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Pathway Description
Steroid Biosynthesis
Arabidopsis thaliana
Metabolic Pathway
The steroid biosynthesis pathway occurs in the endoplasmic reticulum leads to the production of various sterols. In Arabidopsis thaliana, the major products are cholesterol, brassicasterol, stigmasterol, campesterol, and their derivatives. These products serve various roles including signalling and structural support. Stigmasterol is the principal sterol in with campesterol being second most abundant.
The pathway begins with farnesyl pyrophosphate produced in the terpenoid backbone biosynthesis pathway. Squalene synthase catalyzes the reaction of two molecules of farnesyl pyrophosphate to form presqualene diphosphate and then squalene. Squalene is reduced by squalene monoxygenase to form 2,3-epoxysqualene.
2,3-epoxysqualene can be used to form cycloartenol via cycloartenol synthase. This leads to the primary path of steroid biosynthesis in Arabidopsis thaliana. Cycloartenol undergoes a series of reactions to form 24-methylenelophenol. 24-methylenelophenol can be converted to 24-ethylidenelophenol by 24-methylenesterol C-methyltransferase. 24-methylenelophenol and 24-ethylidenelophenol undergo similar separate reactions to form two different sterols. 24-methylenelophenol and 24-ethylidenelophenol are reduced by methylsterol monooxygenase 2 to episterol and δ7-avenasterol respectively. These are oxidized by delta(7)-sterol-C5(6)-desaturase to 5-dehydroepisterol and 5-dehydroavenasterol. These then undergo a series of reductions to form 24-methylenecholesterol and avenasterol; 24-epi-campesterol and β-sitosterol; and finally brassicasterol and stigmasterol. 24-methylenecholesterol may also be reduced by delta(24)-sterol reductase to form campesterol.
Alternatively, lanosterol synthase can use 2,3-epoxysqualene to form lanosterol. Lanosterol is reduced by sterol-14-demethylase to form 4,4-dimethylcholesta-8,14,24-trienol
and then by delta(14)-sterol reductase to form 4,4-dimethyl-5a-cholesta-8,24-dien-3-b-ol. 4,4-dimethyl-5a-cholesta-8,24-dien-3-b-ol then undergoes a series of reactions to for zymosterol. Zymosterol is converted into 5a-Cholest-8-en-3b-ol by delta(24)-sterol reductase, which is converted into lathosterol by 3-beta-hydroxysteroid-delta(8),delta(7)-isomerase, then into 7-dehydrocholesterol by 7-dehydrocholesterol reductase, then into cholesterol by 7-dehydrocholesterol reductase.
References
Steroid Biosynthesis References
Dunkley TP, Hester S, Shadforth IP, Runions J, Weimar T, Hanton SL, Griffin JL, Bessant C, Brandizzi F, Hawes C, Watson RB, Dupree P, Lilley KS: Mapping the Arabidopsis organelle proteome. Proc Natl Acad Sci U S A. 2006 Apr 25;103(17):6518-23. doi: 10.1073/pnas.0506958103. Epub 2006 Apr 17.
Pubmed: 16618929
Kolesnikova MD, Xiong Q, Lodeiro S, Hua L, Matsuda SP: Lanosterol biosynthesis in plants. Arch Biochem Biophys. 2006 Mar 1;447(1):87-95. doi: 10.1016/j.abb.2005.12.010. Epub 2006 Jan 11.
Pubmed: 16445886
Lange BM, Ghassemian M: Genome organization in Arabidopsis thaliana: a survey for genes involved in isoprenoid and chlorophyll metabolism. Plant Mol Biol. 2003 Apr;51(6):925-48.
Pubmed: 12777052
Benveniste P: Biosynthesis and accumulation of sterols. Annu Rev Plant Biol. 2004;55:429-57. doi: 10.1146/annurev.arplant.55.031903.141616.
Pubmed: 15377227
Morikawa T, Mizutani M, Aoki N, Watanabe B, Saga H, Saito S, Oikawa A, Suzuki H, Sakurai N, Shibata D, Wadano A, Sakata K, Ohta D: Cytochrome P450 CYP710A encodes the sterol C-22 desaturase in Arabidopsis and tomato. Plant Cell. 2006 Apr;18(4):1008-22. doi: 10.1105/tpc.105.036012. Epub 2006 Mar 10.
Pubmed: 16531502
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