Catalytic function of proteins
The main and most striking property of certain proteins is their ability to act as biological catalysts. In biochemistry, these proteins are called enzymes or enzymes. Without their involvement, life as we know it would be impossible, since the rates of chemical reactions at body temperature would be too low. According to the fundamental principles of bioorganic chemistry, absolutely all biochemical transformations in the cells and tissues of the body proceed only with the direct participation of the enzymatic apparatus. This means that metabolism is entirely subject to the catalytic action of protein molecules.
Types of catalyzed reactions
Biological catalysts govern a colossal spectrum of chemical transformations. The entire array of enzymatic processes can be divided into several key directions:
- Synthesis and breakdown of biopolymers: enzymes control both the assembly processes of complex macromolecules and their hydrolysis (cleavage involving water). The same applies to the metabolism of complex esters.
- Redox processes: enzymes ensure the transfer of electrons and protons, catalyzing the oxidation and reduction of various organic substances, which is critical for cellular respiration and energy metabolism.
- Isomerization: molecular rearrangements within a single molecule without changing its qualitative and quantitative composition are also governed by specialized proteins.
- Transfer of functional groups: movement of chemical groups from a donor molecule to an acceptor molecule.
- Photosynthesis: the highly complex cascade of reactions converting light energy into chemical bond energy is impossible without the participation of a specific enzymatic complex.
Unique enzyme specificity
Unlike many inorganic catalysts that can accelerate a broad class of reactions, enzymes possess high, often absolute, specificity. This means that for every specific biochemical reaction in the body, there exists a strictly defined, specific enzyme. One reaction — one catalyst.
Furthermore, enzymology distinguishes the concept of species specificity. Enzymes exist in a vast multitude of varieties. The structure and properties of these varieties are finely tuned and strictly correspond to the physiological and evolutionary features of each specific biological species. Thus, an enzyme performing the same function in a bacterium, a plant, and a human will exhibit significant structural differences.
Catalytic efficiency
The efficiency of enzymes astonishes synthetic chemists. Biological catalysts allow reactions to proceed under mild intracellular conditions that are considered extremely difficult to achieve within traditional inorganic chemistry and require extreme temperatures, pressures, or aggressive media.
A classic example of such unsurpassed efficiency is the fixation and reduction of atmospheric nitrogen ($N_2$). The nitrogen molecule possesses an extremely strong triple bond, which is notoriously difficult to cleave in a laboratory setting. However, in the cells of certain bacterial species, this process occurs continuously and without the expenditure of vast amounts of external energy — solely due to the potent catalytic action of specific enzymatic systems.
Briefly: answers to topical questions
How do enzymes differ from inorganic catalysts?
Enzymes differ from inorganic catalysts by their high reaction rate, specificity, and capacity for regulation.
| Sign | Characteristics of enzyme differences |
|---|---|
| Reaction rate | Several thousand times higher |
| Specificity | Highly substrate-specific |
| Operating conditions | Mild intracellular conditions |
| Regulation | The reaction rate can be regulated |
| Dependence on quantity | Rate is proportional to the amount of enzyme |
What are the classes of enzymes according to the international classification?
According to the international classification, all enzymes are divided into 6 main classes depending on the type of catalyzed reaction.
- Oxidoreductases — catalyze oxidation-reduction reactions.
- Transferases — carry out the transfer of groups (amino, phosphate).
- Hydrolases — catalyze the hydrolysis of bonds involving water.
- Lyases — catalyze the non-hydrolytic cleavage of bonds.
- Isomerases — carry out isomeric transformations within a single molecule.
- Ligases — catalyze synthesis coupled with ATP consumption.
What components make up a complex enzyme (holoenzyme)?
A complex enzyme (holoenzyme) consists of a protein part and a non-protein component.
Its structure features the following elements:
- Apoenzyme — the protein part of the enzyme.
- Cofactor — the non-protein part, represented by metal ions or an inorganic substance.
- Coenzyme — an organic non-protein substance (often a vitamin derivative).
- Prosthetic group — a non-protein component tightly and permanently bound to the enzyme.
The combination of an apoenzyme with a cofactor or coenzyme forms the active form — the holoenzyme.
What is the active site of an enzyme, and what regions are distinguished within it?
The active site of an enzyme is a small region composed of amino acid residues, located at the periphery of the molecule or close to it.
Two regions are distinguished within the active site:
- Binding (contact) site — the region where the substrate attaches to the enzyme via weak non-covalent bonds.
- Catalytic site — the region that transforms the substrate held by the binding site.
What factors influence the rate of an enzymatic reaction?
The rate of an enzymatic reaction is influenced by the physicochemical conditions of the medium, the concentration of the reactants, and the presence of regulators.
Main factors:
- Amount of enzyme — the reaction rate is proportional to its concentration.
- Temperature — as it increases up to the optimum, the rate rises, then drops sharply due to thermal denaturation of the protein.
- pH value — the graph has a bell-shaped appearance with an optimum near intracellular values.
- Substrate concentration — velocity is proportional to the product of the concentrations of the reacting substances.
- Effectors — activators and inhibitors alter the affinity of the enzyme for the substrate.
What types of enzyme inhibition exist?
Enzyme inhibition is divided into reversible and irreversible.
- Irreversible inhibition — the inhibitor forms covalent bonds with the enzyme, causing a permanent loss of activity.
- Competitive reversible inhibition — the inhibitor interacts with the active site. The Michaelis constant increases, but the maximum velocity remains unchanged.
- Noncompetitive reversible inhibition — the inhibitor binds to the allosteric site. The Michaelis constant does not change, but the maximum reaction velocity decreases.
- Mixed inhibition — both the Michaelis constant and the maximum velocity values change.
To remember the essence of enzymes, use the rule "SER": Specificity (each reaction has its own), Efficiency (they outperform inorganics, as in the case of nitrogen), Reactions (they accelerate all processes: from hydrolysis to photosynthesis).
Class and exam questions
What is the chemical basis of any enzyme?
All enzymes are proteinaceous in their chemical nature and possess pronounced catalytic activity.
Can a single enzyme catalyze completely different types of reactions?
No, enzymes exhibit strict specificity. For each specific biochemical reaction in the body, there is an individual enzyme.
What is the main advantage of enzymes over inorganic catalysts?
They possess colossal efficiency, allowing hard-to-achieve reactions (such as the fixation of atmospheric nitrogen $N_2$) to be carried out under mild physiological conditions.
What else to study on this topic
- Mechanisms of biopolymer and ester hydrolysis
- Oxidation-reduction enzymatic systems
- The role of enzymes in functional group transfer and isomerization reactions
- The enzymatic apparatus of photosynthesis
- Biochemical mechanisms of atmospheric nitrogen fixation in bacteria
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Other topics in "Chapter 18. Major Classes of Bioorganic Compounds"
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