Iberin In the FARM motifs of

In the FARM motifs of GGPPSs, hydrophobic Iberin are replaced by aliphatic amino acids, which are more flexible residues and allow the entrance of longer substrates into the pocket (Wang and Ohnuma, 2000). It has been reported that the lepidopteran Manduca sexta CA possesses GGPP synthase (GGPPS) activity; significant amounts of GGPP were produced by larval and adult M. sexta CA homogenates when DMAPP was used as the allylic substrate (Sen et al., 2007b). AaFPPS was also able to synthesize traces of GGPP in the presence of IPP and GPP. The function of the divalent metal cation is to anchor the diphosphate moieties and to facilitate ionization of allylic substrate (Brems and Rilling, 1977). Sen et al. (2007b) reported metal-dependent changes of product chain length in CA crude homogenates of M. sexta; showing that FPP formation was stimulated by adding Mg2+, whereas GPP formation increased in the presence of Mn2+. Mosquito FPPS, also produced preferentially FPP in presence of Mg2+, and the affinity for GPP in the presence of Mg2+ was similar to those described in other insect species, such as Drosophila melanogaster (7 μM) (Sen et al., 2007a), M. sexta (0.8 μM) (Sen and Sperry, 2002), Myzus persicae (25.4 μM and 15.4 μM) (Zhang and Li, 2012), Aphis gossypii (12.6 μM) (Ma et al., 2010) and Phaedon cochleariae (1.1 μM) (Frick et al., 2013). AaFPPS only displayed a switch on the product released when Co2+ was used as divalent cofactor, as described in the horseradish leaf beetle P. cochleariae (Frick et al., 2013). Interestingly, the simultaneous presence of both cofactors, Mg2+ and Co2+, did not lead to increased production of a single product; instead it reduced the synthesis of FPP and GPP, which suggests that a competition of both ions for the metal binding motif might interfere with catalysis. Substrate concentration and metal ion might regulate product specificity through changes of the conformation of the catalytic pocket. Frick et al. (2013) reported that the quaternary structure of P. cochleariae FPPS is different when the protein is coordinated with Co2+ or Mg2+; resulting in different abilities to accommodate and process allylic substrates. This metal ion-dependent regulatory mechanism allows a single enzyme to selectively control the metabolites it produces, thus potentially altering the flow of carbon into separate metabolic pathways. Whether this phenomenon is of any functional significance in mosquitoes in vivo remains to be determined, however this type of ‘flexible’ enzyme may provide insects faster mechanisms for the generation of the chemical diversity critical to adjust to developmental or environmental changes (Snyder and Qi, 2013). In summary, mosquito A. aegypti possess a single FPPS, which is able to synthesize GPP and FPP in different proportion depending on the metal cofactor; since the mosquito does not possesses a specific GPPS in its genome, it is not surprising that AaFPPS has evolved to synthesize GPP and FPP under different cellular environmental conditions, since these two metabolites are important precursors of the mevalonate and juvenile hormone pathway in insects.
Acknowledgments This work was supported by NIH Grant No AI 45545 to F.G.N.
Farnesyl diphosphate synthase (FDPS) catalyzes the reaction between two isopentenyl diphosphate (IPP) molecules and one dimethylallyl diphosphate (DMAPP) molecule to produce farnesyl diphosphate (FPP). It does this through a sequential process where one IPP molecule and one DMAPP molecule react to form geranyl diphosphate (GPP), followed by a second reaction between GPP and an additional IPP molecule to create FPP. This 15 carbon diphosphate is a key biosynthetic intermediate in the formation of a variety of small molecules including cholesterol, heme, ubiquinone, and geranylgeranyl pyrophosphate as well as prenylated proteins . FDPS is a promising target for the development of therapeutics because it has been implicated in a number of different diseases including malaria , Chagas’ disease , osteoporosis, hypertension, and heart disease and many different types of cancers . To date, there are only a few FDPS inhibitors on the market, including alendronate (Fosamax), zoledronate (Zometa), and risedronate (Actonel) . These inhibitors target diseases of osteoclasts, mainly because they are all based on a nitrogen-containing bisphosphonate motif. This type of chemical moiety causes the inhibitors to localize in bone tissue which renders them effective in several types of osteopathologies but relatively ineffective against other cell types Such functionality also results in cellular accumulation in certain parasites involved in a number of different diseases, because these types of organisms contain a higher level of pyrophosphates .