Principles of lyase action
These enzymes perform two key, yet functionally opposite tasks. First, they are capable of removing specific groups ($CO_2$, $H_2O$, $NH_2$, $SH_2$) from substrate molecules. A crucial condition is that this occurs via a non-hydrolytic pathway, meaning water does not act as "scissors" (a cleaving agent).
Secondly, lyases can catalyze addition reactions. Most often this involves the addition of a small molecule (such as water) directly across the double bond of the substrate molecule, resulting in a change in its chemical structure.
Decarboxylation reactions
A classic example of group removal is decarboxylation. During this reaction, enzymes remove a carboxyl group (-COOH).
Let's examine this process using a specific example:
- Substrate: Glutamic acid (also known as glutamate).
- Enzyme: Glutamate decarboxylase.
- Coenzyme: Pyridoxal phosphate (PLP).
- Reaction products: $\gamma$-Aminobutyric acid (GABA) is formed and carbon dioxide ($CO_2$) is released.
Mechanism: The carboxyl group is removed from the $\alpha$-position of glutamic acid. This is precisely how a new molecule is formed through the removal of $CO_2$.
Water addition (hydratase) reactions
The second type of lyase reaction is addition. An example of a hydratase reaction is the interaction of a substrate with a water molecule.
- Substrate: Fumarate.
- Enzyme: Fumarate hydratase (or fumarase).
- Reaction equation: `Fumarate + H₂O` $\rightleftarrows$ `Malate`.
Mechanism: Water is added strictly across the double bond of the fumarate molecule. As a result, the double bond region ($CH=CH$) becomes saturated and converts into a single bond region with a hydroxyl group ($CH_2-CH(OH)$), forming malate. This reaction is reversible.
Briefly: answers to topical questions
To which class of enzymes do lyases belong according to the international classification?
According to the international classification, lyases belong to the fourth class of enzymes. All known enzymes are systematized into six main groups based on the type of catalyzed reaction, with lyases assigned a specific ordinal number.
What is the fundamental difference between lyases and hydrolases?
The fundamental difference lies in the mechanism of chemical bond cleavage and the involvement of water molecules in the catalytic process.
| Sign | Lyases | Hydrolases |
|---|---|---|
| Cleavage mechanism | Non-hydrolytic bond cleavage | Bond cleavage involving water |
| Reaction type | Group removal without water or addition across double bonds | Group transfer to water |
Which enzyme subclasses belong to lyases besides decarboxylases and hydratases?
Besides decarboxylases and hydratases, sources list the following as important subclasses of lyases:
- C-C lyases.
- C-O lyases.
- C-N lyases.
- C-S lyases.
Additionally, in the classification, class 4 is designated as lyases (synthases).
In which metabolic pathway does fumarate hydratase operate?
Fumarate hydratase (fumarase) functions in the aspartate-argininosuccinate shunt / cycle of regenerating aspartate from fumarate. [Wait, keeping exact meaning:] functions in the cycle of regenerating aspartate from fumarate. During this process, which is closely coupled with the tricarboxylic acid cycle reactions, this enzyme catalyzes the hydration of fumarate to form malate.
What is the biological role of GABA formed during glutamate decarboxylation?
GABA ($\gamma$-aminobutyric acid), formed during the decarboxylation of glutamic acid, serves as an inhibitory neurotransmitter in higher brain centers.
To remember the core concept, imagine that lyases are "dry scissors" (cleaving groups without water) or "glue for double bonds" (adding molecules across a double bond).
Class and exam questions
Do lyases use water to cleave bonds?
No. Lyases perform the removal of chemical groups ($CO_2$, $NH_2$, etc.) via a non-hydrolytic pathway, meaning water does not participate in the reaction as a cleaving agent.
Which group is removed during glutamate decarboxylation?
Upon the action of glutamate decarboxylase, the carboxyl group (-COOH) is removed from the $\alpha$-position of glutamic acid.
How does the bond structure change upon the action of fumarase?
Fumarase adds water across the double bond of fumarate ($CH=CH$). The double bond is broken, and $CH_2-CH(OH)$ is formed — the malate molecule.
What else to study on this topic
- Role and functions of $\gamma$-aminobutyric acid (GABA)
- Structure and functions of the coenzyme pyridoxal phosphate (PLP)
- Significance of the conversion of fumarate to malate
- Removal of other groups: ammonia ($NH_2$) and hydrogen sulfide ($SH_2$)
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