Redox control and substrate specificity of the glycogen catabolic isoenzymes in Synechocystis sp. PCC 6803

Glycogen serves as the main carbon storage polymer in many organisms and is widespread across all domains of life. In cyanobacteria, glycogen degradation is crucial for metabolic transitions during dark phases or during resuscitation from nitrogen starvation. Like many other cyanobacteria, Synechocystis sp. PCC 6803 possesses multiple homologues of glycogen catabolizing enzymes, though their specific roles and regulatory mechanisms are only partially understood. Here we demonstrate through biochemical analysis that the glycogen phosphorylase GlgP1, known to promote high temperature acclimation, is uniquely regulated by a C-terminal redox switch found certain cyanobacteria. This is the first evidence of redox regulation of a prokaryotic glycogen degrading enzyme. Notably, GlgP1 is activated via oxidation by reactive oxygen species and inactivated by reducing agents, with thioredoxin being the most effective inhibitor tested. The physiological implications of this redox regulation are discussed. Additionally, a biochemically characterization of the two glycogen debranching isoenzymes GlgX1 and GlgX2 revealed that only GlgX1 exhibits debranching activity, while GlgX2 does not. Mutant analysis confirmed that GlgX1 plays an essential role in glycogen mobilization, being crucial for resuscitation from chlorosis and survival during extended dark periods. In contrast the physiological function of GlgX2 remains unclear.