## 3 Terpene Compounds
### Monoterpene Synthes (MTS)  
In addition to GPPS, MTSs and sesquiterpene synthases (SPSs) have been discovered in the hop lupulin gland as HIMITS1, HIMITS2, and HIMITS3, which generate volatile terpenids from GPP and FPP, respectively [46]. HIMITS1's full-length cDNA (2095 bp) encodes a 585-amino acid peptide sequence with an estimated molecular mass of 67,510 Da and a pI of 4.9. The full-length cDNA of HIMITS2 (1953 bp) includes an open reading frame of 1842 nucleotides that encodes a 613 amino acid predicted protein with a pI of 5.68. HIMITS1 and HIMITS2 are identical in 46.7% of their amino acid sequences. MTS1 and MTS2 feature a plastid-targeting peptide at the N terminus (the first 31 and 46 amino acids, respectively). HIMITS1 and HIMITS2 deduced proteins share 48.9% and 52% amino acid similarity with _Vitis vinifera_ (-)-\(\alpha\)-terpineol synthase [99], respectively. HIMITS2 was revealed to be capable of catalyzing the in vitro production of -myrcene. HIMITS3 is an ortholog of LIS that is exhibited to be significantly expressed in floral tissues with the most significant linalool accumulation. MTS1 and MTS2, which encode enzymes implicated in the production of monoterenoid odors, have also been shown to be more abundant in hop cones than in the leaves [17]. By expressing a codon-optimized linalool synthase gene from _Actinidia arguta_, the oleaginous yeast _Y. lipolytica_ was effectively engineered to generate 7 mg L\({}^{-1}\) of linalool [100]. A sesquiterpene synthase (STS) in hops was also found, which catalyze the in vitro synthesis of \(\alpha\)-humulene (70%) and \(\beta\)-caryophyllene (25%) [61].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Hop Compounds** & **Enzyme** & **Organism** & **Expression System** & **Reference** \\ \hline \multirow{6}{*}{Hop bitter acids} & BCAT & _Escherichia_ & \multirow{2}{*}{-} & [26] \\  & BCKDH & _Pseudomonas putida_ & - & [37] \\  & CCL2, CCL4 & _Populus tomettosa_ & _Escherichia coli_ & [50] \\  & VPS & _Humulus lupulus_ & _Escherichia coli_ & [57] \\  & PT & _Humulus lupulus_ & insect cells & [59] \\  & DXS & _Pica abies_ & - & [70] \\  & GPPS & _Menthx x piperita_ & - & [94] \\ compounds & MTS & _Humulus lupulus_ & - & - \\  & LIS, GES & _Mint and basil_ & yeast & [95] \\  & C4H & _Humulus lupulus_ & yeast & [96] \\ Xanthohumol & CHS\_H1 & _Humulus lupulus_ & yeast & - \\ derivatives. & CHIL & _Humulus lupulus_ & yeast & - \\  & OMT & _Mediago truncatula_ & _Escherichia coli_ & [97; 98] \\ \hline \hline \end{tabular}  
* Branched-chain aminotransferase; BCKDH: Branched-chain keto-acid dehydro-rogenzse; CCL: Caroxyl CoA ligase; VPS: Valerophenone synthase; PT: Prebutyltransferase; DXS: 1-deoxyxylulose-5-phosphate synthase; GPPS: Geranyl diphosphate synthase; MTS: Monoterpene synthase; LIS: Linaloal synthase; GES: Geranjol synthase; C4H: Cinamate 4-hydroxylase; CHS: Chalcone synthase; CHIL: Chalcone isomerase (CHI)-like proteins; OMT: O-methyltransferaseerase.  
\end{table}
Table 1: Research closely related to the key enzymes involved with the phytochemical compound’s biosynthesis in hops.  
As for LIS or other monoterpene synthases, few results were obtained in hop studies. According to previous BLAST results [46], the deduced proteins of HIMPS3 are ortholog of linalool synthase and were highly expressed in flower tissues where linalool accumulation was highest. A total of 44 transcripts were identified as putatively involved in the biosynthesis of volatile secondary metabolites [101], including five meridold/linalool synthases. In addition, there is no detailed report about the geraniol synthase (GES) in hops yet, thus more studies should focus on these MTS, including LIS or GES.  
In addition, it was demonstrated that brewer's yeast might be engineered to biosynthesize linalool and geraniol, which lend a hoppy taste to beer, by adding recombinant LIS and GES obtained from mint and basil [95] (Table 1). These findings further demonstrate the significance of MTS research.