Figure 3

Energy fitness and productivities in microbial cell factories. a: The trend of metabolic entropy changes (unit: ATP generation per glucose). In optimal metabolism, one mole glucose generates 38 ATP for biosynthesis. Under constraints of P/O ratios and maintenance loss, less ATP can be generated (increase of metabolic entropy). b: The transition from carbon limitation to energy limitation with the increase of product yield. In many cases, the energy limitation prevents strains from achieving the yield and titer above break-even point. c: Cascade of energy changes (Heat of combustion) during biofuel synthesis from glucose. Energy was calculated based on stoichiometry yields (green text, entire bar) and the practical yields (red text, grey bar). Reported yields: ethanol -- 96% of theoretical yield [29], isobutanol -- 85% of theoretical yield [37], fatty acid --50% of theoretical yield [35], and H₂ (dark fermentation) -- 50% of theoretical yield [38].