# Biosynthesis Progress of High-Energy-Density Liquid Fuels Derived from Terpenes
## 3 Microbial Synthesis of Monoterpene and Sesquiterpene Fuels  
Monoterpenes and sesquiterpenes naturally exist in plants, but their contents are extremely low. Therefore, it is difficult to achieve large-scale production for fuel applications through direct extraction and isolation methods. In addition, slow plant growth and geographical limitations further restrict the extraction of terpen from plant sources [43]. The microbial synthesis of terpen provides an efficient and scalable alternative platform for the production of monoterpene and sesquiterpene fuels. By constructing biosynthetic pathways of the target production in microbial cells, specific synthesis and renewable production of the desired products can be achieved effectively. The biosynthesis of monoterpenes and sesquiterpenes is based on isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). In microorganisms, IPP and DMAPP are usually synthesized through two pathways, the mevalonate (MVA) pathway or the 2-C-methyl-D-erythitol 4-phosphate (MEP) pathway (Figure 1). The MVA pathway exists in fungi, bacteria, the plant cytoplasm and animal cells. It starts with acetyl-CoA and undergoes catalytic reactions to form acetoacetyl-CoA, HMG-CoA and mevalonic acid, ultimately generating the precursors IPP and DMAPP [44]. The MEP pathway mainly exists in bacteria, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Fuel** & **Structure** & **NHOC/(MJ/L)** & **Density/(g/mL)** & **Viscosity/mm\({}^{2}\)/s** & **Freezing Point/\({}^{\circ}\)C** & **Ref.** \\ \hline p-Menthane & \(\circ\)\(\circ\)\(\circ\)\(\circ\) & 34.72 & 0.804 (15 \({}^{\circ}\)C) & 5.19 (\(-\)40 \({}^{\circ}\)C) & \(<\)\(-\)70 & [22; 42] \\ \hline Hydrogenated \(\beta\)-pinene dimer & \(\circ\)\(\circ\)\(\circ\)\(\circ\) & 39.50 & 0.938 (20 \({}^{\circ}\)C) & 35.05 (40 \({}^{\circ}\)C) & \(-\)30 & [16] \\ \hline \(\alpha\)-Pinene dimer & - & 39.31 & 0.935 (20 \({}^{\circ}\)C) & 34.68 (40 \({}^{\circ}\)C) & \(-\)52 & [16] \\ \hline Camphene dimer & - & 39.58 & 0.941 (20 \({}^{\circ}\)C) & 34.96 (40 \({}^{\circ}\)C) & \(-\)54 & [16] \\ \hline Limonene dimer & - & 38.36 & 0.914 (20 \({}^{\circ}\)C) & 25.86 (40 \({}^{\circ}\)C) & \(-\)78 & [16] \\ \hline Farnesane & \(\circ\)\(\circ\)\(\circ\)\(\circ\)\(\circ\) & 33.93 & 0.77 (20 \({}^{\circ}\)C) & 2.35 (40 \({}^{\circ}\)C) & \(-\)52 & [32] \\ \hline Pentalenene & \(\circ\)\(\circ\)\(\circ\)\(\circ\) & 40.73 & 0.91 (20 \({}^{\circ}\)C) & — & \(-\)63.5 & [32] \\ \hline Pentalenane & \(\circ\)\(\circ\)\(\circ\)\(\circ\) & 40.55 & 0.89 (20 \({}^{\circ}\)C) & — & \(<\)\(-\)67 & [32] \\ \hline Pentalenene cyclopropanation & \(\circ\)\(\circ\)\(\circ\)\(\circ\) & 38.43 & 0.90 (20 \({}^{\circ}\)C) & — & \(<\)\(-\)53 & [32] \\ \hline Presilphiperfol-1-ane & \(\circ\)\(\circ\)\(\circ\)\(\circ\) & 39.24 & 0.92 (20 \({}^{\circ}\)C) & — & \(-\)47 & [32] \\ \hline Hydrogenated \(\circ\)edarwood oil & - & 39.25 & 0.92 (20 \({}^{\circ}\)C) & 6.2 (40 \({}^{\circ}\)C) & \(<\)\(-\)80 & [37] \\ \hline Valencane & \(\circ\)\(\circ\)\(\circ\)\(\circ\) & 37.73 & 0.88 (20 \({}^{\circ}\)C) & 50.2 (\(-\)20 \({}^{\circ}\)C) & — & [34] \\ \hline Premnaspirodiane & \(\circ\)\(\circ\)\(\circ\)\(\circ\) & 37.78 & 0.88 (20 \({}^{\circ}\)C) & 42.9 (\(-\)20 \({}^{\circ}\)C) & — & [34] \\ \hline Caryophyllane & \(\circ\)\(\circ\)\(\circ\)\(\circ\)\(\circ\) & 37.01 & 0.85 (20 \({}^{\circ}\)C) & 60.5 (\(-\)20 \({}^{\circ}\)C) & — & [34] \\ \hline Cedrane & \(\circ\)\(\circ\)\(\circ\)\(\circ\) & 39.47 & 0.923 (20 \({}^{\circ}\)C) & 33 (\(-\)20 \({}^{\circ}\)C) & — & [37] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._algae and plant chloroplasts. It starts with pyruvate acid and glyceraldehyde-3-phosphate and undergoes catalytic reactions to form 1-deoxy-D-xylulose-5-phosphate (DXP) and MEP and eventually produces IPP, which is isomerized into DMAPP by isopentenyl diphosphate isomerase [45]. DMAPP and IPP condense to form the precursor geranyl pyrophosphate (GPP) for monoterpene synthesis catalyzed by monoterpene synthase (MTPS).