Non-hydrolytic Cleaving Reactions(NRs) mediate the formation and removal of carbon-carbon double bonds are also common in biological systems and are catalyzed by a class of enzymes called lyases. This reaction often results in the formation of a new cyclic structure or a new double bond, and a reverse reaction called a “Michael addition” might also possibly happen under the catalysis of lyase. To obtain either a double bond or a new ring, lyase acts upon the single substrate and a molecule is eliminated. Lyases are different from other enzymes for only one substrate is required for the reaction in one direction, but two substrates are essential for the reverse reaction. Lyases can be commonly observed in the reactions of the Citric Acid Cycle (Krebs cycle) and in glycolysis. In glycolysis, aldolase could readily and reversibly degrade fructose 1,6-bisphosphate into the products glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, which is an example of a lyase cleaving carbon-carbon bonds. Lyase works without the necessary requirements for cofactor recycling and gives an absolute stereospecificity with a theoretical yield of 100%, being much more efficient compared with enantiomeric resolutions of only 50% productive rate. Therefore, considerable researches have been addicted to the exploration of lyases as biocatalysts to synthesize optically active compounds, which have also been already found application in a few large commercial processes. Lyases are systematically named as ” substrate group-lyase”, such as decarboxylase, dehydratase, aldolase, etc. When a product is more significant, synthase may be expressed in a name like phosphosulfolactate synthase.1
Lyases are sorted as EC 4 in the EC number classification of enzymes and can be further classified into eight subclasses.
The typical reactions are shown as follow:
RCOCOOH → RCOH + CO2
[X-A + B-Y] → [A=B + X-Y]
In the EC number classification of enzymes, EC 4 could represent lyases, which can be further classified into seven subclasses. Lyases in EC 4.1 cleave carbon-carbon bonds, and include decarboxylases (EC 4.1.1), aldehyde lyases (EC 4.1.2) facilitating the reverse reaction of aldol condensations, oxo acid lyases (EC 4.1.3) that catalyzes the cleavage of many 3-hydroxy acids, and others (EC 4.1.99). EC 4.2 contains a group of lyases that break carbon-oxygen bonds, such as dehydratases. Hydro-lyases being a part of carbon-oxygen lyases could facilitate the cleavage of C-O bonds by the elimination of water. Some other carbon-oxygen lyases promote the elimination of a phosphate or the removal of an alcohol from a polysaccharide. Lyases cleaving carbon-nitrogen bonds are sorted into EC 4.3. They could release ammonia with powerful cleaving ability and simultaneously form a double bond or ring. Some of these enzymes can also help to eliminate an amine or amide group. EC 4.4 represents lyases that split carbon-sulfur bonds, which could eliminate or substitute dihydrogen sulfide (H2S) from a reaction. Carbon-halide bonds cleaving enzymes are lyases in EC 4.5 and that utilize an action mode that removes hydrochloric acid from a synthetic pesticide dichloro-diphenyl-trichloroethane (DDT) . EC 4.6 comprises lyases fracturing phosphorus-oxygen bonds, like adenylyl cyclase and guanylyl cyclase, and they eliminate diphosphate from nucleotide triphosphates. EC 4.99 is a group of other lyases.
Narrow substrate specificity is usually considered to be a drawback for the commercialization of an enzyme in that it greatly restricts the flexibility of an enzyme as an assistant in the production of related compounds. Lyases are generally, but not always, found with narrow substrate specificity. Most hydratases and ammonia-lyases indeed possess quite narrow substrate specificity, while the substrate specificity for aldolases, decarboxylases and oxynitrilases is much broader. It is noteworthy here that the substrate specificity of a specific lyase varies depending on its source. However, it is not an absolute prerequisite for enzymes to own unrestricted substrate specificity for their commercial exploitation. In fact, there are several of the lyases in commercial use bearing a rather narrow substrate spectrum.
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“Lyase Introduction.” https://www.creative-enzymes.com/resource/Hydrolase-Introduction_21.html. Retrieved 2020-09-15 ↩