Farnesyl-diphosphate farnesyltransferase

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Squalene synthase
3q30.png
Human Squalene synthase in complex with inhibitor. PDB 3q30[1]
Identifiers
EC number 2.5.1.21
CAS number 9077-14-9
Databases
IntEnz IntEnz view
BRENDA BRENDA entry
ExPASy NiceZyme view
KEGG KEGG entry
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO
farnesyl-diphosphate farnesyltransferase 1
Identifiers
Symbol FDFT1
Entrez 2222
HUGO 3629
OMIM 184420
RefSeq NM_004462
UniProt P37268
Other data
EC number 2.5.1.21
Locus Chr. 8 p23.1-p22

Squalene synthase (SQS) or farnesyl-diphosphate:farnesyl-diphosphate farnesyl transferase is an enzyme localized to the membrane of the endoplasmic reticulum. SQS participates in the isoprenoid biosynthetic pathway, catalyzing a two-step reaction in which two identical molecules of farnesyl pyrophosphate (FPP) are converted into squalene, with the consumption of NADPH.[2] Catalysis by SQS is the first committed step in sterol synthesis, since the squalene produced is converted exclusively into various sterols, such as cholesterol, via a complex, multi-step pathway. SQS belongs to squalene/phytoene synthase family of proteins.

Diversity[edit]

Squalene synthase has been characterized in animals, plants, and yeast.[3] In terms of structure and mechanics, squalene synthase closely resembles phytoene synthase (PHS), another prenyltransferase. PHS serves a similar role to SQS in plants and bacteria, catalyzing the synthesis of phytoene, a precursor of carotenoid compounds.[4]

Structure[edit]

Squalene synthase (SQS) is localized exclusively to the membrane of the endoplasmic reticulum (ER).[5] SQS is anchored to the membrane by a short C-terminal membrane-spanning domain.[6] The N-terminal catalytic domain of the enzyme protrudes into the cytosol, where the soluble substrates are bound.[2] Mammalian forms of SQS are approximately 47kDa and consist of ~416 amino acids. The crystal structure of human SQS was determined in 2000, and revealed that the protein was composed entirely of α-helices. The enzyme is folded into a single domain, characterized by a large central channel. The active sites of both of the two half-reactions catalyzed by SQS are located within this channel. One end of the channel is open to the cytosol, whereas the other end forms a hydrophobic pocket.[5] SQS contains two conserved aspartate-rich sequences, which are believed to participate directly in the catalytic mechanism.[7] These aspartate-rich motifs are one of several conserved structural features in class I isoprenoid biosynthetic enzymes, although these enzymes do not share sequence homology.[5]

Squalene Synthase (Human). Key residues in the central channel are shown as spheres.

Mechanism[edit]

Squalene synthase (SQS) catalyzes the reductive dimerization of farnesyl pyrophosphate (FPP), in which two identical molecules of FPP are converted into one molecule of squalene, via a two-step mechanism. FPP is a soluble allylic compound containing 15 carbon atoms (C15), whereas squalene is an insoluble, C30 isoprenoid.[2][4] This reaction is a head-to-head terpene synthesis, because the two FPP molecules are both joined at the C1 position and form a 1'-1 linkage. 1'-4 linkages are much more common in isoprene biosynthesis than 1'-1 [8][9] The reaction mechanism of SQS requires a divalent cation, often Mg2+, to facilitate binding of the pyrophosphate groups on FPP.[10]

Reaction

FPP condensation[edit]

In the first half-reaction, two identical molecules of farnesyl pyrophosphate (FPP) are bound to squalene synthase (SQS) in a sequential manner. The FPP molecules bind to distinct regions of the enzyme, and with different binding affinities [11] The pyrophosphate group is cleaved from one molecule of FPP, designated as the donor FPP. The resulting allylic carbocation reacts with the C2,3 double bond of the acceptor FPP in a 1',2,3 prenyl transferase reaction. The product of this condensation is presqualene pyrophosphate (PSPP), a stable cyclopropylcarbinyl pyrophosphate intermediate.[9] The condensation of the two FPP molecules releases a pyrophosphate and a proton (H+). The PSPP created remains associated with SQS for the second reaction.[5][10]

Presqualene Pyrophosphate

PSPP rearrangement and reduction[edit]

In the second half-reaction of SQS, presqualene pyrophosphate (PSPP) moves to a second reaction site within SQS. Keeping PSPP in the central channel of SQS is thought to protect the reactive intermediate from reacting with water.[5] The cyclopropyl group is opened, and PSPP is rearranged and reduced using NADPH to produce a linear final product, squalene. SQS releases squalene into the membrane of the endoplasmic reticulum.[2] This reaction also produces pyrophosphate, H+, and NADP+.[10]

Interactive pathway map[edit]

Click on genes, proteins and metabolites below to link to respective articles. [§ 1]

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  1. ^ The interactive pathway map can be edited at WikiPathways: "Statin_Pathway_WP430". 

