Karl Forchhammer | Regulating glycogen metabolism in response to the autotrophy-heterotrophy switch
Channel: CyanoWorld
Date: 2023-02-08

Karl Forchhammer - Eberhard Karl University of Tübingen, Germany

Homepage: https://uni-tuebingen.de/fakultaeten/mathematisch-naturwissenschaftliche-fakultaet/fachbereiche/interfakultaere-einrichtungen/imit/arbeitsgruppen/mikrobiologie-organismische-interaktionen/ag-forchhammer/
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Research Gate: https://www.researchgate.net/profile/Karl-Forchhammer

Regulation of glycogen metabolism in response to the autotrophy-heterotrophy switch in Cyanobacteria

Cyanobacteria constantly oscillate between metabolic phases of autotrophy (CO2 fixation) and heterotrophy (consumption of fixed carbon). In this metabolic concept, the synthesis and turnover of carbon reserve polymers play a pivotal role. Using the model organism Synechocystis PCC 6803, we could show that the two carbon storage polymers, glycogen, and polyhydroxybutyrate (PHB), are interconnected by glycolytic flux through the Emden-Meyerhof-Parnas pathway. Carbon flux between glycogen and PHB is critically controlled at the phosphoglycerate mutase catalyzed interconversion of 3-phosphoglycerate (3-PGA) to 2-PGA, which turns out as a major control point of the heterotrophy-autotrophy switch. Glycogen storage becomes essential when Synechocystis is starved for combined nitrogen sources. Metabolism switches rapidly to the glycogen-consuming, heterotrophic mode when combined nitrogen sources become available again. In this situation, energy homeostasis is initially based on sodium-motif force-dependent ATP regeneration at the cytoplasmic membrane. The activation of nitrogen-assimilatory reactions then initiates glycogen degradation, catalyzed by only one of the two glycogen phosphorylase isoenzymes (GlgP2). We found that the subsequent conversion of Glucose-1P (Glc-1P) to Glc-6P through phosphoglucomutase (PGM) is tightly controlled through newly identified regulatory features, involving a second PGM isoenzyme, which catalyzes the formation of the activating molecule for PGM-1. Moreover, PGM-1 forms a redox-dependent transient complex (a metabolon) with the subsequent catabolic enzyme, the Glc-6P dehydrogenase (G6PDH), mediated by a redox-controlled connector protein (OpcA). We hypothesize that the formation of this complex directs the glycolytic carbon flow toward the oxidative pentose phosphate pathway. This regulatory complex should prevent flux into the glycolytic EMP pathway, dispensable during the re-greening of nitrogen-starved cells.

This talk is part of the CyanoWorld online seminar series, hosted by Nicolas Schmelling and Ilka Axmann, and was held on Zoom on October 28th, 2021.

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