Mechanism of action and distinctive features
Enzymes of the transferase class exhibit strict specialization. Their sole and primary task is to catalyze the transfer of functional groups. When studying biochemical reactions, it is important to clearly distinguish the action of transferases from other enzyme classes.
In particular, transferases do not catalyze the following processes:
- hydrolysis of peptide or any other bonds (which is performed by hydrolases);
- intramolecular isomerization of a substrate molecule (the function of isomerases);
- substrate oxidation or electron transfer (the task of oxidoreductases).
The essence of a transferase reaction always boils down to the interaction of two molecules: the first donates a specific chemical group and the second accepts it, resulting in the formation of new compounds with altered properties.
Classification of transferases
The systematization of enzymes in this class is based on the chemical nature of the group they transfer. Several key subclasses are distinguished:
- Aminotransferases — enzymes specialized in transferring the amino group.
- Acyltransferases — catalyze the transfer of acyl groups (carboxylic acid residues).
- Methyltransferases — ensure the movement of methyl groups from a donor to an acceptor.
- Glycosyltransferases — transfer glycosyl residues, participating in the construction of carbohydrate chains.
- Kinases (or phosphotransferases) — a crucial subclass responsible for the transfer of the phosphate group.
Aminotransferases: transamination reaction
A classic example of transferase action is the transamination reaction catalyzed by the enzyme alanine $\alpha$-ketoglutarate aminotransferase (abbreviated as ALT). According to the classification, this enzyme belongs to the class of transferases, subclass of aminotransferases.
During this reaction, two starting reagents interact:
- Alanine (acts as the amino group donor);
- $\alpha$-Ketoglutarate (accepts the amino group).
As a result of the chemical transfer, new products are formed:
- Pyruvate;
- Glutamic acid (Glutamate).
For the successful catalysis of this reaction, the ALT enzyme vitally requires a specific coenzyme — pyridoxal phosphate (often designated by the abbreviation PLP in diagrams).
Phosphotransferases: mechanism of protein phosphorylation
Another equally important example of a transferase reaction is protein phosphorylation. This process is catalyzed by enzymes belonging to the phosphotransferase subclass (most commonly referred to as protein kinases).
The reaction equation is as follows: `Protein + ATP` $\xrightarrow{\text{Protein kinase}}$ `Phosphoprotein + ADP`
In this biochemical system, the ATP molecule plays a strictly defined role. It is a phosphoric acid residue donor. It is important to understand that in the context of protein kinase function, ATP does not act as an electron acceptor, nor as an amino or methyl group donor. The entire essence of the reaction is that the protein kinase cleaves the phosphate group from ATP and transfers it to the protein molecule, turning it into a phosphoprotein and leaving ADP as a byproduct.
Briefly: answers to topical questions
What products are formed in the transamination reaction under the action of aspartate aminotransferase (AST)?
The products of the transamination reaction under the action of aspartate aminotransferase are α-ketoglutarate and aspartate, and in the case of the reverse reaction — oxaloacetate and glutamate.
Depending on the direction of the process, the following are formed:
- α-Ketoglutarate and Aspartate — synthesized in the forward reaction from glutamate and oxaloacetate.
- Oxaloacetate (OAA) and Glutamate — are products of the reverse reaction upon the interaction of aspartate and α-ketoglutarate.
What coenzyme or group donor do methyltransferases use?
Methyltransferases use the active form of methionine — S-adenosylmethionine (SAM) as a methyl group donor.
Examples of using this donor:
- Epinephrine synthesis — SAM donates a methyl group during the conversion of norepinephrine under the action of phenylethanolamine N-methyltransferase.
- Histamine inactivation — methylation of histamine to methylhistamine by the enzyme histamine N-methyltransferase.
During these reactions, after donating the methyl group, SAM is converted into S-adenosylhomocysteine (SAH).
What is the difference between the mechanism of action of kinases and phosphorylases?
The main difference according to the cited sources lies in the utilization of the phosphate group.
| Characteristic | Kinases (phosphotransferases) | Glycogen phosphorylase |
|---|---|---|
| Source/participant of the phosphate group | ATP is a phosphoric acid residue donor | Glycogenolysis occurs with the participation of inorganic phosphate ($P_i$) |
| Mechanism according to sources | Transfer a phosphate group, for example during protein phosphorylation: `Protein + ATP → Phosphoprotein + ADP` | Catalyzes glycogen breakdown: `Glycogen → Glucose-1-phosphate` |
| Examples | Protein kinase; protein kinase A phosphorylates phosphorylase kinase and glycogen synthase | Glycogen phosphorylase participates in glycogen mobilization |
The term "transferase" is easily deciphered through the English word "transfer" — movement, relocation. Imagine a courier who picks up a package (the functional group) from the sender (the donor molecule) and delivers it to the recipient (the acceptor molecule).
Class and exam questions
What is the main function of transferases?
Transferases catalyze chemical reactions of functional group transfer from one compound to another. They are not involved in bond hydrolysis, oxidation, or molecule isomerization.
What role does ATP play in reactions involving protein kinases?
In kinase reactions (protein phosphorylation), the ATP molecule is exclusively a donor of the phosphoric acid residue, which is transferred to the protein.
What products are formed in the ALT-catalyzed reaction?
Upon the interaction of alanine and $\alpha$-ketoglutarate under the action of alanine $\alpha$-ketoglutarate aminotransferase (ALT), pyruvate and glutamic acid (glutamate) are formed.
Which coenzyme is required for aminotransferase activity?
For transamination reactions involving aminotransferases (including the ALT enzyme) to proceed, the coenzyme pyridoxal phosphate (PLP) is required.
What else to study on this topic
- Structural organization and ALT isoenzymes
- Mechanism of action of the pyridoxal phosphate (PLP) coenzyme
- Phosphorylation cascades and the role of protein kinases in the cell
- Differences between acyltransferases and methyltransferases with specific examples
- Significance of transamination reactions for amino acid metabolism
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