Glycolytic shunts are anaplerotic reactions for the Calvin-Benson-Bassham cycle
In this work, it was shown that the cyanobacterium Synechocystis employs three glycolytic shunts as anaplerotic reactions for the CBB cycle. The three shunts, named 'PGI shunt', 'OPP shunt' and 'ED shunt', not only mediated anaplerosis of glucose that was taken up under mixotrophic conditions but also of glycogen-derived glucose under photoautotrophic conditions. It could be shown that degradation of glycogen was able to improve restarting of the CBB cycle after a period of darkness. This suggests, that the cellular glycogen pool in cyanobacteria functions as a metabolic buffer that can be tapped upon demand in order to improve flux in the CBB cycle. The three glycolytic shunts thereby function as anaplerotic reactions. It can be assumed that glycolytic shunts not only help to control metabolite stoichiometry in the CBB cycle, but also to influence the cellular ATP and NADPH pools and thus provide a tool to improve synchronization with the photosynthetic light reaction.