO₂ fixation. Under these circumstances, cells live photomixotrophically.

An essential difference between cyanobacteria and plants is the complexity of their cells. Cyanobacteria are as prokaryotes in principle not compartmented - with the exception of the carboxysomes, that harbor the CO₂ fixing enzyme Rubsico. In cyanobacteria, photosynthesis and respiratory chain share one membrane and CO₂ fixation and breakdown of carbohydrates both take place in the cytosol. In plants, these processes are distributed among different compartments. Photosynthesis and CO₂ fixation are localized in chloroplasts and respiration is found in mitochondria. However, the breakdown of carbohydrates is localized in the cytosol as well as in chloroplasts and mitochondria.

We would like to understand how these processes in the central carbohydrate metabolism, that run partly in opposite directions, are fine-tuned, how they intertwine and how they regulate each other. A special focus lies on the role of glycolytic routes.

It has been textbook knowledge for many years that animals and plants break down carbohydrates via two glycolytic routes: the Emden-Meyerhoff-Parnas (EMP) pathway (often simply referred to as glycolysis) and the oxidative pentose phosphate (OPP) pathway. However, we discovered that the Entner-Doudoroff (ED) pathway, which is a glycolytic route that was known in archaea and bacteria but had been previously overlooked in oxygenic photoautotrophs, operates both in cyanobacteria and plants (Chen et al., 2016). Glykolytic routes and CO₂ fixation run in opposing directions. In the first case carbohydrates are broken down, in the second case they are synthesized. EMP and OPP pathway share many enzymes with the CBB cycle, that work in opposing directions in the respective processes. Fine-tuning is important in order to prevent futile cycling and mutual inhibition. The ED pathway is special in that sense, that it does not share enzymes with the CBB cycle. When cells are brought from darkness to light, CO₂ fixation via the CBB cycle starts with a certain delay. We could show in the cyanobacterium Synechocystis, that glycolytic routes form shunts under these conditions that support the starting of the CBB cycle (Makowka et al., 2020).

We would like to understand the physiological significance of the ED pathway in cyanobacteria and plants in detail and focus especially on the interplay of glycolytic routes, photosynthesis and CO₂ fixation und the alternation of photoautotrophic, heterotrophic and photomixotrophic metabolism.

References

Chen, X., Schreiber, K., Appel, J., Makowka, A., Fähnrich, B., Roettger, M., Hajirezaei, M.R., Sönnichsen, F.D., Schönheit, P., Martin, W.F. and Gutekunst, K. 2016. The Entner–Doudoroff pathway is an overlooked glycolytic route in cyanobacteria and plants. Proceedings of the National Academy of Sciences 113(19), 5441-5446.

Makowka, A., Nichelmann, L., Schulze, D., Spengler, K., Wittmann, C., Forchhammer, K. and Gutekunst, K. 2020. Glycolytic Shunts Replenish the Calvin–Benson–Bassham Cycle as Anaplerotic Reactions in Cyanobacteria. Molecular Plant 13(3), 471-482.