# Molecular Regulatory Mechanisms Affecting Fruit Aroma
## 3 Main Synthetic Pathways  
### Terpenoid Synthesis Pathway  
The terpenoid secondary metabolism generally has the characteristics of substrate and enzyme diversity. One enzyme can catalyze the same or multiple substrates, and different synthases can also act on the same substrate to produce multiple products [23]. The biosynthesis of terpenoids is mainly divided into the following three stages: (i) the generation stage of the C5 precursor isopentyl diphosphate (IPP) and its double-bond isomer dimethylallyl diphosphate (DMAPP); (ii) the generation stage of direct precursors such as geranyl diphosphate (GPP), famesyl diphosphate (FPP), and geranyl geranyl diphosphate (GGPP); (iii) the terpene generation and modification stage. The current research is relatively clear on the first two stages. terpenoids are produced in plants by two independent pathways, namely, the mexolonate (MVA) pathway and methylerythth phosphate (MEP) pathway, both of which involve the C5 precursor IPP and its allyl isomer DMAPP [9]. The mexolonate pathway and the methyleryththitol pathway are involved in this pathway but are not completely independent (Figure 2a). The main method of terpenoid synthesis is to synthesize IPP and the C5 precursor. Then, as the general framework for terpenoid syntheses, a series of precursor substances is used for catalysis. Finally, the modification stage produces the corresponding derivative. In the MVA pathway, the acetyl CoA is first formed as acetoacetyl-CoA under the catalysis of acetyl CoA acetyl transferase (AACT) catalysis. Then, under the catalysis of hydroxymethylglutaryl CoA synthase (HMGCS), the acetoacetyl-CoA is converted to the 3-hydroxy3-methylglutaryl coenzyme A (HMG-CoA). Subsequently, HMG-CoA is catalyzed by two molecules of NADPH and hydroxymethylglutaryl-CoA reductase (HMGCR) to generate MVA, which is then converted to mexolonate-5-phosphate (MVAP) under the catalysis of mevalonate kinase (MVK). Then, under the catalysis of phosphomevalonate kinase (PMK), MVAP is converted to mevalonate-5-diphosphate (MVAPP), and, finally, MVAP is generated under the catalysis of mevalonate diphosphomevalonate decarboxylase (MVD). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Species** & **Main Aroma Component** & \begin{tabular}{c} **Variety of Aroma** \\ **Substance** \\ \end{tabular} & **Reference** \\ \hline \begin{tabular}{c} Mango \\ Pear \\ Citrus \\ \end{tabular} & \begin{tabular}{c} terpene olefins, 3-pinene, caryophyllene, \(\alpha\)-pinene \\ hexyl acetate, alpha-farresene \\ limonene, citral, naringene \\ methyl butyrate, ethyl butyrate, butyl acetate, methyl \\ caproate, ethyl hexanoate \\ \end{tabular} & \
\begin{tabular}{c} 60 \\ 202 \\ 162 \\ 979 \\ \end{tabular} &
\begin{tabular}{c} [PERSON] et al. [22] \\ [PERSON] et al. [6] \\ [PERSON] et al. [8] \\ [PERSON] et al. [7] \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 1: Main chemical structures of terpenoids (**a**) and benzene compounds (**b**). The red color represents the chemical structure skeleton. In the MEP pathway, pyruvate and glyceraldehyde-3-phosphate first generate 1-deoxy-D-xylose 5-phosphate (DXP) under the action of 1-deoxy-D-xylose 5-phosphate synthase (DXS), and then DXP forms 2C-Methyl-D-erythitol 4-phosphate (MEP) under the catalysis of 1-deoxy-D-Dxylose 5-phosphate reductioisomerase (DXR). Through the 2C methyl-D erythroid 4-phosphate cytidine transferase (MCT) catalysis, MEP is converted to 2C-methyl-D-erythrin-4-cytidine diphosphate, followed by 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase (CMK) catalysis to produce 2-C-methyl-D-erythrin-4-cytidine diphosphate. Then, under the action of 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase (MDS), 2C-methyl-D-erythritol 2,4-cyclic diphosphate is generated, which is catalyzed by 4-hydroxy-3-methylbutene-2-enyl-phosphate synthase (HDS) to form 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate (HMBPP).