Fig. 1

Overview of the key metabolic steps and pathways involved in BC production and energy generation in the presence of glucose and ethanol. The periplasmic oxidation of the sugars, alcohols, and sugar alcohols is linked to the electron transport chain (ETC)—the dehydrogenases transfer the reducing equivalents to ubiquinone (UQ—dark green) converting it to its reduced form, ubiquinol (UQ—light green). UQ can be further reoxidized by terminal oxidases in a process coupled with oxygen (O₂) reduction to water (H₂0) and the generation of a proton (H⁺) gradient. Then, F₁F₀ ATPase facilitates H⁺ translocation, owing to the H⁺ gradient, in a process coupled with ATP generation. In the periplasm, glucose can be partially oxidized to D-gluconate, 2-keto-D-gluconate, and 5-D-ketogluconate by PQQ-dependent glucose dehydrogenase (GDH), flavin-dependent gluconate-2-dehydrogenase (G2D), and PQQ-dependent gluconate-5-dehydrogenase (G5D). Glucose can also be imported and used for cellulose biosynthesis or metabolized through the Entner–Doudoroff (ED), Embden–Meyerhof (EMP), or Pentose Phosphate Pathway (PPP). Ethanol can be oxidized to acetaldehyde and further to acetic acid in the periplasm by PQQ-dependent alcohol dehydrogenase (ADH) and further to acetaldehyde by flavin-dependent aldehyde dehydrogenase (ALDH). When ethanol, acetaldehyde, or acetate are assimilated by the cell, the metabolites are converted to acetyl-CoA for biomass generation, or complete oxidation through the tricarboxylic acid cycle (TCA). This is also coupled with ATP generation at different points. Additionally, NADH can also be oxidized to NAD⁺ in the cytosolic membrane by NADH dehydrogenase (NADH), which is also linked to the ETC.