Glycogen, a randomly branched glucose polymer, provides energy storage in organisms. It forms small β particles which in animals bind to form composite α particles, which give better glucose release. Simulations imply β particle size is controlled only by activities and sizes of glycogen biosynthetic enzymes and sizes of polymer chains. Thus, storing more glucose requires forming more β particles, which are expected to sometimes form α particles. No α particles have been reported in bacteria, but the extraction techniques might have caused degradation. Using milder glycogen extraction techniques on Escherichia coli, transmission electron microscopy and size-exclusion chromatography showed α particles, consistent with this hypothesis for α-particle formation. Molecular density and size distributions show similarities with animal glycogen, despite very different metabolic processes. These general polymer constraints are such that any organism which needs to store and then release glucose will have similar α and β particle structures: a type of convergent evolution.
This study compares four glycogen extraction methods, TCA-HW, TCA-CW, KOH-HW and SGDU-CW, for glycogen yields and structural degradation. Minimal degradation is essential to be able to study the properties of the native glycogen. The criteria for minimal degradation are that the SEC whole-molecule weight distribution w(log Rh) shows more large molecules, and that the CLD of the debranched chains shows the greatest number of long chains; while it was expected that harsh extraction techniques would yield more glycogen, that is likely to be more degraded. SGDU-CW best preserved the molecular structure of bacterial glycogen. The SEC weight distribution suggested the presence of fragile and stable glycogen α particles from SGDU-CW, an inference which was supported by TEM. Based on the combined analysis of SEC results and molecular density distribution results, fragile and dense α particles are suggested to consist of protein-embedded large and dense β particles due to the long ACLs. In contrast, stable light α particles are postulated to comprise small compact β particles with proteins associated at surfaces due to the short ACLs, which contributes to the strong binding of β particles and stability of α particles.
The existence of α particles in any organism which needs to store, and at a later stage use, glucose was inferred from theory, arising from the average size of β particles being limited by general polymer requirements rather than any particularities of a biological system. Supporting this inference, α particles are here seen in bacteria for the first time. The similarity of glycogen structure in two completely different organisms is seen because of the “polymer-physical-chemistry” control to be likely to evolve in any system which has the need to store and gradually release glucose: a type of convergent evolution. Discovery of co-occurrence of fragile and stable α particles opens a novel direction for bacterial glycogen study, such as how glucose concentration influences glycogen structure, how glycogen storage and structure change in different stages of bacterial life, and what proteins are responsible for the formation and fragility of α particles in bacteria. This may shed light on not only bacterial metabolism and physiology but could also add to the understanding of glycogen stability and fragility in healthy and diabetic liver tissue, which is of direct importance to human health.
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