Famesyl pyrophosphate synthetase

Li, C.P. and Larkins, B.A. (1996) Identification of a maize endosperm-specific cDNA encoding famesyl pyrophosphate synthetase. Gene, 171,193-6. 

Ogura et al. (Ogura et al., 1970 1972 Koyama et al.. 1972) showed that not only isopentenyl pyrophosphate, but also its ethyl homologue, 3-butenyl-3-ethyl pyrophosphate, can serve as substrate for famesyl pyrophosphate synthetase, which is responsible for the biosynthesis of the triterpene chain. This has been... 

A prenyl transferase, FPP-synthetase (EC 2.5.1.1), from extracts of germinating castor bean seedlings, has been highly purified both forms obtained synthesized all -fame-syl pyrophosphate (Porter and Spurgeon, 1981). FPP synthetases also have been demonstrated from Gossypium hirsutum and from Pisum sativum (Croteau and Johnson, 1985). E,E-Famesyl pyrophosphate synthetase is proposed to employ a stepwise ionization-condensation mechanism involving a rigid pyrophosphate-carbonium ion pair as an intermediate (Poulter et al., 1981). 

The stereochemistry is well established, and many questions concerning the overall mechanism of the condensation have now been resolved. Famesyl pyrophosphate synthetase (EC 2.5.1.1) is the key enzyme in the biosynthetic pathways for several classes of terpenes. This enzyme catalyzes l -4 condensation between IPP and DMAPP, or geranyl pyrophosphate, polymerizations that constitute the major building steps of terpenoid biosynthesis (Fig. 21.2) (Poulter and Rilling, 1978 Poulter et al., 1978, 1981). The condensation... 

The biosynthesis of sesquiterpenes in plants appears to be isolated from that of either monoterpenes or diterpenes. A prenyl transferase isolated from pumpkin Cucurbita pepo, Cucurbitaceae) converts C5 units into FPP, but not into gera-nylgeranyl pyrophosphate (the precursor to diterpenes). Another enzyme from the same source forms C20 terpenoids from C5 units, but does not accumulate lower homologs (also see Chapters 19 and 22). However, because famesyl pyrophosphate synthetase is a branch-point metabolite for the synthesis of sesquiterpenes, triterpenes, and sterols, this enzyme is ubiquitous in plants (Croteau and Johnson, 1985). 

Both 2- -6- -famesyl (2-trans-6-trans-famesy ) (3) and 2-Z-6- -famesyl (2-cis-6-trans-famesyl) (4) pyrophosphate have been suggested as intermediates for various types of cyclic systems (Fig. 21.3), but, at present, there are no unambiguous examples of conversion of 2-E- to 2-Z-famesyl pyrophosphate. The direct conversion of the 2-E compound to cyclic intermediates without intermediacy of the 2-Z compound has been suggested. This may possibly occur because of the conformational flexibility of the 10-carbon ring (Banthorpe and Charlwood, 1980 Loomis and Croteau, 1980). The status of a 2-Z-famesyl pyrophosphate synthetase is uncertain (Croteau and Johnson, 1985). 

Geranylgeranyl pyrophosphate synthetase activity has been found in carrot, pumpkin, and castor bean endosperm. In preparations from pumpkin and castor bean endosperm, it was possible to separate famesyl pyrophosphate synthetase and geranylgeranyl pyrophosphate synthetase activity. 

Takahashi I, Ogura K. Famesyl pyrophosphate synthetase from Bacillus subtilis, J Biochem Tokyo, 1981 89 1581-1587. 

The enzymes necessary for the conversion of famesyl pyrophosphate to squalene are called squalene synthetase . The enzymes necessary for the two reactions have not been resolved nor has either been purified to a significant extent, and it is not yet certain if the two reactions are catalyzed by two discrete entities. Squalene synthetase has an absolute requirement for a divalent cation, Mg " and Mn " being the best. A reduced pyridine nucleotide (NADH or NADPH) is required for the reduction of presqualene pyrophosphate to squalene. Yeast microsomes with Mn " and no reduced pyridine nucleotide will form dehydrosqualene instead [70]. The conversion of famesyl pyrophosphate to presqualene pyrophosphate is enhanced several-fold by the reduced pyridine nucleotide [71]. Also, some organic solvents as well as detergents increase this activity. 

