On the role and regulation of glycogen catabolic isoenzymes in Synechocystis sp. PCC6803

Glycogen is the main carbon storage polymer in many organisms and widespread throughout all domains of life. In cyanobacteria glycogen degradation plays an essential role when metabolism changes to a heterotrophic mode during dark phases or the resuscitation from nitrogen starvation. Like many other cyanobacteria the genome of Synechocystis sp. PCC 6803 encodes for several homologues of glycogen catabolic enzymes. The role and regulation of this isoenzymes has so far been only partly understood. Here we show via biochemical analysis that the glycogen phosphorylase GlgP1 is regulated by a C-terminal redox switch unique to certain cyanobacteria. This is the first time a redox regulation of a glycogen degrading enzyme has been shown in prokaryotes. Remarkably, GlgP1 is inactivated in its reduced state and gets activated via oxidation by ROS. We conclude that GlgP1 is especially important for rapid glycogen degradation in certain stress conditions during photoautotrophic growth. We suggest that thereby carbon is channeled through the OPP pathway, to stabilize photosynthesis during situations of high ribulose-1,5-BP demands. Furthermore, we suggest that the concomitant formation of NADPH within this pathway is essential to regenerate ROS scavenging compounds like GSH to reduce potential oxidative damage and to improve survivability under prolonged phases of oxidative stress. In addition, we show for the first time the role of the two glycogen debranching isoenzymes where GlgX1 turned out to be the essential glycogen debranching enzyme during resuscitation from chlorosis, while the role of GlgX2 remains elusive.