Biological Function[edit]

Squalene synthase (SQS) is an enzyme participating in the isoprenoid biosynthetic pathway. SQS synthase catalyzes the branching point between sterol and nonsterol biosynthesis, and commits farnesyl pyrophosphate (FPP) exclusively to production of sterols.[2] An important sterol produced by this pathway is cholesterol, which is used in cell membranes and for the synthesis of hormones.[12] SQS competes with several other enzymes for use of FPP, since it is a precursor for a variety of terpenoids. Decreases in SQS activity limit flux of FPP to the sterol pathway, and increase the production of nonsterol products. Important nonsterol products include ubiquinone, dolichols, heme A, and farnesylated proteins [13]

Development of squalene synthase knockout mice has demonstrated that loss of squalene synthase is lethal, and that the enzyme is essential for development of the central nervous system.[14]

Disease Relevance[edit]

Squalene synthase is a target for the regulation of cholesterol levels. Increased expression of SQS has been shown to elevate cholesterol levels in mice.[14] Therefore, inhibitors of SQS are of great interest in the treatment of hypercholesterolemia and prevention of coronary heart disease (CHD).[15] It has also been suggested that variants in this enzyme may be part of a genetic association with hypercholesterolemia.[16]

Squalene synthase inhibitors[edit]

Squalene synthase inhibitors have been shown to decrease cholesterol synthesis, as well as to decrease plasma triglyceride levels.[12][17] SQS inhibitors may provide an alternative to HMG-CoA reductase inhibitors (statins), which have problematic side effects for some patients.[18] Squalene synthase inhibitors that have been investigated for use in the prevention of cardiovascular disease include TAK-475, zaragozic acid, and RPR 107393.[19][20] Despite reaching phase 2 clinical trials, TAK-475 was discontinued and is no longer being investigated for clinical use.[21][22]

Squalene synthase homolog inhibition in Staphylococcus aureus is currently being investigated as a virulence factor-based antibacterial therapy.[23]

References[edit]