The conversion of famesyl pyrophosphate to squalene marks the transition from water-soluble to water-insoluble intermediates in the biosynthesis of cholesterol. When the effect of liver cytosol or SCPj on the conversion of famesyl pyrophosphate to squalene was investigated, neither rat liver cytosol nor partially purified SCPj had any significant effect on this conversion [23]. Therefore, microsomal squalene synthetase performs its catalytic function without responding to the mediating effect of any specific soluble protein. 

The mevalonate pathway in the cytosol is responsible for biosynthesis of sterols, sesquiterpenes, and triterpenoids. After conversion of mevalonic acid to isopentenyl pyrophosphate, three C5 units can be joined head to tail to produce a C15 compound, famesyl pyrophosphate. Two famesyl pyrophosphates are then united head to head to form squalene, the progenitor of the C30 isoprenoids from which sterols are derived. The plant squalene synthetase, like its mammalian homologue, is found in the ER and the reaction proceeds via a presqualene pyrophosphate intermediate (Chapter 14). In the last step prior to cyclization, squalene is converted to squalene 2,3-epoxide. 

Steroids are members of a large class of lipid compounds called terpenes. Using acetate as a starting material, a variety of organisms produce terpenes by essentially the same biosynthetic scheme (Fig. 8). The self-condensation of two molecules of acetyl coenzyme A (CoA) forms acetoacetyl CoA. Condensation of acetoacetyl CoA with a third molecule of acetyl CoA, then followed by an NADPH-mediated reduction of the thioester moiety produces mevalonic acid [150-97-0] (72). Phosphorylation of (72) followed by concomitant decarboxylation and dehydration processes produce isopentenyl pyrophosphate. Isopentenyl pyrophosphate isomerase establishes an equilibrium between isopentenyl pyrophosphate and 3,3-dimethylallyl pyrophosphate (73). The head-to-tail addition of these isoprene units forms geranyl pyrophosphate. The addition of another isopentenyl pyrophosphate unit results in the sesquiterpene (C15) famesyl pyrophosphate (74). Both of these head-to-tail additions are catalyzed by prenyl transferase. Squalene synthetase catalyzes the head-to-head addition of two achiral molecules of famesyl pyrophosphate, through a chiral cyclopropane intermediate, to form the achiral triterpene, squalene (75). 

Geranylgeranyl pyrophosphate synthase (detected in pro-plastids of infected cells of Ricinus communis) had a molecular weight of 72,000 and required Mg (Beale and MacMillan, 1988). The enzymes with geranylgeranyl pyrophosphate synthetase activity also would accept pyrophosphates smaller than famesyl pyrophosphate, indicating that famesyl pyrophosphate probably does not lie at the branch between sterol and carotene metabolism in these organisms (Porter and Spurgeon, 1981) that is, famesyl pyrophosphate is synthesized by enzymes leading to the two series of compounds independently. 

The last step in the biosynthesis of phytoene is the head-to-head condensation of two molecules of GGPP (Fig. 8). In most respects, this reaction is analogous to the condensation of two molecules of famesyl pyrophosphate to form squalene in the sterol biosynthetic pathway. Studies on the latter reaction have provided a great deal of insight into the mechanism of phytoene biosynthesis and are included in the discussion below where relevant. Both reactions involve the formation of a cyclopropylcarbinyl pyrophosphate intermediate. In the biosynthesis of phytoene this compound is pre-phytoene pyrophosphate, the C40 analogue of presqualene pyrophosphate. A proposed mechanism for the formation of prephytoene pyrophosphate is shown in Fig. 9 (Beytia and Porter, 1976). Prephytoene pyrophosphate has been isolated from a Mycobacterium preparation and synthesized chemically. Both the natural and synthetic compounds have been converted to phytoene by a cell-fiee system from Mycobacterium (Altman et al., 1972). Prephytoene pyrophosphate was also formed when GGPP was incubated with yeast squalene synthetase (Qureshi et al., 1972, 1S>73), but in the pres-... 

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