Enzyme structure
By their chemical nature, enzymes are proteins assembled from amino acids on cellular ribosomes. Information about their structure is encoded in the genetic code. To perform their functions, the protein chain folds to form a complex spatial configuration—a tertiary or quaternary structure.
Often, an enzyme requires additional components for proper functioning. In such cases, its molecule is divided into two functional parts:
- Apoenzyme is the purely protein part of the molecule.
- Non-protein part is an additional component necessary for catalysis.
The non-protein part can exist in two forms. If it is an inorganic substance or metal ion, the component is called a cofactor. If it is an organic compound (for example, a vitamin derivative), it is called a coenzyme or coenzyme.
Key properties
The efficiency of biological catalysts is due to their unique properties, which distinguish them from catalysts in inorganic chemistry.
- High specificity. Each enzyme interacts only with a specific substance (substrate). This is because the spatial structure of the enzyme and its substrate correspond strictly to each other. In biology, this ideal match is described by the "lock-and-key" principle, where the lock is the enzyme and the key is the substrate.
- Sequence of action. Biochemical pathways in the cell are organized as chains. Several enzymes work in a strictly defined order: the product of a reaction catalyzed by one enzyme instantly becomes the substrate for the next.
Classification (6 main classes)
In modern biochemistry, the classification of enzymes is based solely on the type of catalyzed reaction. All known enzymes are divided into six major classes:
| Class | Catalyzed process |
|---|---|
| Oxidoreductases | Ensure the occurrence of oxidation-reduction reactions. |
| Transferases | Carry out the transfer of functional groups from one molecule to another. |
| Hydrolases | Catalyze hydrolysis—the breakdown of complex organic substances into simpler ones by adding a water molecule. |
| Lyases | Responsible for the non-hydrolytic addition or removal of various functional groups. |
| Isomerases | Catalyze isomerization reactions (intramolecular rearrangements). |
| Ligases (synthetases) | Ensure the synthesis of complex compounds using ATP energy. |
Biological function
The main task of enzymes is the incredible acceleration of chemical transformations. They can increase the rate of biochemical reactions millions of times compared to conditions without a catalyst.
In a living cell, no vital process can proceed spontaneously without the participation of enzyme systems. They make the following possible:
- DNA replication (doubling);
- Synthesis of all types of RNA and cellular proteins;
- Synthesis of the universal energy source—ATP;
- Photosynthesis processes in plants;
- Cellular respiration.
Briefly: answers to topical questions
What parts does the active site of an enzyme consist of?
The active site of an enzyme is a small region consisting of a combination of 12–16 amino acid residues that provides substrate binding and catalysis. Two parts are distinguished within it:
- Binding (contact, anchor) site fixes and holds the substrate through weak non-covalent bonds; it includes reactive amino acid residues.
- Catalytic site carries out conformational changes and transforms the substrate.
What mechanisms of regulation of enzyme activity exist?
In biological systems, the following mechanisms regulate enzyme activity:
- Allosteric regulation is carried out through the attachment of effectors (activators or inhibitors) to a spatially distant allosteric site.
- Covalent modification (phosphorylation-dephosphorylation) is the rapid reversible addition or removal of a phosphate group.
- Protein-protein interactions.
- Partial proteolysis is the activation of proenzymes (zymogens).
- Inhibition (reversible and irreversible, including competitive and non-competitive).
To memorize the six classes of enzymes, you can use the acronym: Oxidoreductases, Transferases, Hydrolases, Isomerases, Lyases, Ligases.
Class and exam questions
What is the basis for the classification of enzymes?
The classification is based exclusively on the type of chemical reaction catalyzed by the enzyme. According to this criterion, they are divided into 6 classes.
What is the difference between a cofactor and a coenzyme?
A cofactor is a metal ion or other inorganic substance. A coenzyme is an organic substance. Both are the non-protein part of a conjugated enzyme.
Can DNA replicate without enzymes?
No. No intracellular process, including DNA replication, RNA, protein, or ATP synthesis, can proceed without biological catalysts.
What else to study on this topic
- Mechanism of substrate-enzyme interaction ("lock-and-key" theory)
- Tertiary and quaternary structure of the protein molecule (apoenzyme)
- Connection of the genetic code with the synthesis of enzymes on ribosomes
- The role of metal ions and vitamins in the non-protein part
- Enzymatic chains in cellular respiration and photosynthesis
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