This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: C24-alkyl sterol turnover
|Superclasses:||Biosynthesis → Fatty Acids and Lipids Biosynthesis → Sterol Biosynthesis|
Some taxa known to possess this pathway include : Glycine max
Phytosterols are ancient molecules in plants; they are part of the cell membrane architectural components. The specific structural feature of phytosterols are the 24-alkyl group in the sterol side chain [Nes00]. This has evolved into more complex forms from single cellular organisms to vascular plants. Fungi, algae and protozoa, synthesize 24β- methyl sterols or ergosterols, plants synthesize 24 α- ethyl sterols like sitosterols. Alkyl steroids have immense variation in the structures in nature and only small fractions of them have been identified [Neelakandan09].
The sterol methylations are catalyzed by (S)-adenosyl-L-methionine: Δ(24)-sterol methyl transferase (SMT) which are key enzymes in the biosynthesis of plant sterols. Non-photosynthetic organisms do not have these enzymes. Several fungi and plant SMT's have been cloned and extensively characterized [BouvierNave97]. The transformation of olefins into structurally variable C-methylated products by SMT's are complex and under investigations for over 50 years. Although SMT's from lower organisms and plants appear to be distinct their mechanistic behavior and mode of sterol transformation appears to be similar [Nes03].
In this pathway, the SMT isoforms SMT2-1 and SMT2-2 show differential expression in distinct developmental stages, such as flowering and seed development, where SMT2-2 levels during seed development are higher than SMT2-1 and SMT1. This indicates the genes of the SMT's have undergone divergent evolution. The authors of this study hypothesize that the primitive SMT gene to have been bifunctional and catalytically promiscuous [Neelakandan09].
This pathway describes the reaction of sterol 24C methyltransferases (SMT2-1, SMT2-2) from Glycine max with the mechanism-inactivator of SMT's 26,27-dehydrozymosterol. This inhibitor has been used to determine SMT concentrations in soluble fractions and has also been applied as binding and enrichment factor for SMTs in purification procedures of various organisms [Nes02, Neelakandan09]. SMT2 enzymes are thought to be key enzymes in diverting the carbon flux from C24 methyl sterols (campesterol) towards the C24 ethyl sterols (sitosterol) formation (see plant sterol biosynthesis) [Neelakandan10]. The latter are very important molecules for the food industry due to the cholesterol lowering properties of phytosterols such as sitosterol [Weststrate98]. In this pathway SMT2-1 and SMT2-2 react with the mechanism-based inhibitor 26,27-dehydrozymosterol which either binds the SMT enzymes covalently and forms 24-alkyl sterol 1 and 24-alkyl sterol 3 when treated with methanol and/or potassium hydroxide or is converted into enzyme-free 24-alkyl sterol 2 which can enter the sterol pathway [Neelakandan09].
BouvierNave97: Bouvier-Nave P, Husselstein T, Desprez T, Benveniste P (1997). "Identification of cDNAs encoding sterol methyl-transferases involved in the second methylation step of plant sterol biosynthesis." Eur J Biochem 246(2);518-29. PMID: 9208946
Neelakandan09: Neelakandan AK, Song Z, Wang J, Richards MH, Wu X, Valliyodan B, Nguyen HT, Nes WD (2009). "Cloning, functional expression and phylogenetic analysis of plant sterol 24C-methyltransferases involved in sitosterol biosynthesis." Phytochemistry 70(17-18);1982-98. PMID: 19818974
Neelakandan10: Neelakandan AK, Kumar R, Tran LS, Guttikonda SK, Quach TN, Aldrich DL, Nes WD, Nguyen HT (2010). "Molecular characterization and functional analysis of Glycine max sterol methyl transferase 2 genes involved in plant membrane sterol biosynthesis." Plant Mol Biol 74(4-5);503-18. PMID: 20865301
Nes02: Nes WD, Marshall JA, Jia Z, Jaradat TT, Song Z, Jayasimha P (2002). "Active site mapping and substrate channeling in the sterol methyltransferase pathway." J Biol Chem 277(45);42549-56. PMID: 12196515
Weststrate98: Weststrate JA, Meijer GW (1998). "Plant sterol-enriched margarines and reduction of plasma total- and LDL-cholesterol concentrations in normocholesterolaemic and mildly hypercholesterolaemic subjects." Eur J Clin Nutr 52(5);334-43. PMID: 9630383
BouvierNave98: Bouvier-Nave P, Husselstein T, Benveniste P (1998). "Two families of sterol methyltransferases are involved in the first and the second methylation steps of plant sterol biosynthesis." Eur J Biochem 256(1);88-96. PMID: 9746350
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