If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: onion lachrymatory factor biosynthesis
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Sulfur-Containing Secondary Compounds Biosynthesis|
Expected Taxonomic Range: Allioideae
Plants belonging to the Allioideae family have been valued both for flavor and medicinal purposes throughout the world. The domestic Alliums (such as onion, garlic, chives and leek) contain high concentrations of organic sulfur compounds especially in their bulbs and leaves. These compounds consist mostly of S-alkyl-L-cysteine S-oxides such as alliin, (+)-trans-isoalliin, and methiin, as well as γ-glutamyl peptides (together these compounds make up over 70% of the total sulfur in garlic [Lawson96]).
These compounds, which are found in the cytoplasm, are produced constitutively. The S-alkyl cysteine S-oxides are substrates for the alliinase enzymes (EC 18.104.22.168, alliin lyase), which convert them to S-alkylsulfenates. However, the enzymes are kept in vacuoles, and are thus sequestered from their substrates [Lancaster81, Pickering09]. Only when the tissue is injured, the alliinase enzymes are released from the vaculoes and interact with their substrates, producing S-alkylsulfenates. [Rose05]. Once formed, S-alkylsulfenates are very active and tend to condense spontaneously, forming thiosulfinates [Aoyagi11]. Depending on the Allium species, and under differing conditions, the thiosulfinates can decompose to form additional sulfur constituents including diallyl, methyl allyl, and diethyl mono-, di-, tri-, tetra-, penta-, and hexasulfides, vinyldithiins, and (E)- and (Z)-ajoene [Rose05].
In some species, dedicated enzymes commonly known as "lachrymatory-factor synthases" act on the S-alkylsulfenates rapidly, before they condense, producing volatile compounds that cause the eyes of animals to tear (lachrymatory factors) - a further defense mechanism against herbivory.
About This Pathway
It is well known that when onions (Allium cepa) are chopped, they release a volatile compound that causes tearing in animals. This compound, commonly referred to as a lachrymatory-factor, was shown to be (Z)-propanethial S-oxide [Brodnitz71].
The path of synthesis of (+)-trans-isoalliin is still speculative. An early research paper reported that γ-glutamyl peptides are not the immediate precursors for its synthesis [Edwards97]. However, later work has suggested that the pathway does involve γ glutamyl peptides, starting with glutathione and a carboxypropyl source. Radiolabeled L-valine has been shown to give rise to to radiolabeled methacrylate [Granroth70], and thus L-valine degradation is a potential source for the side group of the S-alk(en)yl-L-cysteine S-oxides [Jones04, Hughes05].
As described above, (+)-trans-isoalliin is constitutively present in the cytoplasm of onion cells, but is sequestered from the alliinase, which is kept in vacuoles. Upon injury alliinase is released, converting it to (E) 1-propenylsulfenate [Lancaster00], which is rapidly converted to the lachrymatory-factor (Z)-propanethial S-oxide by sulfenic acid isomerase. In the absence of the enzyme, (E) 1-propenylsulfenate dimerizes spontaneously to form thiosulfinate [Aoyagi11].
Superpathways: superpathway of Allium flavor precursors
Aoyagi11: Aoyagi M, Kamoi T, Kato M, Sasako H, Tsuge N, Imai S (2011). "Structure and bioactivity of thiosulfinates resulting from suppression of lachrymatory factor synthase in onion." J Agric Food Chem 59(20);10893-900. PMID: 21905712
Edwards97: Edwards S. J., Britton G., Collin H. A. (1997). "The biosynthetic pathway of the S-alk(en)yl-L-cysteine sulphoxides (flavour precursors) in species of Alium." Plant Cell, Tissue and Organ Culture 38:181-188.
Lancaster00: 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
Lancaster81: Lancaster JE, Collin HA (1981). "Presence of alliinase in isolated vacuoles and of alkyl cysteine sulphoxides in the cytoplasm of bulbs of onion (Allium cepa)." Plant Science Letters 22(2);169-176.
Lawson96: Lawson, L.D. (1996). "The composition and chemistry of garlic cloves and processed garlic." in H.P. Koch, L.D. Lawson (Eds.), Garlic: The Science and Therapeutic Application of Allium sativum L. and Related Species (second ed.), Williams and Wilkins, Baltimore, USA. pp. 37-108.
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
Pickering09: Pickering IJ, Sneeden EY, Prince RC, Block E, Harris HH, Hirsch G, George GN (2009). "Localizing the chemical forms of sulfur in vivo using X-ray fluorescence spectroscopic imaging: application to onion (Allium cepa) tissues." Biochemistry 48(29);6846-53. PMID: 19463015
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
Eady08: Eady CC, Kamoi T, Kato M, Porter NG, Davis S, Shaw M, Kamoi A, Imai S (2008). "Silencing onion lachrymatory factor synthase causes a significant change in the sulfur secondary metabolite profile." Plant Physiol 147(4);2096-106. PMID: 18583530
Norris90: Norris PG, Nunn AV, Hawk JL, Cox TM (1990). "Genetic heterogeneity in erythropoietic protoporphyria: a study of the enzymatic defect in nine affected families." J Invest Dermatol 95(3);260-3. PMID: 2384686
Rubio06: Rubio S, Larson TR, Gonzalez-Guzman M, Alejandro S, Graham IA, Serrano R, Rodriguez PL (2006). "An Arabidopsis mutant impaired in coenzyme A biosynthesis is sugar dependent for seedling establishment." Plant Physiol 140(3);830-43. PMID: 16415216
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