Mechanism of Squalene Cyclization

Mulheirn, L. J.; Caspi, Eliahu

doi:10.1016/s0021-9258(18)62315-4

Cited by 18 publications

(3 citation statements)

References 34 publications

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“…The precursor of fusidanes is supposed to be protosta-17(20) Z ,24-dien-3β-ol ( 1 ), which is formed through cyclization of 2,3-oxidosqualene folded in pre chair−boat−chair conformation (Scheme ). After formation of a tetracyclic protosteryl cation, the reaction terminates by loss of H-17α proton without any methyl and/or hydride shifts . Except for lanosterol synthase and cycloartenol synthase for sterol biosynthesis, cucurbitadienol synthase is the only identified OSC that involves protosteryl cation as an intermediate .…”

Section: Introductionmentioning

confidence: 99%

Biosynthesis of Steroidal Antibiotic Fusidanes: Functional Analysis of Oxidosqualene Cyclase and Subsequent Tailoring Enzymes from Aspergillus fumigatus

et al. 2009

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Three putative oxidosqualene cyclase (OSC) genes exist in the genome of the fungus Aspergillus fumigatus that produces a steroidal antibiotic, helvolic acid. One of these genes, Afu4g14770, designated AfuOSC3, is clustered with genes of cytochrome P450 monooxygenases (P450s), a short-chain dehydrogenase/reductase (SDR), and acyltransferases, which presumably function in triterpene tailoring steps, suggesting that this gene cluster codes for helvolic acid biosynthesis. AfuOSC3 was PCR amplified from A. fumigatus IFO8866 genomic DNA and expressed in yeast. The yeast transformant accumulated protosta-17(20)Z,24-dien-3beta-ol, an established precursor for helvolic acid. Its structural isomer, (20R)-protosta-13(17),24-dien-3beta-ol, was also isolated from the transformed yeast. To further identify the function of triterpene tailoring enzymes, four P450 genes (CYP5081A1-D1) and a SDR gene (AfuSDR1) in the cluster were each coexpressed with AfuOSC3 in yeast. As a result, coexpression of AfuSDR1 gave a 3-keto derivative of protostadienol. On the other hand, coexpression with CYP5081A1 gave protosta-17(20)Z,24-diene-3beta,29-diol and protosta-17(20)Z,24-dien-3beta-ol-29-oic acid. These metabolites are in well accord with the oxidative modification involved in helvolic acid biosynthesis. AfuSDR1 and CYP5081A1 presumably function together to catalyze demethylation of C-29 methyl group. These results provided a firm ground for identification of the present gene cluster to be involved in helvolic acid biosynthesis.

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Section: Introductionmentioning

confidence: 99%

Biosynthesis of Steroidal Antibiotic Fusidanes: Functional Analysis of Oxidosqualene Cyclase and Subsequent Tailoring Enzymes from Aspergillus fumigatus

et al. 2009

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“…When we approached the problem, for reasons given earlier, an a priori decision was made to use an in vivo system. In a search for an appropriate model, we concentrated on fusidic acid which is efficiently biosynthesized from MVA by the mold F. coccineum, [25][26] The use of fusidic acid presented certain additional distinct advantages. First it is apparently a metabolite of the protosterol (18) which most probably is formed by the stabilization of the cation 16 without a backbone rearrangement.23 Secondly, it has an 1 la-hydroxy group which provides an easy entry for the evaluation of the labeling pattern at C-l 1 and 12.…”

Section: Resultsmentioning

confidence: 99%

Mechanism of oxidative cyclization of squalene. Evidence for cyclization of squalene from either end of the squalene molecule in the in vivo biosynthesis of fusidic acid by Fusidium coccineum

