Lyases: Function, Classification, and Biological Roles

Lyases are a major class of enzymes that break chemical bonds through a mechanism fundamentally different from most other biological catalysts: instead of adding water across a bond (hydrolysis) or transferring electrons (oxidation-reduction), they strip atoms from a molecule to create a new double bond, or work in reverse to add atoms across an existing one. This elimination-based chemistry gives lyases a distinctive niche, and they turn up in processes ranging from the most basic steps of sugar metabolism to DNA repair, breathing, neurotransmitter production, and plant defense. Their classification is broad, spanning at least five recognized subclasses defined by the type of bond they act on, and their medical relevance stretches from rare genetic disorders to cancer biology to approved pharmaceuticals.

What Makes Lyases Different From Other Enzymes

Most enzymes that break molecules apart do so by recruiting water. Digestive enzymes, for instance, snap peptide bonds or sugar linkages by slipping a water molecule into the break. Lyases skip that step entirely. They remove a group of atoms from a substrate to generate a double bond (or, in the reverse direction, add a group across a double bond). The result is an elimination reaction rather than a hydrolytic one. Polysaccharide lyases, for example, cleave sugar chains using elimination rather than the hydrolytic mechanism employed by glycoside hydrolases, producing an unsaturated product at the cleavage site.1Biochemical Journal. A hierarchical classification of polysaccharide lyases for glycogenomics

Because they do not need water or a separate oxidant, lyases can operate under cellular conditions where those co-substrates might be limiting. They can also run in both directions. Many lyases catalyze bond formation just as readily as bond cleavage, and which direction predominates depends on the concentrations of reactants in the cell. This reversibility is not just a laboratory curiosity; it is central to how several metabolic pathways operate in living organisms.

How Lyases Are Classified

The Enzyme Commission system groups lyases as EC 4 and then subdivides them by which bond they break. The five main subclasses cover most known lyases, though the boundaries can blur when an enzyme acts on more than one bond type.

These categories are tidy on paper, but individual lyases can surprise researchers. Aldolase, for instance, is classified as a carbon-carbon lyase for its glycolytic role, yet it participates in numerous non-enzymatic functions in the cell that have nothing to do with bond cleavage.6PubMed Central. Multifunctional Fructose 1,6-Bisphosphate Aldolase as a Therapeutic Target Enzymes do not always respect the categories we assign them.

Lyases in Everyday Metabolism

Several of the most heavily trafficked metabolic pathways depend on lyases at key steps. Aldolase sits in the fourth reaction of glycolysis, the pathway every cell uses to extract energy from glucose. It splits fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, two smaller molecules that continue through the pathway to yield ATP.2PubMed. Snapshots of catalysis: the structure of fructose-1,6-(bis)phosphate aldolase covalently bound to the substrate dihydroxyacetone phosphate Without aldolase, glycolysis stalls at step four and the cell cannot finish extracting energy from sugar.

Fumarase occupies a similarly critical junction in the citric acid cycle, the hub of aerobic energy metabolism. It catalyzes the reversible hydration of fumarate to malate, with the direction of the reaction depending on which form of the enzyme is protonated and which substrate is available.7PubMed Central. Identification of the catalytic mechanism and estimation of kinetic parameters for fumarase Fumarase also shows up later in the article in connection with the urea cycle, where it recycles one of the cycle’s byproducts back into mainstream metabolism.

A third metabolically pivotal lyase is ATP-citrate lyase (ACLY), a cytoplasmic enzyme that generates acetyl-CoA, the building block cells use to manufacture fatty acids, cholesterol, and related lipids.8PubMed. ATP citrate lyase knockdown induces growth arrest and apoptosis through different cell- and environment-dependent mechanisms ACLY essentially bridges the energy-generating citric acid cycle and the biosynthetic pathway for fats, making it a gatekeeper for lipid production in the cell.

