Note: a dashed line (without arrowheads) between two compound names is meant to imply that the two names are just different instantiations of the same compound -- i.e. one may be a specific name and the other a general name, or they may both represent the same compound in different stages of a polymerization-type pathway. 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: glucosinolate hydrolysis, glucosinolate activation, glucosinolate degradation
|Superclasses:||Activation/Inactivation/Interconversion → Activation|
|Degradation/Utilization/Assimilation → Secondary Metabolites Degradation → Nitrogen Containing Secondary Compounds Degradation → Nitrogen Containing Glucosides Degradation → Glucosinolates Degradation|
Expected Taxonomic Range: Brassicales
Glucosinolates are substituted β-thioglucoside N-hydroxysulfates, formed by certain planst 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 [Hayes, 2008]. 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 220.127.116.11 ), 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 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].
Unification Links: AraCyc:PWY-5267
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
Bernardi03: Bernardi R, Finiguerra MG, Rossi AA, Palmieri S (2003). "Isolation and biochemical characterization of a basic myrosinase from ripe Crambe abyssinica seeds, highly specific for epi-progoitrin." J Agric Food Chem 51(9);2737-44. PMID: 12696966
Burow06: Burow M, Markert J, Gershenzon J, Wittstock U (2006). "Comparative biochemical characterization of nitrile-forming proteins from plants and insects that alter myrosinase-catalysed hydrolysis of glucosinolates." FEBS J 273(11);2432-46. PMID: 16704417
Shikita99: Shikita M, Fahey JW, Golden TR, Holtzclaw WD, Talalay P (1999). "An unusual case of 'uncompetitive activation' by ascorbic acid: purification and kinetic properties of a myrosinase from Raphanus sativus seedlings." Biochem J 341 ( Pt 3);725-32. PMID: 10417337
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