Fig. 1: Biochemical pathways involved in glycerol conversion in Bacteria and Eukarya forming dihydroxyacetone phosphate (DHAP) which is channelled into central metabolism.

Glycerol metabolism starts with its uptake either via facilitated diffusion mediated mainly by the glycerol uptake facilitator (GlpF) (encoded by the glpF gene) or via (protein-independent) simple diffusion through the cytoplasmic membrane. Following uptake, glycerol is converted finally to DHAP via two basic pathways: In respiring organisms, glycerol is first phosphorylated by the glycerol kinase (GlpK) (encoded by the glpK gene) to G3P which is further oxidized by two different membrane bound FAD-dependent G3P dehydrogenases (G3PDH), i.e. GlpD (encoded by the glpD gene, e.g. aerobic conditions E. coli, mitochondria) and GlpABC (encoded by the glpA, B, and C genes, e.g. anaerobic conditions E. coli, Haloarchaea). Electrons are transferred via the G3PDH bound FAD cofactor to ubiquinone (UQ) by GlpD, or to menaquinone (MQ) by GlpABC components in the respiratory chain. A third mechanism of G3P oxidation mainly known from aerotolerant lactic acid bacteria as well as Mycoplasma species is catalysed by a soluble, cytoplasmic FAD-dependent G3P oxidase (GlpO, encoded by the glpO gene) which directly utilizes molecular oxygen as electron acceptor. The second basic glycerol converting pathway is known from anaerobic, fermentatively growing organisms. Here, glycerol is first oxidized via an NAD⁺-dependent G3PDH to dihydroxyacetone (GldA, encoded by the gldA gene) which is subsequently phosphorylated by dihydroxyacetone kinase (DhaK, encoded by dhaK gene) with phosphoenolpyruvate (less frequently ATP) as phosphoryl donor. In all cases, DHAP is further degraded via the lower common shunt of the ED and EMP pathway or utilized for gluconeogenesis. G3P serves as building block for phospholipid/membrane synthesis in Bacteria and Eukarya.