What is a Lyase?
In physiology, Lyase meaning is defined as any member of an enzyme class that catalyzes the removal or addition of the elements of ammonia (nitrogen, hydrogen), carbon dioxide (carbon, oxygen), or water (oxygen, hydrogen) at double bonds in different ways other than hydrolysis and oxidation. For example, dehydrases remove water and decarboxylases remove carbon dioxide from amino acids.
Nomenclature
The systematic names can be formed as "substrate group-lyase." Common names are aldolase, dehydratase, decarboxylase, and so on. Synthase may be used in the name of a product when it is very essential, such as phosphosulfolactate synthase sulfite's Michael addition to phosphoenolpyruvate). A combination of both - a Michael and elimination addition is seen in the O-succinylhomoserine (thiol) -lyase (either MetY or MetZ) that catalyze first the γ-elimination of O-succinyl homoserine (succinate as a leaving group) and after that, the sulphide addition to the vinyl intermediate. First, this particular reaction was classified as a lyase, but then, it was reclassified as a transferase.
Examples of Lyase
A few examples of lyase include phenylalanine ammonia-lyase, citrate lyase, isocitrate lyase, hydroxynitrile, pectate lyase, argininosuccinate lyase, pyruvate formate lyase, alginate lyase, and pectin lyase.
Classification
In the classification of enzymes, lyases may be classified further into seven subclasses.
The first group includes Lyases that cleave carbon-carbon bonds and also include decarboxylases, aldehyde lyases by facilitating the aldol condensation's reverse reaction, oxo acid lyases (EC 4.1.3), which catalyzes the cleavage of several 3-hydroxy acids, including others.
The second one has a group of lyases, which break carbon-oxygen bonds like dehydratases. Hydro-lyases, being a part of carbon-oxygen lyases, may facilitate the cleavage of C-O bonds by water elimination. Phosphate reduction or the removal of alcohol from a polysaccharide was promoted by a few other carbon-oxygen lyases.
Lyases cleaving the carbon-nitrogen bonds are. And, they could release ammonia with the powerful cleaving ability and simultaneously produce a double ring or bond. A few of these enzymes may also help to eliminate an amide or amine group.
The fourth group shows lyases that split carbon-sulphur bonds that could either substitute or eliminate hydrogen sulphide (H₂S) from a reaction.
Carbon-halide bonds, by cleaving enzymes, are the lyases in the fifth group and which utilize an action mode, which removes hydrochloric acid from dichloro-diphenyl-trichloroethane (DDT), a synthetic pesticide.
Sixth group comprises the bonds of lyases fracturing phosphorus-oxygen, such as guanylyl cyclase and adenylyl cyclase, and they eliminate phosphate from the nucleotide triphosphates.
Example of Lyases in Each Category
Carbon-Carbon Lyases
Carboxy-Lyases: Pyruvate decarboxylase, acetoacetate decarboxylase, Oxaloacetate decarboxylase, Glutamate decarboxylase, Malonyl-CoA decarboxylase, Ornithine decarboxylase, Phosphoribosylaminoimidazole carboxylase, Lysine decarboxylase, Histidine decarboxylase, Aromatic L-amino acid decarboxylase, Uridine monophosphate synthetase/Orotidine 5'-phosphate decarboxylase, Phosphoenolpyruvate carboxylase, Uroporphyrinogen III decarboxylase, Pyrophosphomevalonate decarboxylase, RUBISCO, Adenosylmethionine decarboxylase, phosphoenolpyruvate carboxykinase
Aldehyde-Lyases: 2-hydroxyphytanoyl-CoA-lyase, aldolase A, aldolase B
Oxo-Acid-Lyases: 3-hydroxy-3-methylglutaryl-CoA lyase
Carbon-Oxygen Lyases: Fumarase, carbonic anhydrase, Enolase (Alpha), aconitase, Enoyl-CoA hydratase/3-Hydroxyacyl ACP dehydrase, Methylglutaconyl-CoA hydratase, Tryptophan synthase, Porphobilinogen synthase, cystathionine beta-synthase, 3-isopropyl malate dehydratase, nitrile hydratase, Urocanate hydratase, Uroporphyrinogen III synthase.
Carbon-Nitrogen Lyases:
Ammonia-Lyases: Histidine ammonia-lyase, Formiminotransferase cyclodeaminase, Serine dehydratase
Amidine-Lyases: Argininosuccinate lyase, Adenylosuccinate lyase
Carbon-Sulphur Lyases: Cystathionine gamma-lyase, Cystathionine-beta-lyase, Leukotriene C4 synthase
Carbon-Halide Lyases: Dichloromethane dehalogenase, Halohydrin dehalogenase
Phosphorus-Oxygen Lyases: guanylate cyclase, adenylate cyclase
Substrate Specificity
Usually, the specificity of a narrow substrate is considered to be a drawback for any enzyme's commercialization because it greatly restricts the enzyme's flexibility as an assistant in the related compound's production. Generally, lyases, but not all the time, are found with narrow substrate specificity. Most of the ammonia and hydratases-lyases possess a quite narrow substrate specificity, whereas the substrate specificity for decarboxylases, oxy nitrilases, and aldolases is much broader.
Note that the substrate specificity of a particular lyase varies based on its source. However, it is not given as an absolute prerequisite for enzymes to own the unrestricted substrate specificity for their commercial exploitation. Also, there are many lyases in commercial use bearing a narrow substrate spectrum.
