## 5 Fuels from Isoprenoid and Fatty Acid Pathways
### Biofuels from Isoprenoids  
Isoprenoids are one of the most appropriate compounds naturally available for biofuel production as they have good traits that a fuel needs to have, like high energy, low freezing points, high cetane and octane numbers, etc. They are a set of various long-chained molecules and are also sometimes referred to as terpenoids [105; 106]. Depending on the carbon atom number present, they are divided into sesquiterpenoids with 15 carbons (C15), monoterreponoids with 10 carbons (C10), and heimterpenoids with 5 carbons C5. Terpenoids are synthesized by a five-carbon molecule condensation; the C5 precursor involved is isopentenyl diphosphate (IPP) along with dimethyllyl diphosphate (DMAPP; isomer of IPP). IPP and DMAPP can be synthesized through both the MEP (2-methyl-d-erythnitro-4-phosphate) and MVA (mevalonate) pathways. Monoterpenes and sesquiterpenes are formed when two molecules of IPP are condensed with one molecule of DMAPP, whereas hemiterpenes are formed from DMAPP alone [105]. In general, the ratio of IPP and DMAPP is less than one, which can limit the synthesis of isoprenoids in the cells. This is because the primary reactant in the synthesis of isoprenoid is DMAPP alone [107]. For this reason, the target molecules for large-scale terpenoid microbial production are IPP and DMAPP [4]. A simple five-carbon hemiterpene that is used extensively in pesticide, rubber, and adhesive industries has been considered as an alternative fuel due to its enormous density of energy [1]. The global rubber production is about 0.012 billion tons per annum [108], and almost 95% of isoprene is used for its synthesis [109]. Though strains of _S. cerevisiae_, _E. coli_, and _Synechocystis_ were engineered to synthesize isoprene, the insignificant production amount is insufficient for commercializing it. In _Synechocystis_, the underexpression of the enzyme isoprene synthase (IspS) [110] along with a reduced cellular IPP and DMAPP ratio [107] are the key limiting parameters for isoprene synthesis. Engineering _Synechocystis_ by introducing the heterologous gene fri from _Streptococcus pneumoniae_ and the ISPC-cpcB gene and expressing them has displayed a 62-fold enhancement in isoprene synthesis [111]. Apart from hemiterepenes, there are few monoterpenes (limonene andpinene) and sesquiterpen (bisabolene and farnesene) that are being targeted for advanced biofuel production through the engineered microorganisms. However, monoterpenes have great industrial importance, as they are employed not only in the biofuel industry but also in the pharmaceutical, flavour, solvent and perfume industries [7; 65]. In _S. cerevisiae_ as well as _E. coli_, there exists a distinct monoterpene pathway, in which the substrate molecule geranyl pyrophosphate (GPP) is synthesized as the lone by-product for the sesquiterpene precursor molecule farnesyl pyrophosphate (FPP). So, there exists a competition for GPP between monoterpene synthases and sesquiterpene precursor FPP synthases (FPPS) [112]. Consequently, for enhancing the production of monoterpenes in _S. cerevisiae_ as well as _E. coli_, novel approaches like downregulating FPPS and upregulating GPP synthases (GPPS) can be applied [113]. Furthermore, for addressing these issues, efforts were made to fuse a combination of three GPPS with three different pinene synthases. This resulted in an 6-fold-enhanced pinene production [114]. Terpenoids, which have wide applications in food industries, biofuels for jet planes, and personal care, are sesquiterpenes. These sesquiterpenes are formed when one molecule of DMAPP along with two IPP become condensed in the presence of farnesyl-diphosphate-isomerase to finally synthesize FPP [115; 116]. Attempts were made to enhance the levels of cytosolic DMAPP along with upregulating bisabolene synthase or FPPS for the synthesis of sesquiterpenes [115; 117].