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.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Sulfur-Containing Secondary Compounds Biosynthesis|
Some taxa known to possess this pathway include : Allium aflatunense [Rose05], Allium altaicum [Rose05], Allium altyncolicum [Rose05], Allium ampeloprasum [Rose05], Allium ascalonicum [Rose05], Allium cepa [Rose05], Allium chinense [Rose05], Allium ochotense [Rose05], Allium tuberosum [Manabe98], Allium victorialis [Rose05]
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
Ethiin is a minor S-alkyl-L-cysteine S-oxide found in most members of the domesticated Allioideae. Its presence has been suggested already in 1968, although not verified at the time [Horhammer68]. In 1997 it was shown that feeding ethanethiol to onion root cultures resulted in production of ethiin [Prince97]. The final proof was provided by utilization of a new sensitive method for detection of S-alkyl-L-cysteine S-oxides [Kubec00, Rose05]. Since ethiin accounts for less than 4% of the total S-alkyl-L-cysteine S-oxides, it probably does not contribute significantly to the aroma formation of common Allium species [Kubec00].
The path leading to ethiin biosynthesis is still speculative. Two possible routes that may coexist have been suggested - one from serine and allyl thiol (shown here), and the other from γ glutamyl peptides [Hughes05, Rose05].
Like other S-alkyl-L-cysteine S-oxides, ethiin is a substrate for the allinase enzyme [Manabe98]. Sequestration of the enzyme and substrate to the vacuoles and cell cytoplasm, respectively, prevents interaction in undisturbed tissue. However, upon tissue injury the enzyme and substrate come into contact and ethiin is converted to ethylsulfenate, which condenses spontaneously with other S-alkylsulfenates to form thiosulfinate compounds [Rose05].
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
Horhammer68: Horhammer L, Wagner H, Seitz M, Vejdelek ZJ (1968). "[On determination of the value of garlic preparations. 1. Chromatographic studies on the genuine contents of Allium sativum L]." Pharmazie 23(8);462-7. PMID: 5715558
Kubec00: Kubec R, Svobodova M, Velisek J (2000). "Distribution of S-Alk(en)ylcysteine sulfoxides in some Allium species. Identification Of a new flavor precursor: S-ethylcysteine sulfoxide (Ethiin)." J Agric Food Chem 48(2);428-33. PMID: 10691652
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
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
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.
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
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