## 3 Terpene Compounds
### Geranyl Diphosphate Synthase (GPPS)  
Geranyl diphosphate (GPP) is a 10-carbon monoterpene precursor that is usually generated from 5-carbon isoprenoid diphosphate units of the MEP pathway. Angiosperms have hetero- and homodimers of GPPS, which catalyzes the condensation of one IPP molecule and one DMAPP molecule into GPP [91]. As a homodimer, the small subunit of GPPS is inactive. As a homodimer, the big subunit from Mentha x piperita is inert, but the dimer of the large subunit in _Antirrhinum majus_ generates geranylgeranyl pyrophosphate (GGPP) from IPP and DMAPP in vitro [92; 93]. Furthermore, in _A. majus_, the GPPS small subunit is strongly related to spatial and temporal monoterpene emission, but the large subunit is not, hence demonstrating the small subunit's regulatory function in the heterodimeric complex [93]. A similar finding was attained in hops, where the small subunit of GPPS acts as a regulator of the activity of the functional heterodimer, favoring GPP synthesis over GGPP production [91]. A GPPS small component was found in hop glandular trichomes [61] in accordance with the associated gene's quantitative real-time PCR findings [91], which is only found in trichomes where monoterpene are produced. Furthermore, when this protein was fused to an enhanced green fluorescent protein (EGFP), it was shown to be localized near the chloroplast edge in tobacco leaves and not transported into chloroplasts [91]. It would be fascinating to look into the exact targeting of this trichome-specific GPPS small subunit, as GPP is important in the production of sesquiterpenoid.  
A crystal structure of heterotetrameric GPPS (PDB code 3 KRC, Figure 5) was reported for mint (_Mentha piperita_) with two large/small subunits (LSU/SSU) [94] (Table 1), the LSU and SSU were found to be in charge of catalysis and regulation, respectively. SSU, surprisingly, lacks the critical catalytic amino acid residues present in LSU and other PTSs, and no activity was identified when LSU or SSU were produced separately. At extended reaction periods, the complete (LSUSU)2 tetramer generated not only C10-GPP, but also C20-GGPP.  
Figure 5: Crystal structure of geranyl diphosphate synthase (PDB: 3 KRC). The different colors indicate the different subunits of the enzyme.  
It comprises the catalytically essential DD(X)nD motifs as the active site of the LSU of GPPS, as well as three noteworthy areas around the active site cavity (AC) of LSU, designated as AC loops 1, 2, and 3. Asp-83, Asp-84, Asp-89, Asp-91, Arg-94, and Arg-95 are essential residues that interact with both allylic and homoallylic substrates in AC loop 1 between helices D and F. AC loop 2 serves as a gate for allylic substrate entrance, while AC loop 3 participates in homoallylic substrate binding. Clearly visible conformational changes in the catalytic site produce a shift in the side chain orientation of Lys-44, Arg-95, and Lys-235 for interaction with substrates.  
Despite extensive study into the structure of the enzyme, given the various molecular configurations (heterodimeric and heterotetrameric) in other plants, the structure of these enzymes in hops has seldom been reported.