Plant Cell Physiology and Biotechnology Group

Central carbon metabolism in cyabobacteria and plants

Revision of the central carbohydrate metabolism in cyanobacteria and plants

 

Cyanobacteria and plants are photoautotrophs that rely on photosynthesis to fix CO2 and synthesize carbohydrates. These carbohydrates need to be broken down during the night to provide energy (ATP), reduction equivalents (NAD(P)H, red. Ferredoxin) and precursors for the synthesis of nucleic acids, amino and fatty acids.

 

Figure 1: The central carbohydrate metabolism is studied in the cyanobacterium Synechocystis and in the crop plant Hordeum vulgare in the group.

 

Whereas heterotrophs as animals (including us humans) and many bacteria rely exclusively on carbohydrate break down for their energy and carbon metabolism, photoautotrophs as cyanobacteria and plants rely on both photosynthesis and carbohydrate oxidation. The heterotrophic metabolism is fueled by carbohydrates that eventually originate from photosynthesis. The circuit of photosynthesis and carbohydrate break down thus substantially fuels all life on our planet.

It has long been thought that plants and cyanobacteria, like animals, rely exclusively on two glycolytic routes for their carbohydrate break down: (1) classical glycolysis (Emden-Meyerhof-Parnas pathway (EMP)) and the (2) oxidative pentose phosphate pathway (OPP).

However, we found recently that cyanobacteria rely on a third glycolytic route in addition: (3) the Entner-Doudoroff pathway (ED).

 

Figure 2: Glycolytic routes in cyanobacteria and plants. Classical glycolysis (EMP) is shown in red, the oxidative pentose phosphate pathway in blue and the Entner-Doudoroff pathway in green.

 

We furthermore found that cyanobacteria, that are regarded as ancestors of plant chloroplasts, transferred the key enzymes of the Entner-Doudoroff pathway (namely EDD and EDA, (see Figure 2) to the plant lineage via endosymbiotic gene transfer. The key enzyme of the ED pathway (Eda) was biochemically characterized and shown to be expressed both in the cyanobacterium Synechocystis and the plant Hordeum vulgare (Figure 1). Furthermore, KDPG which is a characteristic metabolite of the ED pathway (see Figure 2), was detected in both organisms as well, proving the operation of this route in cyanobacteria and plants. Blast analyses furthermore revealed that the key enzyme of the ED pathway is widespread in both cyanobacteria and plants and furthermore also present in algae, moss and fern. The ED pathway has obviously been widely overlooked in photoautotrophs that rely on oxygenic photosynthesis. Therefore, a revision of the central carbohydrate metabolism of photoautotrophs, which substantially supports all life on Earth, is required to understand the physiology of the different glycolytic routes and especially its interconnection with photosynthesis. For further reading please check Chen et al 2016, pdf and for a more popular scientific presentation Research features, pdf.