Identification of the Genes Related to the Glycogen Metabolism in Hyperthermophilic Archaeon, Sulfolobus acidocaldarius

AmyA contributes to the glycogen synthesis in Sulfolobus acidocaldarius

Glycogen, an α-1,4 linked glucose polymer with α-1,6 linked branches, accumulates in Sulfolobus acidocaldarius in granular form and contributes to stress resistance. While the glg operon responsible for glycogen metabolism has been studied, the gene responsible for branch formation remained elusive. Interestingly, the ΔAmyA mutant failed to accumulate glycogen. We hypothesized that amyA is responsible for branch formation in glycogen. In this study, AmyA was characterized to have dual activities as an α-amylase and a glycogen-branching enzyme. Glycogen extracted from S. acidocaldarius exhibited α-1,6 linked glucose branches, with most branches containing 5-13 glucose units. AmyA showed a preference for synthesizing branches with a degree of polymerization of 6. Structural modeling of AmyA, in comparison with GH57 glycogen-branching enzymes (GBE), revealed the presence of key amino acids essential for branching activity, located in positions structurally analogous to those in GH57 GBEs, enabling AmyA to function as a glycogen-branching enzyme. Alignment of the glg operons showed that amyA is conserved, while glgB is absent in most Crenarchaeota. Based on these findings, we propose that AmyA synthesizes α-1,6 branches in glycogen, substituting for the role of GlgB in Crenarchaeota.

Impact of nutrient excess on physiology and metabolism of Sulfolobus acidocaldarius

Overflow metabolism is a well-known phenomenon that describes the seemingly wasteful and incomplete substrate oxidation by aerobic cells, such as yeasts, bacteria, and mammalian cells, even when conditions allow for total combustion via respiration. This cellular response, triggered by an excess of C-source, has not yet been investigated in archaea. In this study, we conducted chemostat cultivations to compare the metabolic and physiological states of the thermoacidophilic archaeon Sulfolobus acidocaldarius under three conditions, each with gradually increasing nutrient stress. Our results show that S. acidocaldarius has different capacities for the uptake of the two C-sources, monosodium glutamate and glucose. A saturated tricarboxylic acid cycle at elevated nutrient concentrations affects the cell's ability to deplete its intermediates. This includes deploying additional cataplerotic pathways and the secretion of amino acids, notably valine, glycine, and alanine, while glucose is increasingly metabolized via glycogenesis. We did not observe the secretion of common fermentation products, like organic acids. Transcriptomic analysis indicated an upregulation of genes involved in fatty acid metabolism, suggesting the intracellular conservation of energy. Adapting respiratory enzymes under nutrient stress indicated high metabolic flexibility and robust regulatory mechanisms in this archaeon. This study enhances our fundamental understanding of the metabolism of S. acidocaldarius.

Architecture, Function, Regulation, and Evolution of α-Glucans Metabolic Enzymes in Prokaryotes

Bacteria have acquired sophisticated mechanisms for assembling and disassembling polysaccharides of different chemistry. α-d-Glucose homopolysaccharides, so-called α-glucans, are the most widespread polymers in nature being key components of microorganisms. Glycogen functions as an intracellular energy storage while some bacteria also produce extracellular assorted α-glucans. The classical bacterial glycogen metabolic pathway comprises the action of ADP-glucose pyrophosphorylase and glycogen synthase, whereas extracellular α-glucans are mostly related to peripheral enzymes dependent on sucrose. An alternative pathway of glycogen biosynthesis, operating via a maltose 1-phosphate polymerizing enzyme, displays an essential wiring with the trehalose metabolism to interconvert disaccharides into polysaccharides. Furthermore, some bacteria show a connection of intracellular glycogen metabolism with the genesis of extracellular capsular α-glucans, revealing a relationship between the storage and structural function of these compounds. Altogether, the current picture shows that bacteria have evolved an intricate α-glucan metabolism that ultimately relies on the evolution of a specific enzymatic machinery. The structural landscape of these enzymes exposes a limited number of core catalytic folds handling many different chemical reactions. In this Review, we present a rationale to explain how the chemical diversity of α-glucans emerged from these systems, highlighting the underlying structural evolution of the enzymes driving α-glucan bacterial metabolism.