## 5 Fuels from Isoprenoid and Fatty Acid Pathways
### Yeast Biofuels from Fatty Acids  
In bacteria, FAS is encoded by different genes (approximately ten), whereas in yeasts, it is determined by only two genes which make the engineering process easy [96]. Though _S. cerevisiae_ does not produce more FAs naturally, it has been metabolically altered to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Molecule** & **Host Organism** & **Genetic Alteration** & **Ref.** \\ \hline \multirow{8}{*}{FFA} & \multirow{3}{*}{_E. coli_} & Expressing FASI from _Corynebacterium glutamicum_[88] \\ \cline{3-4}  & & Development of FFA biosensor mediated PopQC method to & [89] \\  & & select the high-performing cells & \\ \cline{2-4}  & & Overexpression acyltransferases and lipases, \(\Delta\)PXA1, \(\Delta\)POX1, & [90] \\  & & AFA1, \(\Delta\)FA44, and \(\Delta\)FAT & \\ \cline{2-4}  & _S. cerevisiae_ & Expressing _Rhodsosporidium toruloides_ FAS-ACP domains, & \\ \cline{2-4}  & & AFA1, \(\Delta\)FA4, \(\Delta\)POX1, \(\Delta\)HFD1; upregulating ACL, Ctp1 & [91] \\ \cline{2-4}  & & Mutations in the active sites of keto synthase and malonyl- & [92] \\  & & palmitoyltransferase subunits of FASI & \\ \hline \multirow{3}{*}{FAEE} & \multirow{3}{*}{_S. cerevisiae_} & Chromosomal integration of WS2, ADH2, ADH6, ACC1, and & [93] \\  & & mutated acetyl CoA synthetase & \\ \cline{2-4}  & & Insertion of _Brevibacterium ammonigenes_ FASI and bacterial WS2 & [94] \\ \hline \multirow{3}{*}{Alkane} & \multirow{3}{*}{_S. cerevisiae_} & FAR and FADO from _Synechococcus elongates_; \(\Delta\)HFD1 & [95] \\ \cline{2-4}  & & Expressing MmCAR and _Aspergillus nidulans_ & \\ \cline{1-1} \cline{3-4}  & & 4\({}^{\prime}\)-phosphopantetheinyl transferase (NpgA) & [96] \\ \hline \multirow{8}{*}{Fatty alcohol} & \multirow{3}{*}{_E. coli_} & _C. glutamicum_ FAS1A into _E. coli_ and _Marinobacter aquaeolei_ & [88] \\  & & ACP/CoA-reductase & \\ \cline{1-1} \cline{3-4}  & & _Oryza sativa_\(\Delta\)DOX; overexpression \({}^{\prime}\)tesA, AHR, and fadR & [97] \\ \cline{1-1} \cline{3-4}  & & \(\Delta\)’tesA, heterologous FAR & [98] \\ \cline{1-1} \cline{3-4}  & & AFA1, \(\Delta\)FA4, \(\Delta\)HFD1; _Mycobacterium marinum_ MmCAR and & \\ \cline{1-1} \cline{3-4}  & & _M. aquaeolei_ FAR & \\ \cline{1-1} \cline{3-4}  & & _M. aquaeolei_ FAR (FaCoAR); upregulated Adh5 and MmCAR & [99] \\ \cline{1-1} \cline{3-4}  & & Upregulation of MmFAR, mutated ACC1, Ole1; \(\Delta\)ADH6, & [11] \\ \cline{1-1} \cline{3-4}  & & \(\Delta\)DGA1, and \(\Delta\)HFD1 & \\ \hline Methyl ketones & _E. coli_ & Upregulation of ‘tesA, fadD, fadM, fadB, and mult\_11700 genes & [88] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Metabolic engineering for biofuels from fatty acids (in host organisms _E. coli_ and _S. cerevisiae_) (source: [65]). enhance the FAs production because of the ease of the engineering process. It was observed in _S. cerevisiae_ that upregulating genes like DGTA, FAS2, ACC1, and FAS2 has enhanced the production of tri-acyl glycerol (TAG), which led to the accretion of lipid content by 17%. Further, the same strain was altered by knocking out genes like ROX, FAA1, and FAA4, and upregulating heterologous 'tesA (_E. coli_) to produce higher amounts of FFA (400 mg/L) [96]. [PERSON] et al. reported that knocking out genes FAA4, PXA4, FAA1, FAT1, and POX1, as well as upregulating lipases (that degrade TAG) and acyltransferases, produced enhanced levels of FFA (2.2 g/L) [90]. In a study, a fed-batch process was designed which employed an engineered _S. cerevisiae_. The engineering involved the overexpression of heterologous genes like ACP:citrate lyase (ACL), Malic enzymes (ME) from _Rhodosporidium toruloides_, and 'tesA from _E. coli_ along with knocking out HFD1, FAA4, POX1, and FAA1 genes. These alterations resulted in the enhanced production of FFA (10.4 g/L) [91]. A tabulation of a few modifications for fatty acid oleochemical production in _S. cerevisiae_ and _E. coli_ are presented in Table 1.  
Apart from _S.