If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: allium thiosulfinates metabolism, garlic thiosulfinate metabolism
|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
In garlic the predominant S-alkyl-L-cysteine S-oxide is alliin, with lower concentrations of (+)-trans-isoalliin and methiin and trace amounts of propiin. Alliin is produced from S-allyl-L-cysteine, presumably by the action of an alliin oxidase [Ohsumi93]. The path of synthesis of S-allyl-L-cysteine is still speculative. An early research paper reported that γ-glutamyl peptides are not the immediate precursors for its synthesis [Edwards97a]. However, later work has suggested two possible routes that may coexist - one from serine and allyl thiol, and the other (which is shown here) from γ glutamyl peptides. 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].
In garlic allinase and its substrate are contained within a specific tissue, the bundle sheath cells of the clove [Ellmore94]. They are separated from each other by compartmentalization in the vacuole (alliinase) and cell cytoplasm (alliin) [Shimon07]. Once the tissue is injured, allinase is released and rapidly converts alliin to allylsulfenate, which then condenses with a variety of other S-alkylsulfenates, forming allicin and other assorted thiosulfinate compounds spontaneously. Alliin is odorless, and the characteristic pungent odor of garlic is produced only when the tissue is crushed and the thiosulfinates are formed [Van92].
Allicin is a defense molecule with a broad range of biological activities. It is physiologically active in microbial, plant and mammalian cells, due to its ability to participate in redox-reactions with thiol groups in glutathione and proteins [Borlinghaus14]. In a dose-dependent manner allicin can inhibit the proliferation of both bacteria and fungi, and even kill cells, including antibiotic-resistant strains like methicillin-resistant Staphylococcus aureus (MRSA) [Borlinghaus14]. Allicin is also a parent compound of a number of other sulfur-containing compounds such as thiosulfinates, allyl sulfides, dithiines and ajoenes.
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
Edwards97a: 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.
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.
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
Nock87: Nock LP, Mazelis M (1987). "The C-S Lyases of Higher Plants : Direct Comparison of the Physical Properties of Homogeneous Alliin Lyase of Garlic (Allium sativum) and Onion (Allium cepa)." Plant Physiol 85(4);1079-83. PMID: 16665807
Rabinkov94: Rabinkov A, Zhu XZ, Grafi G, Galili G, Mirelman D (1994). "Alliin lyase (Alliinase) from garlic (Allium sativum). Biochemical characterization and cDNA cloning." Appl Biochem Biotechnol 48(3);149-71. PMID: 7979352
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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