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Lyase – Definition, Function & Importance

Lyases are enzymes that break or form chemical bonds without the use of water. They play a central role in human metabolism and biochemical pathways.

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Things worth knowing about "Lyase"

Lyases are enzymes that break or form chemical bonds without the use of water. They play a central role in human metabolism and biochemical pathways.

What Is a Lyase?

A lyase is an enzyme that cleaves or forms chemical bonds – particularly carbon-carbon, carbon-oxygen, or carbon-nitrogen bonds – without requiring water or other molecules as co-reactants. This distinguishes lyases from hydrolases, which break bonds using water. According to the international enzyme classification system (EC nomenclature), lyases belong to enzyme class 4.

Function and Mechanism of Action

Lyases catalyze elimination reactions, in which a single substrate is split into two products, or the reverse reaction, in which two molecules are joined together. The products formed during cleavage often contain a new double bond or ring structure. Because many lyases can also catalyze the reverse reaction, they are sometimes referred to as synthases when acting in the biosynthetic direction.

Classification of Lyases

Lyases are classified into subclasses based on the type of bond they cleave:

Carbon-carbon lyases (EC 4.1): Cleave C-C bonds, e.g., decarboxylases that remove carboxyl groups.
Carbon-oxygen lyases (EC 4.2): Cleave C-O bonds, e.g., hydro-lyases and dehydratases.
Carbon-nitrogen lyases (EC 4.3): Cleave C-N bonds, e.g., ammonia lyases.
Carbon-sulfur lyases (EC 4.4): Cleave C-S bonds.
Carbon-halide lyases (EC 4.5): Cleave bonds between carbon and halogen atoms.
Phosphorus-oxygen lyases (EC 4.6): Cleave P-O bonds, e.g., adenylyl cyclase.

Role in Metabolism

Lyases are active in numerous vital metabolic pathways. Key examples include:

Aconitase: In the citric acid cycle, aconitase catalyzes the conversion of citrate to isocitrate.
Pyruvate decarboxylase: A carbon-carbon lyase that converts pyruvate into acetaldehyde and CO₂.
Aldolase: A carbon-carbon lyase that splits fructose-1,6-bisphosphate into two three-carbon compounds during glycolysis.
Adenylyl cyclase: A phosphorus-oxygen lyase that converts ATP into the important second messenger cAMP, regulating numerous signaling pathways.
Carbonic anhydrase: Catalyzes the interconversion of carbon dioxide and water to bicarbonate – essential for acid-base balance in the blood.

Clinical Relevance

Defects or dysfunction in lyases can lead to serious metabolic disorders. Examples include:

Phenylketonuria (PKU): A defect in phenylalanine hydroxylase (a lyase-related enzyme) leads to the accumulation of phenylalanine in the blood, causing neurological damage if untreated.
Fumarase deficiency: A rare defect in fumarate hydratase (fumarase), a carbon-oxygen lyase in the citric acid cycle, causes severe neurological impairment.
Primary hyperoxaluria: Caused by a defect in alanin-glyoxylate aminotransferase, leading to overproduction of oxalate and kidney damage.

Lyases are also pharmacologically important. Several medications work by inhibiting specific lyases. For example, carbonic anhydrase inhibitors such as acetazolamide block the corresponding lyase and are used in the treatment of glaucoma and epilepsy.

References

Nelson D. L., Cox M. M.: Lehninger Principles of Biochemistry. 8th edition. W. H. Freeman, New York 2021.
Stryer L., Berg J. M., Tymoczko J. L.: Biochemistry. 9th edition. W. H. Freeman, New York 2019.
International Union of Biochemistry and Molecular Biology (IUBMB): Enzyme Nomenclature. Available at: https://iubmb.qmul.ac.uk/enzyme/ (accessed 2024).

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