Breathing and Blood Chemistry

Carbonic anhydrase is one of the fastest enzymes known, and it functions as a carbon-oxygen lyase that interconverts carbon dioxide and bicarbonate. Every breath you take depends on it. In red blood cells, carbonic anhydrase converts dissolved CO₂ into bicarbonate for transport to the lungs; in the lungs, it reverses the reaction so CO₂ can be exhaled. Mice engineered to lack carbonic anhydrase II develop a mixed respiratory and metabolic acidosis, with a blood pH around 7.18 (normal is roughly 7.4) and elevated CO₂ levels, because their bodies cannot efficiently move carbon dioxide out of the tissues and into the air.9PubMed. Respiratory acidosis in carbonic anhydrase II-deficient mice The CO₂ retention in those animals stems from the enzyme’s absence in both red blood cells and lung tissue.

That same enzyme is a drug target. Inhibitors of carbonic anhydrase reduce the production of aqueous humor in the eye, lowering intraocular pressure in glaucoma. Systemic inhibitors such as acetazolamide are potent but carry more side effects, so topical versions like dorzolamide and brinzolamide are more commonly prescribed.10Медицинская этика. Carbonic anhydrase inhibitors for the treatment of glaucoma

Signaling Molecules and Neurotransmitters

Lyases do not just move metabolites around. They also produce molecules that cells use to communicate. Glutamate decarboxylase (GAD) is a carbon-carbon lyase that removes a carboxyl group from the amino acid glutamate, releasing CO₂ and producing gamma-aminobutyric acid (GABA), the brain’s primary inhibitory neurotransmitter.11PubMed Central. Insights and progress on the biosynthesis, metabolism, and physiological functions of gamma-aminobutyric acid (GABA): a review GAD requires pyridoxal 5′-phosphate (a form of vitamin B₆) as a cofactor, which is why severe B₆ deficiency can cause seizures: without enough cofactor, GABA production drops and the brain loses its main brake on neural excitation.

On the vascular side, cystathionine gamma-lyase (CSE) is the major source of hydrogen sulfide (H₂S) in blood vessels.12PubMed. Dysregulation of cystathionine γ-lyase (CSE)/hydrogen sulfide pathway contributes to ox-LDL-induced inflammation in macrophage H₂S has emerged as a gaseous signaling molecule with anti-inflammatory and anti-atherosclerotic effects. CSE also participates in the transsulfuration pathway, converting cystathionine into the amino acid cysteine; during that process it can generate cysteine persulfide, a reactive sulfur species that actually feeds back to inactivate the enzyme itself, providing a built-in off switch.13PubMed Central. Cystathionine γ-Lyase Self-Inactivates by Polysulfidation during Cystine Metabolism

Adenylyl cyclase, the phosphorus-oxygen lyase that makes cyclic AMP, rounds out the signaling trio. cAMP is one of the most widespread second messengers in animal cells, relaying signals from hormones and neurotransmitters to the cell interior. Without lyase activity at this step, entire cascades of hormone signaling would go silent.

Repairing Damaged DNA

Ultraviolet light fuses adjacent thymine bases in DNA into cyclobutane pyrimidine dimers, one of the most common forms of DNA damage. Photolyase is a lyase that uses blue light to split those dimers back into normal bases. The enzyme transfers an electron from a light-excited flavin cofactor to the dimer, breaking the cyclobutane ring; the electron then returns to the cofactor, leaving the DNA repaired and the enzyme unchanged.14PubMed Central. Direct observation of thymine dimer repair in DNA by photolyase The entire photocycle finishes in well under a nanosecond. Ultrafast spectroscopy shows that the dimer splits in two sequential steps within about 90 picoseconds, with the electron tunneling between cofactor and substrate through a route that involves an intervening adenine residue.15PubMed Central. Dynamics and mechanism of cyclobutane pyrimidine dimer repair by DNA photolyase

Humans lack functional photolyase (we rely on other repair systems for UV damage), but many bacteria, plants, and some animals retain it. Its speed and precision have made it a model system for studying how enzymes manipulate electrons at ultrafast timescales.

