Lanosterol synthase

lanosterol synthase
Identifiers
EC no.5.4.99.7
CAS no.9032-71-7
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
LSS
PDB 1w6j EBI.jpg
Available structures
PDBOrtholog search: PDBe RCSB
Identifiers
AliasesLSS, OSC, CTRCT44, lanosterol synthase (2,3-oxidosqualene-lanosterol cyclase), lanosterol synthase, HYPT14, APMR4
External IDsOMIM: 600909 MGI: 1336155 HomoloGene: 37408 GeneCards: LSS
Orthologs
SpeciesHumanMouse
Entrez
Ensembl
UniProt
RefSeq (mRNA)

NM_001001438
NM_001145436
NM_001145437
NM_002340

NM_146006

RefSeq (protein)

NP_001001438
NP_001138908
NP_001138909
NP_002331

NP_666118

Location (UCSC)Chr 21: 46.19 – 46.23 MbChr 10: 76.53 – 76.56 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Lanosterol synthase is an oxidosqualene cyclase (OSC) enzyme that converts (S)-2,3-oxidosqualene to a protosterol cation and finally to lanosterol.[5] Lanosterol is a key four-ringed intermediate in cholesterol biosynthesis.[6][7] In humans, lanosterol synthase is encoded by the LSS gene.[8][9]

In eukaryotes, lanosterol synthase is an integral monotopic protein associated with the cytosolic side of the endoplasmic reticulum.[10] Some evidence suggests that the enzyme is a soluble, non-membrane bound protein in the few prokaryotes that produce it.[11]

Due to the enzyme's role in cholesterol biosynthesis, there is interest in lanosterol synthase inhibitors as potential cholesterol-reducing drugs, to complement existing statins.[12]

Mechanism[]

Full-length profile.
Figure 1: Lanosterol synthase mechanism. The discrete carbocation intermediates show the non-concerted nature of the mechanism.

Though some data on the mechanism has been obtained by the use of suicide inhibitors, mutagenesis studies, and homology modeling, it is still not fully understood how the enzyme catalyzes the formation of lanosterol.[12]

Initial epoxide protonation and ring opening[]

Before the acquisition of the protein's X-ray crystal structure, site-directed mutagenesis was used to determine residues key to the enzyme's catalytic activity. It was determined that an aspartic acid residue (D455) and two histidine residues (H146 and H234) were essential to enzyme function. Corey et al. hypothesized that the aspartic acid acts by protonating the substrate's epoxide ring, thus increasing its susceptibility to intramolecular attack by the nearest double bond, with H146 possibly intensifying the proton donor ability of the aspartic acid through hydrogen bonding.[13] After acquisition of the X-ray crystal structure of the enzyme, the role of D455 as a proton donor to the substrate's epoxide was confirmed, though it was found that D455 is more likely stabilized by hydrogen bonding from two cysteine residues (C456 and C533) than from the earlier suggested histidine.[12]

Ring formation cascade[]

Epoxide protonation activates the substrate, setting off a cascade of ring forming reactions. Four rings in total (A through D) are formed, producing the cholesterol backbone.[12] Though the idea of a concerted formation of all four rings had been entertained in the past, kinetic studies with (S)-2,3-oxidosqualene analogs showed that product formation is achieved through discrete carbocation intermediates (see Figure 1). Isolation of monocyclic and bicyclic products from lanosterol synthase mutants has further weakened the hypothesis of a concerted mechanism.[14][15] Evidence suggests that epoxide ring opening and A ring formation is concerted, though.[16]

Structure[]

Lanosterol synthase is a two-domain monomeric protein[10] composed of two connected (α/α) barrel domains and three smaller β-structures. The enzyme active site is in the center of the protein, closed off by a constricted channel. Passage of the (S)-2,3-epoxysqualene substrate through the channel requires a change in protein conformation. In eukaryotes, a hydrophobic surface (6% of the total enzyme surface area) is the ER membrane-binding region (see Figure 2).[12]

The enzyme contains five fingerprint regions containing Gln-Trp motifs, which are also present in the highly analogous bacterial enzyme squalene-hopene cyclase.[12] Residues of these fingerprint regions contain stacked sidechains which are thought to contribute to enzyme stability during the highly exergonic cyclization reactions catalyzed by the enzyme.[17]

Function[]

Catalysis of lanosterol formation[]

Lanosterol synthase catalyzes the conversion of (S)-2,3-epoxysqualene to lanosterol, a key four-ringed intermediate in cholesterol biosynthesis.[6][7] Thus, it in turn provides the precursor to estrogens, androgens, progestogens, glucocorticoids, mineralocorticoids, and neurosteroids. In eukaryotes the enzyme is bound to the cytosolic side of the endoplasmic reticulum membrane.[10] While cholesterol synthesis is mostly associated with eukaryotes, few prokaryotes have been found to express lanosterol synthase; it has been found as a soluble protein in Methylococcus capsulatus.[11]

Catalysis of epoxylanosterol formation[]

Lanosterol synthase also catalyzes the cyclization of 2,3;22,23-diepoxysqualene to 24(S),25-epoxylanosterol,[18] which is later converted to 24(S),25-epoxycholesterol.[19] Since the enzyme affinity for this second substrate is greater than for the monoepoxy (S)-2,3-epoxysqualene, under partial inhibition conversion of 2,3;22,23-diepoxysqualene to 24(S),25-epoxylanosterol is favored over lanosterol synthesis.[20] This has relevance for disease prevention and treatment.

