The (Glg) <scp>ABCs</scp> of cyanobacteria: modelling of glycogen synthesis and functional divergence of glycogen synthases in <i>Synechocystis</i> sp. <scp>PCC</scp> 6803

Glycogen is the principal carbon reserve in Synechocystis sp. PCC 6803. We reconstituted its biosynthetic pathway in vitro-GlgC (glucose-1-phosphate adenylyltransferase), two glycogen synthase isoenzymes (GlgA1, GlgA2) and the branching enzyme GlgB-to define how supply, polymerization and branching set flux and product structure. GlgA2 shows higher specific activity and cooperates with GlgB-generated branched primers, whereas GlgA1 has higher substrate affinity and responds more to primer concentration. Product profiling links mechanism to architecture: GlgA1 produces more-branched glycogen, while GlgA2 yields longer, less-branched polymers, with GlgB biasing utilization toward GlgA2. The complementary behaviors of GlgA1 and GlgA2 provide capacity for rapid accumulation versus steady-state maintenance and offer dynamic metabolic levers to tune glycogen content and architecture in cyanobacteria.