  1. ^ Ichikawa M, Yokomizo A, Itoh M, Sugita K, Usui H, Shimizu H, Suzuki M, Terayama K, Kanda A (March 2011). "Discovery of a new 2-aminobenzhydrol template for highly potent squalene synthase inhibitors". Bioorg. Med. Chem. 19 (6): 1930–49. doi:10.1016/j.bmc.2011.01.065. PMID 21353782. 
  2. ^ a b c d e Tansey TR, Shechter I (December 2000). "Structure and regulation of mammalian squalene synthase". Biochim. Biophys. Acta 1529 (1–3): 49–62. doi:10.1016/S1388-1981(00)00137-2. PMID 11111077. 
  3. ^ Nakashima T, Inoue T, Oka A, Nishino T, Osumi T, Hata S (March 1995). "Cloning, expression, and characterization of cDNAs encoding Arabidopsis thaliana squalene synthase". Proc. Natl. Acad. Sci. U.S.A. 92 (6): 2328–32. Bibcode:1995PNAS...92.2328N. doi:10.1073/pnas.92.6.2328. PMC 42476. PMID 7892265. 
  4. ^ a b Tansey TR, Shechter I (2001). "Squalene synthase: structure and regulation". Prog. Nucleic Acid Res. Mol. Biol. Progress in Nucleic Acid Research and Molecular Biology 65: 157–95. doi:10.1016/S0079-6603(00)65005-5. ISBN 9780125400657. PMID 11008488. 
  5. ^ a b c d e Pandit J, Danley DE, Schulte GK, Mazzalupo S, Pauly TA, Hayward CM, Hamanaka ES, Thompson JF, Harwood HJ (September 2000). "Crystal structure of human squalene synthase. A key enzyme in cholesterol biosynthesis". J. Biol. Chem. 275 (39): 30610–7. doi:10.1074/jbc.M004132200. PMID 10896663. 
  6. ^ Jennings SM, Tsay YH, Fisch TM, Robinson GW (July 1991). "Molecular cloning and characterization of the yeast gene for squalene synthetase". Proc. Natl. Acad. Sci. U.S.A. 88 (14): 6038–42. Bibcode:1991PNAS...88.6038J. doi:10.1073/pnas.88.14.6038. PMC 52017. PMID 2068081. 
  7. ^ Gu P, Ishii Y, Spencer TA, Shechter I (May 1998). "Function-structure studies and identification of three enzyme domains involved in the catalytic activity in rat hepatic squalene synthase". J. Biol. Chem. 273 (20): 12515–25. doi:10.1074/jbc.273.20.12515. PMID 9575210. 
  8. ^ Poulter CD (1990). "Biosynthesis of non-head-to-tail terpenes. Formation of 1'-1 and 1'-3 linkages". Accounts of Chemical Research 23 (3): 70–77. doi:10.1021/ar00171a003. 
  9. ^ a b Lin FY, Liu CI, Liu YL, Zhang Y, Wang K, Jeng WY, Ko TP, Cao R, Wang AH, Oldfield E (December 2010). "Mechanism of action and inhibition of dehydrosqualene synthase". Proc. Natl. Acad. Sci. U.S.A. 107 (50): 21337–42. Bibcode:2010PNAS..10721337L. doi:10.1073/pnas.1010907107. PMC 3003041. PMID 21098670. 
  10. ^ a b c Beytia E, Qureshi AA, Porter JW (March 1973). "Squalene synthetase. 3. Mechanism of the reaction". J. Biol. Chem. 248 (5): 1856–67. PMID 4348553. 
  11. ^ Mookhtiar KA, Kalinowski SS, Zhang D, Poulter CD (April 1994). "Yeast squalene synthase. A mechanism for addition of substrates and activation by NADPH". J. Biol. Chem. 269 (15): 11201–7. PMID 8157649. 
  12. ^ a b Kourounakis AP, Katselou MG, Matralis AN, Ladopoulou EM, Bavavea E (2011). "Squalene synthase inhibitors: An update on the search for new antihyperlipidemic and antiatherosclerotic agents". Curr. Med. Chem. 18 (29): 4418–39. doi:10.2174/092986711797287557. PMID 21864285. 
  13. ^ Paradise EM, Kirby J, Chan R, Keasling JD (June 2008). "Redirection of flux through the FPP branch-point in Saccharomyces cerevisiae by down-regulating squalene synthase". Biotechnol. Bioeng. 100 (2): 371–8. doi:10.1002/bit.21766. PMID 18175359. 
  14. ^ a b Okazaki H, Tazoe F, Okazaki S, Isoo N, Tsukamoto K, Sekiya M, Yahagi N, Iizuka Y, Ohashi K, Kitamine T, Tozawa R, Inaba T, Yagyu H, Okazaki M, Shimano H, Shibata N, Arai H, Nagai RZ, Kadowaki T, Osuga J, Ishibashi S (September 2006). "Increased cholesterol biosynthesis and hypercholesterolemia in mice overexpressing squalene synthase in the liver". J. Lipid Res. 47 (9): 1950–8. doi:10.1194/jlr.M600224-JLR200. PMID 16741291. 
  15. ^ Davidson MH (January 2007). "Squalene synthase inhibition: a novel target for the management of dyslipidemia". Curr Atheroscler Rep 9 (1): 78–80. doi:10.1007/BF02693932. PMID 17169251. 
  16. ^ Do R, Kiss RS, Gaudet D, Engert JC (January 2009). "Squalene synthase: a critical enzyme in the cholesterol biosynthesis pathway". Clin. Genet. 75 (1): 19–29. doi:10.1111/j.1399-0004.2008.01099.x. PMID 19054015. 
  17. ^ Hiyoshi H, Yanagimachi M, Ito M, Saeki T, Yoshida I, Okada T, Ikuta H, Shinmyo D, Tanaka K, Kurusu N, Tanaka H (November 2001). "Squalene synthase inhibitors reduce plasma triglyceride through a low-density lipoprotein receptor-independent mechanism". Eur. J. Pharmacol. 431 (3): 345–52. doi:10.1016/S0014-2999(01)01450-9. PMID 11730728. 
  18. ^ Seiki S, Frishman WH (2009). "Pharmacologic inhibition of squalene synthase and other downstream enzymes of the cholesterol synthesis pathway: a new therapeutic approach to treatment of hypercholesterolemia". Cardiol Rev 17 (2): 70–6. doi:10.1097/CRD.0b013e3181885905. PMID 19367148. 
  19. ^ Charlton-Menys V, Durrington PN (2007). "Squalene synthase inhibitors : clinical pharmacology and cholesterol-lowering potential". Drugs 67 (1): 11–6. doi:10.2165/00003495-200767010-00002. PMID 17209661. 
  20. ^ Amin D, Rutledge RZ, Needle SN, Galczenski HF, Neuenschwander K, Scotese AC, Maguire MP, Bush RC, Hele DJ, Bilder GE, Perrone MH (May 1997). "RPR 107393, a potent squalene synthase inhibitor and orally effective cholesterol-lowering agent: comparison with inhibitors of HMG-CoA reductase". J. Pharmacol. Exp. Ther. 281 (2): 746–52. PMID 9152381. 
  21. ^ Gibbs, Edwina (29 October 2007). "UPDATE 2-US FDA tells Takeda to stop some TAK-475 trials". Reuters. Retrieved 5 March 2013. 
  22. ^ "Discontinuation of Development of TAK-475, A Compound for Treatment of Hypercholesterolemia". Takeda Pharmaceutical Company Limited. 28 March 2008. Retrieved 5 March 2013. 
  23. ^ Liu CI, Liu GY, Song Y, Yin F, Hensler ME, Jeng WY, Nizet V, Wang AH, Oldfield E (March 2008). "A cholesterol biosynthesis inhibitor blocks Staphylococcus aureus virulence". Science 319 (5868): 1391–4. Bibcode:2008Sci...319.1391L. doi:10.1126/science.1153018. PMC 2747771. PMID 18276850. 

External links[edit]