Ebersole¹,

Godtfredsen²,

Vagnedal³

et al. 1973

J. Am. Chem. Soc.

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phenyl phosphide was prepared by the reaction of diphenylphosphine with n-butyllithium in THF. Lithium diphenylamide was prepared by the reaction of diphenylamine with methyllithium in THF. Sodium dimsylate was prepared as described by Greenwald, Chaykovsky, and Corey.80 All of the above were reacted with carbon monoxide at 200 psi; only sodium dimsylate took up an appreciable quantity of carbon monoxide (0.5 equiv of carbon monoxide per equivalent of dimsylate over a 24-hr period). Dilithium Benzophenone Dianion. Butyllithium in hexane (6.0 mmol) was added slowly to 1,Zdibromoethane (0.957 g, 5.1 mmol) in a 40-ml centrifuge tube capped with a No-Air stopper. The resulting precipitate of anhydrous lithium bromide was washed four times with 20-ml portions of pentane, dissolved in 7 ml of hot DME, and added to 2 mmol of dipotassium benzophenone dianion in 30 ml of D M E containing 0.1534 g of n-nonadecane as internal standard in a stoppered 40-ml centrifuge tube. The solution was thoroughly mixed by shaking and the precipitate of potassium bromide separated by centrifugation. The resulting solution of dilithium benzophenone79 was added to a solution of a,a-diphenyla-hydroxyacetophenone (0.0967 g, 0.336 mmol) in 10 ml of DME. The red color of dilithium benzophenone persisted after the addition. After stirring at room temperature for 2 hr, the reaction mixture was hydrolyzed with aqueous potassium hydroxide solution; the aqueous layer was separated and extracted with one 50-ml portion of ethyl ether, and the organic layers were combined and dried (Na2SOa). Analysis by glpc indicated the presence of a,adiphenylacetophenone (72 %) and ap-diphenyl-a-hydroxyacetophenone (9 %).Preparation of Other Anions. Lithium phenyl acetylide and lithium butyl acetylide were prepared by reaction of the corresponding acetylene with methyllithium in ether solution. Lithium di-(79) Solutions of 18 prepared using this procedure were analyzed for potassium by hydrolysis and treatment with lithium tetraphenylborate: cf. D. N. Bhattacharyya, C. L. Lee, J. Smid, and M. Szwarc, J. Phys. Chem., 69, 608 (1965). Less than 1 % of the potassium originally present as dipotassium benzophenone remained in solution after addition of the lithium bromide.Abstract: Fusidic acid biosynthesized by F. coccineum from (3RS,5S)-[2-' 4C,5-3H]me~alonic acid was shown to contain 0.5 atom of tritium at the C-llp and C-12 positions. From the known mechanism of squalene formation the 0.5 atom of tritium at C-12 must have the LY configuration. Our results indicate that either one of the terminal double bonds of squalene is epoxidized to an equal degree, and that the ensuing cyclization to prosterol occurs from either end of the squalene molecule. This shows that the geometrical asymmetry imparted to the squalene on the squalene synthethase is not retained during the conversion to oxidosqualene. These observations are consistent with the hypothesis of the release of squalene into a free squalene pool prior to epoxidation. he biosynthetic steps involved in the formation...

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“…The fourth compound (18%) was not identified. The third compound (22%) was identified as 2-(l-propenvl)phenyl acetate (3): ir (neat) 5.75 (-OAc), 6.1 µ (C=C); nmr (CDC1S) S 1.89 (d, 3 H, CH3C=C-), 2.32 (s, 3 H, CH3COO-). 6.3 (m, 2 H, -CH=CH-), and 7.25 (m, 4 H, aromatic).…”

Section: Methodsmentioning

confidence: 99%

Aromatization of cyclic ketones. I. Alkylcyclohexanone

Kablaoui¹

1974

J. Org. Chem.

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Mechanism of Squalene Cyclization

Cited by 18 publications

References 34 publications

Biosynthesis of Steroidal Antibiotic Fusidanes: Functional Analysis of Oxidosqualene Cyclase and Subsequent Tailoring Enzymes from Aspergillus fumigatus

Biosynthesis of Steroidal Antibiotic Fusidanes: Functional Analysis of Oxidosqualene Cyclase and Subsequent Tailoring Enzymes from Aspergillus fumigatus

Mechanism of oxidative cyclization of squalene. Evidence for cyclization of squalene from either end of the squalene molecule in the in vivo biosynthesis of fusidic acid by Fusidium coccineum

Aromatization of cyclic ketones. I. Alkylcyclohexanone

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