GPP synthases

The IBP and its products are displayed in Figure 12.1. HMG-CoA, ultimately derived from acetyl-CoA is converted to mevalonate via the enzyme HMG-CoA reductase (HMGR) [8]. This reaction is the rate-limiting step in the pathway. Mevalonate is then phosphorylated via mevalonate kinase (MK) to yield 5-phosphomevalonate [9]. IPP is formed following additional phosphorylation and decarboxylation steps [10]. Isomerization of IPP via the enzyme IPP isomerase yields DMAPP [11]. In mammals, the enzyme farnesyl pyrophosphate synthase (FDPS) catalyzes the synthesis of both GPP and FPP [12]. In plants, a separate GPP synthase has been identified [13]. GPP is a key intermediate in plants as it serves as the precursor for all monoterpenes. In animals, however, GPP appears to serve only as an intermediate in the synthesis of FPP. Very low basal levels of GPP have been measured in cell culture, although cellular GPP levels can become markedly increased in the setting of FDPS inhibition [14]. 

The prenyltransferases that catalyse the s)mtheses of GPP, FPP and GGPP may be important regulatory enz)mies in plant terpenoid bios)mthesis since they are situated at the primary branch points of the pathway, directing flux among the various major classes of terpenoids. The level of prenyltransferase activity is, in fact, closely correlated with the rate of terpenoid formation in many experimental systems (Dudley et at, 1986 Hanley et at, 1992 Hugueney et at, 1996) consistent with the regulatory importance of these catalysts. The localization of specific prenyltransferases in particular types of tissue or subcellular compartments may control the flux and direction of terpenoid synthesis at these sites. For example, the GPP synthase in Salvia officinalis is restricted to the secretory cells of the glandular trichomes, which are the sole site of monoterpene bios)mthesis in this species (Croteau and Purkett, 1989). 

Mevalonate kinase Phosphomevalonate kinase Mevalonate PP decarboxylase GPP synthase IPP isomerase FPP synthase Squalene synthase... 

GPP synthase AnUrrhinum majus large su GPP synthase Mentha x piperita large su... 

GPP synthase2 Abies grandis GPP synthaseS Abies grandis PalDSI Picea abies GPP synthaset Abies grandis... 

GPP synthase Citrus sinensis GPP synthase Quercus robur GPP synthase Arabidopsis thaliana PalDSS Picea abies PPP synthase Arabidopsis thaliana FPP synthase Musa acuminata FPP synthase Artemisia annua FPP synthase Gingko biloba... 

Fig. 1.6 Phylogenetic tree of gymnosperm and angiosperm isoprenyl diphosphate synthase sequences. The isolated Picea ahies sequences are marked other isoprcnyl diphosphate synthases sequences are listed according to the major reaction product of the recombinant protein. Abbreviations GPP synthase, geranyl diphosphate synthase FPP synthase, famesyl diphosphate synthase and GGPP synthase, geranyl geranyl diphosphate synthase large su, large subunit of protein small su, small subunit of protein.

The key prenyltransferase in the biosynthesis of monoterpenes is GPP synthase. To date, only a few GPP synthases have been fully characterized (Burke et al. 1999 Bouvier et al. 2000). Enzyme-catalysed isomerization of GPP via loss of pyrophosphate as a leaving group produces the allylic linalyl cation (16). Attack by an extraneous nucleophile can take place at either end of the allylic system and ultimately gives rise to linear monoterpenes such as geraniol (17), linalool (18) and linalyl acetate (19), all common components of a number of essential oils (Figure 3.9). 

Like the E. coli equivalent, the yeast EPPS is a bifunctional enzyme, and no dedicated GPP synthase exists similar problems are therefore encountered when attempting to produce monoterpenes in yeast. Mutation of the native EPPS to promote the release of GPP can result in production of low levels of monoterpenes... 

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