Table 1 Compilation of the studies showing the metabolic engineering strategies implied to improve the yield of biofuels

Alcohol based products

Ethanol

Synechocystis sp. PCC 6803

Glucose

Double homologous recombination technology was used to integrate the alcohol dehydrogenase II (adh) and pyruvate decarboxylase (pdc) genes from Zymomonas mobilis into the Synechocystis PCC 6803 

5.2 mmol OD₇₃₀ /unit/L/day

[34]

Ethanol

Synechococcus sp. PCC 7942

Glucose

Transformation of Synechococcus sp. strain PCC 7942 using the bacterium Zymomonas mobilis by cloning the coding sequences of alcohol dehydrogenase II (adh) and pyruvate decarboxylase (pdc) into the shuttle vector pCB4.

6 mmol OD₇₃₀/unit/L/day

[35]

Ethanol

Synechocystis PCC 6803

Glucose

Engineering the pathway involving ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), fructose-1,6/sedoheptulose-1,7-bisphosphatase (FBP/SBPase), transketolase (TK) and aldolase (FBA)

Enhanced by 43.6% for EtOH-rbcSC, 45.2% for EtOH-70glpX, 38.4% for EtOH-tktA and 47.4% for EtOH-fbaA.

[36]

Butanol

Clostridium acetobutylicum

Glucose

The pta and buk genes were disrupted that encoded for phosphotransacetylase and butyrate kinase, while the gene adhE1^D485G encoding for aldehyde/alcohol dehydrogenase, was overexpressed

0.71 mol butanol/mol glucose

[37]

Butanol

Clostridium tyrobutyricum

Lactose

cat1 gene responsible for production of butyrate was replaced by adhE1 or adhE2

26.2 g/L

[38]

Butanol

Clostridium acetobutylicum

Glucose

AAD (aldehyde/alcohol dehydrogenase) variants using substrate specificity feature analysis, random mutagenesis, and structure-based butanol selectivity design were prepared

17.47 and 15.91 g butanol/g ethanol for AAD_F716L and AAD_N655H

[39]

Propanol

Escherichia coli

Glucose

Deletion of rpos gene and overexpression of cimA and ackA gene that encodes for citramalate synthase and acetate kinase A/propionate kinase II, by introducing a modified adhE gene

0.107 g/g and 0.144 g/L/h⁻¹

[40]

Propanol

Escherichia coli

Glycerol

Deletion of rpos gene and overexpression of cimA and ackA gene that encodes for citramalate synthase and acetate kinase A/propionate kinase II, by introducing a modified adhE gene

0.259 g/g and 0.083 g/L/h

[40]

Fatty acid based products

Triacylglycerol

Rhodococcus opacus

Glucose

Deletion of acyl-coenzyme A (CoA) synthetases and over-expression of three lipases with lipase-specific foldase

82.9 g/L

[41]

Lipid

Yarrowia lipolytica

Glucose, glycerol

Overexpression of 148 putative transcription factor

Increase was 90.9% using glucose while 74% using glycerol

[42]

Free fatty acid

Yarrowia lipolytica

Glucose

Approaches were used to increase the NAD(P)H utilization while removing the efficiency to use formic acid using synthetic biology-based approaches

98.9 g/L

[43]

Free fatty acid

Rhodococcus opacus

Glucose

Deletion of acyl-CoA dehydrogenases and overexpression of lipases, foldase, acyl-CoA synthetase and wax ester synthase.

21.3 g/L

[41]

Alkane

Rhodococcus opacus

Glucose

Deletion of acyl-CoA dehydrogenases and alkane-1 monooxygenase and overexpression of lipases, foldase, acyl-CoA synthetase and heterologous acyl-CoA reductase, acyl-ACP reductase and aldehyde deformylating oxygenase

5.2 g/L

[41]

Free fatty acid

Saccharomyces cerevisiae

Glucose

Designing and alteration in fine-tuned NADPH, subcellular metabolic trafficking, and ATP supply, while declining the carbon flux to biomass. Moreover, replacement of lipogenesis metabolism with fermentation process.

33.4 g/L

[44]

Alkane

Cupriavidus necator

Glucose

Hetreologous ferredoxin (Fd)–Fd reductase was overexpressed

1.48 g/L

[45]

Isoprenoid based product

Isoprene

Escherichia coli

Glucose

Redox cofactor balancing

0.665 g/L

[46]

α-Santalene

Yarrowia lipolytica

Glucose

Overexpression of ERG8, ERG10, ERG12, ERG13, ERG19, ERG20, HMG1, and tHMG1

13.31 mg/L

[47]

Geraniol

Escherichia coli

Glycerol

Alternative pathway i.e. isoprenoid alcohol (IPA) pathway was used that focuses on the synthesis and subsequent IPAs phosphorylation.

0.6 g/L

[48]

Gaseous products

Hydrogen

Thermoanaerobacterium aotearoense

Rice straw

Deletion of Fd:NADP⁺ oxidoreductase (nfnAB)

0.381–0.419 g/L

[49]

Hydrogen

Escherichia coli

Xylose and Glucose

Deletion in gene ptsG (phosphotransferase system) for as well as ldhA and frdD

0.284 g/L

[50]

Hydrogen

Chlamydomonas reinhardtii

CO₂

Repression of the psbA (PSII D1 protein gene)

60% enhancement in hydrogen production

[51]