Fig. 4

Biosynthesis pathway of expanding carbon chain for producing advanced biofuels using 2-keto acid intermediates. This biosynthesis pathway can be subdivided into three distinct sub-pathways (I, II, III), each of which results in a corresponding and structurally distinct advanced biofuel. a The I sub-pathway is that the carbon chain is extended via adding two carbon atoms (+ 2) with pyruvate as carbon extension unit; the key enzymes are acetohydroxy acid synthase (AHAS) and IlvIHCD. b The II sub-pathway is that the carbon chain is lengthened via adding one carbon atom (+ 1) with CoA-dependent molecule as carbon extension unit. Advanced biofuels with a branched chain are obtained via integrating the following natural biosynthetic pathways: α-keto acid-based pathway and CoA-dependent extension pathway. c Carbon chain extension of the Type III sub-pathway also occurs involving the addition of one carbon atom (+ 1) using CoA-dependent molecule as the carbon extension unit. Advanced biofuels without branched chains were obtained via utilizing this subtype pathway and conducting a condensation reaction with the overexpression of leuABCD. Green arrows represent using pyruvate as carbon extension unit. Blue arrows represent using CoA-dependent molecule as carbon extension unit. Red arrows represent exogenous decarboxylation and reduction. ilvA threonine deaminase; ilvC acetohydroxy acid isomeroreductase; ilvD dihydroxy acid dehydratase; ilvGM acetohydroxybutanoate synthase; kivd ketoisovalerate decarboxylase; leuA, 2-isopropylmalate synthase; leuB 3-isopropylmalate dehydrogenase; leuCD 2-isopropylmalate isomerase; thrA aspartate kinase, homoserine dehydrogenase; thrB homoserine kinase; thrC threonine synthase; Ac-CoA acetylCoA; ACP acyl carrier protein; CoA coenzyme A; OAA oxaloacetate; ADH2 alcohol dehydrogenase; alsS acetolactate synthase; asd aspartate semialdehyde dehydrogenase; aspC aspartate aminotransferase; cimA citramalate synthase. KIC:2-ketoisocaproate, precursor to l-leucine; KMV 2-keto-3-methylvalerate, precursor of l-isoleucine; PaaH1 β-ketothiolase (BKtB)-3-hydroxy-acyl-CoA dehydrogenase; Crt crotonase; Ter trans-enoyl-CoA reductase; adhE alcohol dehydrogenase. The red arrow shows the oxidation–reduction pathway to higher biofuel. The green arrow is the carbon chain extension pathway