Plant Defense Through Phenylalanine Ammonia-Lyase

In plants, phenylalanine ammonia-lyase (PAL) sits at a metabolic crossroads. It strips ammonia from the amino acid phenylalanine, shunting the carbon skeleton into the phenylpropanoid pathway, which produces lignin for structural support, pigments like anthocyanins, and a battery of antimicrobial and antioxidant compounds.16PubMed Central. Phenylalanine Ammonia-Lyase: A Core Regulator of Plant Carbon Metabolic Flux Redistribution PAL essentially decides how a plant distributes its carbon between growth and defense.

That trade-off plays out vividly in plant-microbe interactions. In the legume Lotus japonicus, knocking down PAL reduces salicylic acid production (a key defense hormone) and loosens cell walls, which allows symbiotic root-nodule bacteria to infect more easily. Overexpressing PAL has the opposite effect: thicker, more lignified cell walls and fewer bacterial infections.17PubMed. The Phenylalanine Ammonia Lyase Gene LjPAL1 Is Involved in Plant Defense Responses to Pathogens and Plays Diverse Roles in Lotus japonicus-Rhizobium Symbioses PAL activity thus shapes both the physical structure and the chemical defenses of plant tissues.

When Lyases Malfunction

Genetic defects in lyases cause several recognized metabolic diseases. Argininosuccinate lyase (ASL) catalyzes the fourth step of the urea cycle, breaking argininosuccinic acid into arginine and fumarate. Deficiency of ASL is the second most common urea cycle disorder, occurring in roughly 1 in 70,000 live births.18PubMed Central. Argininosuccinate lyase deficiency It can present as a severe neonatal crisis with dangerously high blood ammonia in the first days of life, or as a milder late-onset form with episodic vomiting, developmental delay, and liver problems.19PubMed Central. Argininosuccinate lyase deficiency: longterm outcome of 13 patients detected by newborn screening Even children detected early through newborn screening may develop long-term complications including cognitive difficulties and high blood pressure.

ACLY, the lipid-pathway lyase described earlier, has drawn attention in cancer research. Tumor cells frequently ramp up ACLY to feed their voracious appetite for fatty acids and cholesterol. Knocking down ACLY in cancer cells can trigger growth arrest and cell death, though the exact mechanism varies between cell types and environments.8PubMed. ATP citrate lyase knockdown induces growth arrest and apoptosis through different cell- and environment-dependent mechanisms A sugar-sensing modification on ACLY has been found to couple glucose availability directly to lipid production, helping explain why cancer cells that consume large amounts of glucose can also synthesize fats so rapidly.20PubMed Central. O-GlcNAcylation of ATP-citrate lyase couples glucose supply to lipogenesis for rapid tumor cell proliferation

Microbial Lyases as Virulence Factors

Some pathogenic bacteria weaponize lyases to invade host tissues. Streptococcus agalactiae, a leading cause of neonatal sepsis and meningitis, secretes a hyaluronate lyase that degrades hyaluronan, a major structural component of connective tissue, along with certain chondroitin sulfates. By enzymatically dissolving these tissue barriers, the bacterium clears a path deeper into the host.21Journal of Biological Chemistry. Hyaluronan Binding and Degradation by Streptococcus agalactiae Hyaluronate Lyase Similar hyaluronate lyases appear in other streptococcal species and in Staphylococcus aureus, making them potential targets for anti-virulence therapies that would disarm bacteria without necessarily killing them.

