Schematic diagram of natural and engineered pathways to biofuels and biofuel precursors in microbial platforms. Note, not all pathways are simultaneously present in any single microbe. Metabolic pathways are coloured thus — Core metabolism: light blue. Short chain alcohol pathways: purple. The Ehrlich degradative pathway to fusel alcohols: orange. Fatty acid metabolism: yellow. Isoprenoid MVA and MEP pathways: green. Terpenoid synthetic pathways: blue. The various biofuels are highlighted in red. Black arrows represent enzymatic transformations, with the enzymes or enzyme classes indicated. Blue arrows represent multiple enzymatic transformations in the metabolic pathways indicated. The red arrow represents abiotic, chemical conversion of TAGs to FAMEs and FAAEs. Abbreviations for enzymes are: PDC, Pyruvate dehydrogenase complex; ALS, Acetolactate (acetohydroxyacid) synthase; ADH, Alcohol dehydrogenase; KDC, α-keto acid decarboxylase; PDH, Pyruvate dehydrogenase; FabH, 3-oxoacyl-[acyl-carrier-protein] synthase; FabD, Malonyl CoA-acyl carrier protein transacylase; Acc A/B/C/D, Acetyl-CoA carboxylase; ACat, Acetyl transferase; AAS, Acyl-ACP synthase; TES, Thioesterase; AAR, Acyl-ACP reductase; FadD, acyl-CoA synthase; OleT_JE, CYP152L1(cytochrome P450 fatty acid peroxygenase); FAP, fatty acid photodecarboxylase; CAR, carboxylic acid reductase; CER, Wax ester synthase; DGAT, Diglyceride acyltransferase; FAR, Fatty acid reductase; ACR, acyl-CoA reductase; AHR, Aldehyde reductase; ADO, aldehyde deformylating oxygenase; HMGS, 3-hydroxy-3-methylglutaryl-CoA synthase; HMGR, HMG-CoA reductase; MK, mevalonate kinase; PMK, phosphomevalonate kinase; PMD, phosphomevalonate decarboxylase; DXR/ispC, DXP reductoisomerase; IPPS, IPP isomerase; IDI, Isopentenyl-diphosphate delta isomerase; GPPS, geranyl diphosphate synthase; PS, pinene synthase; LS, limonene synthase; agBIS, bisabolene synthase; FS, farnesene synthase.