enzymology Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

, an alliin lyase () is an enzyme that

catalyzes Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

the chemical reaction :an ''S''-alkyl-L-cysteine ''S''-oxide

\rightleftharpoons

an alkyl sulfenate + 2-aminoacrylate Hence, this enzyme has one

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

, ''S''-alkyl-L-

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

''S''-oxide, and two products, alkyl sulfenate and

2-aminoacrylate Dehydroalanine (Cα,β-didehydroalanine, α,β-di-dehydroalanine, 2-aminoacrylate, or 2,3-didehydroalanine) is a dehydroamino acid. It does not exist in its free form, but it occurs naturally as a residue found in peptides of Microorganism, microb ...

. This enzyme belongs to the family of

lyase In biochemistry, a lyase is an enzyme that catalyzes the breaking (an elimination reaction) of various chemical bonds by means other than hydrolysis (a substitution reaction) and oxidation, often forming a new double bond or a new ring structure. ...

s, specifically the class of carbon-sulfur lyases. The systematic name of this enzyme class is ''S''-alkyl-L-cysteine ''S''-oxide alkyl-sulfenate-lyase (2-aminoacrylate-forming). Other names in common use include alliinase, cysteine sulfoxide lyase, alkylcysteine sulfoxide lyase, ''S''-alkylcysteine sulfoxide lyase, L-cysteine sulfoxide lyase, ''S''-alkyl-L-cysteine sulfoxide lyase, and alliin alkyl-sulfenate-lyase. It employs one cofactor, pyridoxal phosphate. Many alliinases contain a novel ''N''-terminal epidermal growth factor-like domain ( EGF-like domain).

Occurrence

These enzymes are found in plants of the genus '' Allium'', such as garlic and onions. Alliinase is responsible for catalyzing chemical reactions that produce the volatile chemicals that give these foods their flavors, odors, and tear-inducing properties. Alliinases are part of the plant's defense against herbivores. Alliinase is normally sequestered within a plant cell, but, when the plant is damaged by a feeding animal, the alliinase is released to catalyze the production of the pungent chemicals. This tends to have a deterrent effect on the animal. The same reaction occurs when onion or garlic is cut with a knife in the kitchen.

Chemistry

In garlic, an alliinase enzyme acts on the chemical

alliin Alliin is a sulfoxide that is a natural constituent of fresh garlic. It is a derivative of the amino acid cysteine. When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of ...

converting it into

allicin Allicin is an organosulfur compound obtained from garlic, a species in the family Alliaceae. It was first isolated and studied in the laboratory by Chester J. Cavallito and John Hays Bailey in 1944. When fresh garlic is chopped or crushed, the ...

. The process involves two stages: elimination of 2-propenesulfenic acid from the amino acid unit (with α-aminoacrylic acid as a byproduct), and then condensation of two of the

sulfenic acid In chemistry, a sulfenic acid is an organosulfur compound and oxoacid with the general formula . It is the first member of the family of organosulfur oxoacids, which also include sulfinic acids () and sulfonic acids (), respectively. The base ...

molecules. Alliin and related substrates found in nature are chiral at the

sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...

position (usually having the ''S'' absolute configuration, and alliin itself was the first natural product found to have both carbon- and sulfur-centered stereochemistry. However, the sulfenic acid intermediate is not chiral, and the final product's stereochemistry is not controlled. There are a range of similar enzymes that can react with the cysteine-derived sulfoxides present in different species. In onions, an isomer of alliin, isoalliin, is converted to 1-propenesulfenic acid. A separate enzyme, the lachrymatory factor synthase or LFS, then converts this chemical to ''syn''-propanethial-''S''-oxide, a potent lachrymator. The analogous butyl compound, ''syn''-butanethial-''S''-oxide, is found in '' Allium siculum'' species.

Structural studies

As of late 2007, 3 structures have been solved for this class of enzymes, using X-ray crystallography. The PDB accession codes are , , and .

References

Bibliography

* * *

External links

Genuine Garlic:Alliinase from ''Allium sativum''.
{{Portal bar, Biology, border=no EC 4.4.1 Enzymes of known structure Pyridoxal phosphate enzymes