The effect of externally supplied pyruvate on the IDH enzyme of the cyanobacterium Nostoc muscorum Meg 1 directly reflects on its photosynthetic performance and diazotrophy

Pyruvate, the end product of glycolysis, is an important metabolite for the tricarboxylic acid (TCA) cycle and lipid metabolism. Pyruvate is converted to acetyl-CoA, which enters the TCA cycle for citrate formation. Citrate is the precursor for isocitrate that undergoes oxidative decarboxylation reaction forming α-ketoglutarate (α-KG) coupled with NADP⁺ reduction catalyzed by the enzyme isocitrate dehydrogenase (IDH). In cyanobacteria, α-KG donates its carbon skeleton to the glutamine synthetase-glutamine oxoglutarate aminotransferase (GS-GOGAT) pathway, thereby linking carbon and nitrogen metabolisms. In this study, the effect of pyruvate on IDH activity, its protein content, and its mRNA expression level were analyzed. Amongst the concentrations studied (100 µM, 250 µM, and 500 µM), in 250 µM externally supplied pyruvate, IDH registered the highest activity as well as maximum protein and mRNA contents when compared to the control. At this concentration, all biochemical parameters of nitrogen and carbon metabolisms- heterocyst frequency, nitrogenase, and glutamine synthetase activities, photosynthetic pigments, OEC (oxygen evolving complex) activity, carbohydrate content, proteins, and total biomass were also found to be highest. However, at an exposure to 500 µM pyruvate, there was a reduction in all the above-mentioned parameters indicating that although pyruvate is a key metabolite in the cellular metabolism, it tends to become toxic beyond this concentration. The compromised state of the cells was also evident from the micrographs developed from SEM (Scanning electron microscope) and TEM (Transmission electron microscope) studies under 500 µM pyruvate exposure.