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: thiosulfinate biosynthesis, allium thiosulfinates, onion thiosulfinates, cysteine sulfoxide degradation
|Superclasses:||Degradation/Utilization/Assimilation → Inorganic Nutrients Metabolism → Sulfur Compounds Metabolism|
|Degradation/Utilization/Assimilation → Secondary Metabolites Degradation|
Expected Taxonomic Range: Allioideae
Plants belonging to Allioideae, have been valued both for flavor and medicinal purposes throughout the world. Uncrushed whole onion bulbs contain cysteine sulfoxides and γ-glutamylcysteines as the major organosulfur compounds, in equal amounts. The odorless non protein isoalliin is the substrate which is acted upon by the allinase enzyme [Van92]. The enzyme and substrate are compartmentalized in the vacuoles and cell cytoplasm respectively [Shimon07]. The characteristic pungent odor is emitted only when the tissue is crushed and the enzyme interacts with the substrate.
The degradation of allium organosulphur compounds occurs mainly in bulbs of many of the Allioideae and the allinase enzyme has been purified from bulbs as well as leaves and also from a number of other plants including Acacia and Brassicaceae members [Van92]. Allinases are now reported from a wide variety of organisms including bacteria and fungi [Rössner02]. The variety and composition of organosulfur compounds in Allium is explained by the kinetics of cysteine sulfoxide hydrolysis and the reactivity of the initial sulfenic acids [Rose05]. Allium cepa contains S-alkyl(methyl, propyl) cysteine sulfoxide and the S-1-propenyl derivative, but not the S-2-propenyl(allyl) cysteine sulfoxide [Manabe98]
Allinases require pyridoxal 5'-phosphate as a cofactor, after binding alliin it is converted to allylsulfenate [Shimon07], which reacts spontaneously to form allicin, a parent compound of a number of other sulfur-containing compounds such as thiosulfinates, allyl sulfides, dithiines and ajoenes [Shimon07]. The irritating (lachrymatory factor) propanethiol S-oxide in chopped onions is a organosulfur compound that is released by the action of alliinase after cutting [Imai02]. The propanethiol S-oxide is formed only in the presence of lachrymatory factor synthase, alliinase and isoalliin, the absence of any one component prevents the formation of propanethiol S-oxide [Imai02].
The biosynthesis of the flavor precursors isoalliin is still not completely understood [Jones04] although, there are proposed pathways with partial experimental proof . Hence, currently only the degradation of the isoalliin [Hughes05] is shown here.
Superpathways: superpathway of alliin degradation
Manabe98: Manabe T, Hasumi A, Sugiyama M, Yamazaki M, Saito K (1998). "Alliinase [S-alk(en)yl-L-cysteine sulfoxide lyase] from Allium tuberosum (Chinese chive)--purification, localization, cDNA cloning and heterologous functional expression." Eur J Biochem 257(1);21-30. PMID: 9799098
Rose05: Rose P, Whiteman M, Moore PK, Zhu YZ (2005). "Bioactive S-alk(en)yl cysteine sulfoxide metabolites in the genus Allium: the chemistry of potential therapeutic agents." Nat Prod Rep 22(3);351-68. PMID: 16010345
Shimon07: Shimon LJ, Rabinkov A, Shin I, Miron T, Mirelman D, Wilchek M, Frolow F (2007). "Two structures of alliinase from Alliium sativum L.: apo form and ternary complex with aminoacrylate reaction intermediate covalently bound to the PLP cofactor." J Mol Biol 366(2);611-25. PMID: 17174334
Van92: Van Damme EJ, Smeets K, Torrekens S, Van Leuven F, Peumans WJ (1992). "Isolation and characterization of alliinase cDNA clones from garlic (Allium sativum L.) and related species." Eur J Biochem 209(2);751-7. PMID: 1385120
Lancaster00a: Lancaster JE, Shaw ML, Joyce MD, McCallum JA, McManus MT (2000). "A novel alliinase from onion roots. Biochemical characterization and cDNA cloning." Plant Physiol 122(4);1269-79. PMID: 10759524
Selby80: Selby, C, Turnbull, A, Collin, H. A (1980). "Comparison of the onion plant (Allium cepa) and onion tissue culture. II. Stimultaion of flavour precursor synthesis in onion tissue cultures." New Phytol. 84, 307-312.
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