Mechanism of Action and Distinctive Features
Enzymes of the transferase class exhibit strict specialization. Their primary and sole 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.
Specifically, transferases do not catalyze the following processes:
- Hydrolysis of peptide or any other bonds (performed by hydrolases);
- Intramolecular isomerization of a substrate molecule (function of isomerases);
- Oxidation of a substrate or electron transfer (task of oxidoreductases).
The essence of a transferase reaction always reduces to the interaction between two molecules: the first donates a specific chemical group, and the second accepts it, resulting in the formation of new compounds with modified properties.
Classification of Transferases
The systematic classification of enzymes in this class is based on the chemical nature of the group being transferred. Several key subclasses are distinguished:
- Aminotransferases — enzymes specialized in the transfer of amino groups.
- Acyltransferases — catalyze the transfer of acyl groups (carboxylic acid residues).
- Methyltransferases — facilitate the movement of methyl groups from a donor to an acceptor.
- Glycosyltransferases — transfer glycosyl residues, participating in the synthesis of carbohydrate chains.
- Kinases (or phosphotransferases) — a vital subclass responsible for the transfer of phosphate groups.
Aminotransferases: The Transamination Reaction
A classic example of transferase activity is the transamination reaction catalyzed by alanine-$\alpha$-ketoglutarate aminotransferase (abbreviated as ALT). According to classification, this enzyme belongs to the transferase class, aminotransferase subclass.
During this reaction, two initial reactants 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 successful catalysis of this reaction, ALT strictly requires a specific coenzyme — pyridoxal phosphate (often abbreviated as PLP in reaction schemes).
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 serves as the donor of a phosphoric acid residue. It is important to understand that in the context of protein kinase activity, ATP does not act as an electron acceptor, nor as an amino or methyl group donor. The core of the reaction is that the protein kinase cleaves a phosphate group from ATP and transfers it to a protein molecule, converting it into a phosphoprotein while leaving ADP as a byproduct.