Co-factor Requirements
The enzyme's commercial potential may be severely limited by the expensive cofactor's requirement. Since the addition catalyzed by lyase does not implicate the mere reduction or oxidation, it is not an important requirement for cofactors. However, till now, many lyases identified do require cofactors that are involved in the stabilization of reaction intermediates, substrate binding, substrate polarisation, temporary binding of the nucleophile, and more.
The majority of these cofactors are covalently bound to the enzyme and are not very expensive. Thereby, the lyases' cofactors do not establish a barrier to their commercialization. The requirements for lyases' cofactors differ as per their different sources.
Lyase Deficiency Disorder
Lyase deficiency, which is also referred to as HMG-CoA lyase deficiency, is given as a rare inherited disorder, which causes a mess in the metabolism of amino acid leucine and also prevents the body from synthesizing ketones, which are used for energy production during the body's food lacking periods. This condition could be inherited in an autosomal recessive pattern, which means that both copies of the gene in every cell undergo mutations.
Usually, the lyase deficiency symptoms express within the life's first year and primarily include vomiting, diarrhea, dehydration, lethargy, and weak muscular development. During the lyase deficiency episode, blood sugar concentrations may be either extremely low or hypoglycemic and a few harmful compounds would be accumulated to cause the blood to become quite acidic.
Whereas, if it is left untreated, the disorder could even initiate convulsions, coma, breathing problems, and also death. Infection, strenuous exercise, and other physical stresses may probably give rise to bouts with the symptoms of lyase deficiency.
FAQs on Lyase
1. What is a lyase enzyme and what is its main function in biochemistry?
A lyase is an enzyme that catalyses the breaking of various chemical bonds by means other than hydrolysis and oxidation, often forming a new double bond or a new ring structure. Its main function is to split a larger molecule into two smaller molecules without the addition of water, a process known as an elimination reaction.
2. Can you explain the action of a lyase with a specific example?
A classic example of a lyase is aldolase, which functions during glycolysis. It catalyses the cleavage of fructose-1,6-bisphosphate into two smaller sugar molecules: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. This reaction is a non-hydrolytic cleavage, meaning it does not use a water molecule to break the bond.
3. What is the primary difference between a lyase and a hydrolase?
The primary difference lies in their mechanism. A lyase breaks chemical bonds without using water (non-hydrolytic cleavage). In contrast, a hydrolase (EC 3) specifically uses a water molecule to break a bond, a process called hydrolysis. For example, a lyase might remove a group to form a double bond, while a hydrolase adds water to split a molecule.
4. How do lyases differ from ligases in their catalytic action?
Lyases and ligases perform opposite functions.
- Lyases (EC 4) break chemical bonds, splitting one molecule into two. For example, decarboxylase is a lyase that removes CO₂ from a substrate.
- Ligases (EC 6) join two molecules together, forming a new chemical bond. This process requires energy, usually supplied by ATP. For example, DNA ligase joins fragments of DNA.
5. How are lyase enzymes classified according to the CBSE syllabus for 2025-26?
As per the CBSE/NCERT curriculum for 2025-26, lyases (Enzyme Class 4) are classified based on the type of bond they cleave:
- Carbon-Carbon Lyases: These include decarboxylases (removing CO₂) and aldolases (cleaving aldehyde groups).
- Carbon-Oxygen Lyases: An example is dehydratase, which removes a water molecule.
- Carbon-Nitrogen Lyases: For instance, ammonia-lyases that eliminate ammonia.
- Other types: Including Carbon-Sulphur, Carbon-Halide, and Phosphorus-Oxygen lyases.
6. Why is aldolase, an enzyme in glycolysis, classified as a lyase?
Aldolase is classified as a lyase because it performs a non-hydrolytic cleavage of a carbon-carbon bond. During the fourth step of glycolysis, it splits the 6-carbon molecule, fructose-1,6-bisphosphate, into two 3-carbon products (DHAP and G3P). This reaction does not involve the addition of water (hydrolysis) or electron transfer (oxidation-reduction), which is the defining characteristic of a lyase enzyme.
7. How do lyases catalyse the formation of a double bond or a ring structure in a substrate?
Lyases catalyse elimination reactions by removing a group from a substrate, which results in the rearrangement of electrons to form a double bond or a new ring structure. The mechanism does not require an external co-substrate like water or an oxidant/reductant. For instance, a dehydratase (a type of lyase) removes a hydroxyl group (-OH) and a hydrogen atom (H) from adjacent carbons, leading to the formation of water and a C=C double bond in the substrate molecule.
8. What is the importance of lyase enzymes like adenylate cyclase in cellular signalling?
Lyases like adenylate cyclase are crucial for cellular communication. Adenylate cyclase catalyses the conversion of ATP into cyclic AMP (cAMP). cAMP acts as a vital second messenger in many biological processes. It amplifies signals from hormones and neurotransmitters, triggering a cascade of intracellular events that regulate metabolism, gene transcription, and other key cellular functions.
9. Under what conditions might a lyase-catalysed reaction be reversible?
A lyase-catalysed reaction can be reversible under specific cellular conditions, particularly when the concentration of the products is high. The reverse reaction, where a lyase joins two molecules, is known as a Michael addition. This reversibility is governed by the principles of chemical equilibrium (Le Chatelier's principle). If the products of the cleavage reaction accumulate, the equilibrium can shift, favouring the reverse reaction where the lyase synthesizes the larger molecule from its smaller components.
