Phosphoglucose isomerase is important for carbon distribution over central carbon metabolic pathways in cyanobacteria

Phosphoglucose isomerase (PGI) catalyzes the reversible interconversion of glucose 6-phosphate (G6P) to fructose 6-phosphate (F6P), classically considered a non-regulatory step in the Embden-Meyerhof-Parnas pathway for glycolysis and gluconeogenesis. However, in Synechocystis sp. PCC 6803, ¹³C flux analysis has identified the "PGI shunt" as a major route under photomixotrophic conditions, in which F6P is funnelled into the regenerative phase of the Calvin-Benson-Bassham (CBB) cycle to enhance CO₂ fixation. To elucidate the biochemical basis of this regulation, we characterized the cyanobacterial PGI, SynPGI in detail. SynPGI can reversibly convert G6P and F6P but shows a strong preference for the gluconeogenic reaction (F6P → G6P), with a ∼threefold higher catalytic efficiency for F6P, compared to G6P, in agreement with equilibrium assays, which confirmed that the reaction favors G6P accumulation (75%, K_eq = 0.3). Effector studies revealed two potent inhibitors: erythrose 4-phosphate (E4P) and 6-phosphogluconate (6PG). E4P, an intermediate of the regenerative CBB (rCBB) cycle, inhibited SynPGI at μM concentrations and 6PG, a metabolite of the oxidative pentose phosphate pathway, inhibited the enzyme at mM concentrations. Kinetic modeling indicated that a mixed-type inhibition model best describes the inhibitory effects of both metabolites. Enzymatic, structural and phylogenetic analyses reveal that cyanobacterial SynPGI closely resembles plastidic PGIs from plants and suggest that it may function as a metabolic control node, integrating environmental and intracellular signals to fine-tune carbon flux and enhance metabolic efficiency in Synechocystis.