Fructose-1,6-bisphosphatase is involved in heterotrophic growth and glycogen metabolism in cyanobacteria

Cyanobacteria switch between photoautotrophic and heterotrophic carbon metabolism during diurnal cycles. A classical control point is characterized by two glycolytic phosphofructokinases (PFKs) and a bifunctional fructose-1,6-biphosphatase/sedoheptulose-1,7-biphosphatase (F/SBPase; slr2094), which catalyses two reactions in the Calvin-Benson-Bassham (CBB) cycle. In addition, Synechocystis possesses a fructose-1,6-bisphosphatase (FBPase; slr0952) with yet unknown physiological function and biochemical properties. Our aim was to elucidate the physiological role of FBPase, in combination with the above-mentioned enzymes. We discovered that FBPase is specific for fructose 1,6-bisphosphate (FBP), showing no SBPase activity, and unlike F/SBPase does not exhibit any biochemical regulatory properties. In contrast to F/SBPase, FBPase is not involved in the CBB cycle, but instead affects growth and glycogen metabolism under heterotrophic conditions. We hypothesize that FBPase may influence glycogen turnover by controlling cellular levels of its substrate, FBP, since FBP is involved in the formation of glucose 1,6-bisphosphate, which is a regulatory metabolite for the control point between glycogen and central carbon metabolism at the level of phosphoglucomutases. Our data indicate that PFK and FBPase act as an antagonistic enzyme couple in darkness. Furthermore, we found redox-insensitive FBPases from plant chloroplasts to be closely related to Synechocystis FBPase, indicating that they might serve a similar function.