The early evolution of the glycolytic pathway from autotrophic origins to glycogen and back.

Glycolysis stops where gluconeogenesis starts-at pyruvate, the central metabolite of biosynthesis. The early history of carbon metabolism is preserved in archaeal and bacterial enzymes for glucose synthesis and breakdown. Here, we summarize the distribution and phylogeny of enzymes involved in glycolysis, gluconeogenesis and glycogen metabolism from genomes of cultured prokaryotes. The presence of glycolytic pathways in H2-dependent chemolithoautotrophs, including methanogens, which cannot grow on exogenous glucose, correlates with their use of glycogen for intracellular carbon storage. Glycogen synthesis and gluconeogenesis are universal among prokaryotes, but glycolysis is not, indicating that the enzymatic conversions of glycolysis arose in the gluconeogenic direction encompassing three phases: (1) an autotrophic origin from H2 and CO2 to pyruvate and triosephosphate (trunk glycolysis) fulfilling basic amino acid and cofactor synthesis in the last universal common ancestor, (2) from triosephosphate to glucose supplying cell wall (murein and pseudomurein) and nucleic acid biosynthetic requirements in the first free-living autotrophs, also giving rise to intracellular carbon reserves (glycogen), followed by (3) diversification and transfer of enzymes for glycogen-mobilizing glycolytic routes. An autotrophic origin of trunk glycolysis followed by glycogen-dependent origin of glucose utilization account for conservation, distribution and diversity of enzymes observed in microbial sugar phosphate pathways.