Various metabolic pathways towards ethylene production. De novo synthesis of ethylene by biological systems can be realized by using either organic or inorganic substrates (for example, glucose, xylose and CO2) in a global metabolic network (top right). Detailed in panels are the metabolic routes applying various combinations of substrates: (A) CO2 only (autotrophic), (B) Glucose only (heterotrophic), (C) xylose only (heterotrophic), (D) glucose + xylose (heterotrophic), (E) glucose + CO2 (mixotrophic), and (F) xylose + CO2 (mixotrophic). Corresponding carbon efficiency or yield (carbon stored in ethylene/carbon uptake), CO2 release/uptake, and cofactor balances in each panel are normalized to the formation of 1 mole of ethylene, and presented in the table (bottom right). Note that positive cofactor balances represent net production, while negative ATP or NADPH balances require cofactor supply from elsewhere (for example photosynthetic light reactions). The stoichiometries are calculated with computational analysis through determination of elementary modes for a given reaction system [34, 35]. For computational analysis, all possible routes for conversion of organic/inorganic carbons to ethylene were considered. The reaction for ethylene production in panels (A-F) is defined as: α-ketoglutarate = ethylene + 3 CO2. Side reaction of ELE is not taken into account, because of the controversial and uncertain stoichiometry. CO2, carbon dioxide; DHAP, dihydroxyacetone phosphate; E4P, erythrose-4-phosphate; FBP, Fructose 1,6-bisphosphate; GAP, glyceraldehyde-3-phosphate; Hexose-P, hexose 6-phosphate; Pentose-P, pentose 5-phosphate; PGA, phosphoglycerate; RuBP, ribulose 1,5-bisphosphate; S7P, sedoheptulose-7-phosphate; SBP, sedoheptulose 1,7-bisphosphate.