Industrial and Pharmaceutical Uses

The reversibility of lyase reactions makes them attractive tools for chemical manufacturing. Running a carbon-nitrogen lyase in the bond-forming direction allows chemists to attach an amine group across a double bond with precise three-dimensional control, producing optically pure amino acids and heterocyclic compounds that are difficult to make by traditional synthesis.22PubMed Central. Recent Applications of Carbon‐Nitrogen Lyases in Asymmetric Synthesis of Noncanonical Amino Acids and Heterocyclic Compounds Hydroxynitrile lyases are used similarly: they catalyze the stereoselective formation of cyanohydrins and can also perform nitroaldol reactions, creating chiral building blocks for pharmaceuticals and agrochemicals.23European Journal of Organic Chemistry. Hydroxynitrile Lyase Discovery, Engineering, and Promiscuity towards Asymmetric Synthesis: Recent Progress

On the pharmaceutical side, PAL has made the unusual journey from plant enzyme to approved human drug. Researchers recognized decades ago that PAL could degrade phenylalanine, the amino acid that accumulates to toxic levels in people with phenylketonuria (PKU). Early attempts to give the enzyme by mouth showed minimal effect. Success came only after cloning the gene, expressing the protein in engineered cells, and coating it with polyethylene glycol (PEGylation) so the immune system would tolerate it by injection. The result, pegvaliase (brand name Palynziq), is now approved as an enzyme substitution therapy for adults with PKU.24PubMed. Phenylalanine ammonia lyase (PAL): From discovery to enzyme substitution therapy for phenylketonuria In animal models of PKU, treatment with pegvaliase increased the number of certain neurons in multiple brain regions compared to untreated controls, suggesting the therapy may partially rescue some neurological damage caused by elevated phenylalanine.25PubMed. Partial rescue of neuropathology in the murine model of PKU following administration of recombinant phenylalanine ammonia lyase (pegvaliase)

Cofactors, Metal Dependencies, and Structural Flexibility

Lyases rely on a varied toolkit of cofactors. GAD needs vitamin B₆ derivatives. Photolyase uses a flavin cofactor. PAL depends on a self-generated MIO cofactor built from three of its own amino acids. Many polysaccharide lyases require metal ions, and the specific metal matters for both activity and evolutionary history. Polysaccharide lyase family 2 enzymes, for instance, are divided into two subfamilies: one that uses magnesium and works outside the cell, cleaving sugar chains internally (endolysis), and another that uses manganese, operates inside the cell, and trims chains from the ends (exolysis). The two subfamilies likely arose by gene duplication and then diverged in function.26PubMed Central. Functional Analyses of Resurrected and Contemporary Enzymes Illuminate an Evolutionary Path for the Emergence of Exolysis in Polysaccharide Lyase Family 2

Metal identity does not always dictate activity, though. A dimethylsulfoniopropionate (DMSP) lyase involved in marine sulfur cycling was tested with six different metal ions and showed similar reaction energy barriers for five of them, as long as they shared the same three-histidine-one-glutamate coordination. Only copper, which lost one histidine ligand and adopted a two-histidine-one-glutamate arrangement, showed reduced activity. And a strongly oxidizing ion like ferric iron inactivated the enzyme altogether by stealing an electron from a nearby amino acid.27PubMed. Reaction mechanism of the PuDddK dimethylsulfoniopropionate lyase and cofactor effects of various transition metal ions The implication is that for some lyases, the protein scaffold around the metal matters more than the metal itself.

Beyond cofactor chemistry, some lyases depend on large-scale protein motions to function. An allosteric polysaccharide lyase studied using biophysical techniques revealed two distinct modes of structural dynamics: an intrinsic flexibility that allows the enzyme to toggle between open and closed conformations, and a separate substrate-triggered enhancement of movement within the closed state.28PubMed Central. Distinct Modes of Hidden Structural Dynamics in the Functioning of an Allosteric Polysaccharide Lyase Both modes are required for the enzyme to work properly. This kind of hidden choreography is increasingly recognized as important across enzyme families, but lyases have become a particularly productive system for studying it because their elimination mechanism is sensitive to even small changes in how the active site is positioned.