Enzymes: Biological Catalysts and Their Mechanisms
Enzymes: Biological Catalysts
Enzymes are biological catalysts (proteins). They do not disturb the balance of the reaction. Enzymes contain an active site within the globular protein structure. They are highly specific. Specificity is determined by the complementarity between the active site and the substrate. They possess great catalytic power (106 or more) and are not permanently altered by the reaction.
The enzyme provides necessary proximity and orientation for the reaction to occur. There is a complementary structural, stereospecific, and electrostatic interaction between the substrate and the active site.
Types of Enzymes
- Oxidoreductases (Dehydrogenases): Catalyze oxidation-reduction reactions.
- Transferases: Catalyze the transfer of functional groups.
- Hydrolases: Catalyze hydrolysis reactions, where water is the acceptor of a transferred group.
- Lyases: Catalyze the breaking of bonds with electronic rearrangement.
- Isomerases: Catalyze isomerization reactions.
- Ligases: Catalyze the joining or ligation of two substrates, requiring energy (ATP).
Enzyme Kinetics and Inhibition
Competitive Inhibition: An inhibitor structurally similar to the substrate competes for binding to the active site.
The enzyme binds selectively to the substrate, creating suitable environmental conditions for the substrate to be changed and transformed into product. The enzyme forms a complex with the substrate, leading to an orientation of the molecules and multiple weak bonds between the enzyme and substrate. Once the complex is formed, catalysis is favored under tension of the substrate and the active site amino acids are capable of donating or capturing atoms.
Consequently, the enzyme-product (EP) complex dissociates. The enzyme binds via weak bonds, dissociates, and releases the product and free enzyme. The free enzyme can then bind to another substrate molecule.
Factors Affecting Enzyme Activity
pH: pH values modify enzyme activities. Changes in pH alter the state of ionization of functional groups, so small variations can produce significant changes in reaction rates.
The addition of a competitive inhibitor reduces the reaction rate but does not affect the maximum velocity (Vmax). The Michaelis constant (Km) is higher in the presence of the inhibitor.
Non-competitive Inhibition: Km is not affected, but Vmax decreases because the enzyme is not as catalytically efficient in the presence of the inhibitor.
Regulation of Enzymatic Activity
- Covalent modification
- Allosteric modulation
- Synthesis or degradation of the enzyme
- Partial proteolysis
Allosteric Enzymes
The activity of an enzyme is regulated by modulators. An allosteric inhibitor (negative allosteric modulation) binds to a different location than the active site, causing a conformational change that renders the enzyme inactive. Many allosteric enzymes have a quaternary structure and exhibit cooperativity in substrate binding. An activator stabilizes the active form of the enzyme, while an inhibitor stabilizes the inactive form. In some allosteric enzymes with quaternary structure, one or more subunits may have no catalytic role but serve as regulators. These subunits contain a site for the allosteric binding of modulators.
Allosteric Regulation
Allosteric regulation often presents a form of feedback regulation. An allosteric enzyme produces a product that another enzyme uses as a substrate, and so on. Thus, the final product can regulate the initial enzyme in the pathway. If the cell uses a large quantity of the product, it may be toxic. The product can then act as an allosteric inhibitor of the first enzyme, reducing the amount of product produced.
Cofactors
Some enzymes require an additional component in the active site to perform their catalytic activity.
These can be iometallic ions or complex molecules called coenzymes (e.g., NAD, FAD, ATP). The binding may involve weak bonds.
Many enzymes require the presence of other non-protein substances, called cofactors, for their performance.
The metal ion cofactors can be large and complex molecules (cofactors).
The protein that binds to the cofactor is called the apoenzyme. The holoenzyme is the complex of the apoenzyme and cofactor: Holoenzyme = apoenzyme + cofactor.