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: biosynthesis of sulphur volatiles
|Superclasses:||Degradation/Utilization/Assimilation → Inorganic Nutrients Metabolism → Sulfur Compounds Metabolism|
Expected Taxonomic Range: Brassicaceae
Glucosinolates are substituted β-thioglucoside N-hydroxysulfates, formed by certain plants from any one of eight amino acids, namely, alanine, valine, leucine, isoleucine, phenylalanine, methionine, tyrosine and tryptophan. Over 115 naturally occurring glucosinolates have been identified [Hayes08]. Much of the diversity amongst glucosinolates arises from the addition of different sized alkyl groups to the side chain of the amino acids, principally valine, phenylalanine and methionine.
Glucosinolates are found prominently in the order Brassicaceae, which includes cabbage, mustard, oilseed rape, broccoli, and the model plant Arabidopsis, and are responsible for the typical sharp taste and odor of these plants.
The glucosinolates are hydrolyzed by the plant to compounds that are toxic to herbivores, and play an important role in plant defense. The hydrolysis of is catalyzed by myrosinases (b-thioglucoside glucohydrolase, EC 126.96.36.199 ), enzymes that are physically segregated from glucosinolates within the intact plant in specialized ''myrosin'' cells, and are brought into contact by tissue damage [Bones96].
The products of the myrosinase reaction are unstable aglycons, thiohydroximate-O-sulfates, that decompose to different types of products, depending on the chemical nature of the side chain of the parent glucosinolate and the conditions of hydrolysis. Products include nitriles, isothiocyanates, thiocyanates, oxazolidine-2-thiones, and epithionitriles.
Glucosinolates with an aliphatic side chain are generally hydrolyzed to yield isothiocyanates at a neutral pH. However, acidic pH or the presence of Fe2+ ions favors the production of nitriles [Gil80].
In some plants, additional proteins are involved in glucosinolate hydrolysis. For example, it has been shown that the presence of an epithiospecifier protein (ESP) results in conversion of thiohydroximate-O-sulfates to bioactive nitriles or epithionitriles [Lambrix01]. If the parent glucosinolate possessed a terminal alkene group, epithionitriles are formed (by transfer of the sulfur atom from the basic glucosinolate backbone to the terminal alkene residue of the side chain), whereas other glucosinolates are converted to simple nitriles. The protein appears to have no catalytic activity in the absence of myrosinase [Daxenbichler68, Tookey73, MacLeod85, Lambrix01, Zabala05]. In the absence of ESP, as in some Capparales species, isothiocynates are the only breakdown products [Lambrix01].
About this pathway : The hydrolysis products of glucosinolates are methylated to volatile sulfur compounds by five distinct isoforms of thiol methyltransferases (TMT) in Brassica oleracea. The produced sulfur volatiles play a role in anti-insect and anti-pathogen activities. The expression pattern of TMT's in cabbage correlated with the glucosinolate accumulation. These TMT's possess the methyltransferase signature in their sequence but had distinct functions from them [Attieh02].
Attieh02: Attieh J, Djiana R, Koonjul P, Etienne C, Sparace SA, Saini HS (2002). "Cloning and functional expression of two plant thiol methyltransferases: a new class of enzymes involved in the biosynthesis of sulfur volatiles." Plant Mol Biol 50(3);511-21. PMID: 12369626
Lambrix01: Lambrix V, Reichelt M, Mitchell-Olds T, Kliebenstein DJ, Gershenzon J (2001). "The Arabidopsis epithiospecifier protein promotes the hydrolysis of glucosinolates to nitriles and influences Trichoplusia ni herbivory." Plant Cell 13(12);2793-807. PMID: 11752388
Zabala05: Zabala Mde T, Grant M, Bones AM, Bennett R, Lim YS, Kissen R, Rossiter JT (2005). "Characterisation of recombinant epithiospecifier protein and its over-expression in Arabidopsis thaliana." Phytochemistry 66(8);859-67. PMID: 15845404
Attieh00: Attieh J, Sparace SA, Saini HS (2000). "Purification and properties of multiple isoforms of a novel thiol methyltransferase involved in the production of volatile sulfur compounds from Brassica oleracea." Arch Biochem Biophys 380(2);257-66. PMID: 10933880
BrazierHicks08: Brazier-Hicks M, Evans KM, Cunningham OD, Hodgson DR, Steel PG, Edwards R (2008). "Catabolism of glutathione conjugates in Arabidopsis thaliana. Role in metabolic reactivation of the herbicide safener fenclorim." J Biol Chem 283(30);21102-12. PMID: 18522943
Zhao12: Zhao N, Ferrer JL, Moon HS, Kapteyn J, Zhuang X, Hasebe M, Neal Stewart C, Gang DR, Chen F (2012). "A SABATH Methyltransferase from the moss Physcomitrella patens catalyzes S-methylation of thiols and has a role in detoxification." Phytochemistry 81;31-41. PMID: 22795762
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