Clinical significance[]

Enzyme inhibitors as cholesterol-lowering drugs[]

Interest has grown in lanosterol synthase inhibitors as drugs to lower blood cholesterol and treat atherosclerosis. The widely popular statin drugs currently used to lower LDL (low-density lipoprotein) cholesterol function by inhibiting HMG-CoA reductase activity.[6] Because this enzyme catalyzes the formation of precursors far upstream of (S)-2,3-epoxysqualene and cholesterol, statins may negatively influence amounts of intermediates required for other biosynthetic pathways (e.g. synthesis of isoprenoids, coenzyme Q). Thus, lanosterol synthase, which is more closely tied to cholesterol biosynthesis than HMG-CoA reductase, is an attractive drug target.[21]

Lanosterol synthase inhibitors are thought to lower LDL and VLDL cholesterol by a dual control mechanism. Studies in which lanosterol synthase is partially inhibited have shown both a direct decrease in lanosterol formation and a decrease in HMG-CoA reductase activity. The oxysterol 24(S),25-epoxylanosterol, which is preferentially formed over lanosterol during partial lanosterol synthase inhibition, is believed to be responsible for this inhibition of HMG-CoA reductase activity.[22]

Evolution[]

It is believed that oxidosqualene cyclases (OSCs, the class to which lanosterol cyclase belongs) evolved from bacterial squalene-hopene cyclase (SHC), which is involved with the formation of hopanoids. Phylogenetic trees constructed from the amino acid sequences of OSCs in diverse organisms suggest a single common ancestor, and that the synthesis pathway evolved only once.[23] The discovery of steranes including cholestane in 2.7-billion year-old shales from Pilbara Craton, Australia, suggests that eukaryotes with OSCs and complex steroid machinery were present early in earth's history.[24]

References[]

  1. ^ a b c ENSG00000281289 GRCh38: Ensembl release 89: ENSG00000160285, ENSG00000281289 - Ensembl, May 2017
  2. ^ a b c GRCm38: Ensembl release 89: ENSMUSG00000033105 - Ensembl, May 2017
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  4. ^ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
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  8. ^ Baker CH, Matsuda SP, Liu DR, Corey EJ (Aug 1995). "Molecular cloning of the human gene encoding lanosterol synthase from a liver cDNA library". Biochemical and Biophysical Research Communications. 213 (1): 154–160. doi:10.1006/bbrc.1995.2110. PMID 7639730.
  9. ^ Young, Michele; Chen, Haiming; Lalioti, Maria D.; Antonarakis, Stylianos E. (May 1996). "The human lanosterol synthase gene maps to chromosome 21q22.3". Human Genetics. 97 (5): 620–624. doi:10.1007/BF02281872. PMID 8655142. S2CID 21051816.
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  20. ^ Boutaud O, Dolis D, Schuber F (Oct 1992). "Preferential cyclization of 2,3(S):22(S),23-dioxidosqualene by mammalian 2,3-oxidosqualene-lanosterol cyclase". Biochemical and Biophysical Research Communications. 188 (2): 898–904. doi:10.1016/0006-291X(92)91140-L. PMID 1445330.
  21. ^ Telford DE, Lipson SM, Barrett PH, Sutherland BG, Edwards JY, Aebi JD, Dehmlow H, Morand OH, Huff MW (Dec 2005). "A novel inhibitor of oxidosqualene:lanosterol cyclase inhibits very low-density lipoprotein apolipoprotein B100 (apoB100) production and enhances low-density lipoprotein apoB100 catabolism through marked reduction in hepatic cholesterol content". Arteriosclerosis, Thrombosis, and Vascular Biology. 25 (12): 2608–14. doi:10.1161/01.ATV.0000189158.28455.94. PMID 16210564.
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  23. ^ Pearson A, Budin M, Brocks JJ (Dec 2003). "Phylogenetic and biochemical evidence for sterol synthesis in the bacterium Gemmata obscuriglobus". Proceedings of the National Academy of Sciences of the United States of America. 100 (26): 15352–7. Bibcode:2003PNAS..10015352P. doi:10.1073/pnas.2536559100. PMC 307571. PMID 14660793.
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