The effect of denaturing enzyme protein and causing loss of enzyme activity is called enzyme inactivation.
The effect of reducing enzyme activity without causing enzyme protein denaturation is called inhibition. Enzyme inhibitory effects include irreversible inhibition and reversible inhibition.
Some substances do not cause the enzyme protein to denature, but they can change certain essential groups (some groups on the active center) on the enzyme molecule, causing the enzyme activity to decline or even be lost, which are called enzyme inhibitors.
Scientists can use inhibitors to study enzymes; drug manufacturers can use inhibitors as drugs to treat diseases; and inhibitors can also be toxic.
Here in this article, enzyme inhibition are mainly discussed. Enzyme inhibition refers to the effect that the functional group of an enzyme is affected by a certain substance, resulting in a decrease or loss of enzyme activity. This substance is called an enzyme inhibitor.
Enzyme inhibitors are selective for enzymes and are an important tool for studying the mechanism of enzyme action. Many drugs, poisons and poisons used in chemical warfare are enzyme inhibitors. In addition, some biological macromolecules that exist in animals and plants with certain functions are also enzyme inhibitors. When the enzyme is inhibited, its protein portion is not denatured. Enzyme inactivation due to enzyme protein denaturation, and the reduction or loss of enzyme activity caused by the removal of activators (such as metal ions necessary for enzyme activity) are not included in the scope of enzyme inhibition.
Inhibitors can be divided into reversible inhibitors and irreversible inhibitors.
- Irreversible inhibitor
Irreversible inhibitors are mainly covalently bound to enzymes, reducing enzyme activity. Covalent binding is tightly bound, and physical effects such as simple dialysis and dilution cannot be used to remove the inhibitory effect.
- Reversible inhibitor
Reversible inhibitors bind through non-covalent bonds and have weak binding power, so they can both bind and dissociate easily, and quickly reach equilibrium. Reversible inhibitors are divided into two categories: competitive inhibitors and non-competitive inhibitors.
(1) The structure of competitive inhibitors is similar to that of substrates. It mainly binds to the binding groups of essential groups and competes with substrates for enzymes. The ability of the inhibitor to compete with the substrate for the enzyme's binding site depends on the concentration of both. If the inhibitor concentration is constant and the substrate concentration is low, the inhibitory effect is most obvious. As the substrate concentration increased, the enzyme-substrate complex concentration increased, and the inhibitory effect weakened. When the substrate concentration is much higher than the inhibitor concentration, almost all enzymes are taken by the substrate. At this time, the Vmax of the enzymatic reaction remains unchanged, but the Km value becomes larger.
Many drugs are competitive inhibitors of enzymes. Sulfa drugs have a similar structure to para-aminobenzoic acid, which is a substrate for dihydrofolate synthetase. Therefore, sulfa drugs competitively inhibit dihydrofolate synthase, causing bacteria to lack dihydrofolate or even tetrahydrofolate. Inability to synthesize nucleic acids and inhibit proliferation.
(2) The combination of non-competitive inhibitors with sites outside the active center of the enzyme does not affect the binding of the enzyme to the substrate, and the substrate does not affect the binding of the enzyme to the inhibitor. There is no competitive relationship between the substrate and the inhibitor. However, the substrate-enzyme-inhibitor complex cannot further release the product, so it is called non-competitive inhibitory effect, which shows a decrease in Vmax value and constant Km value.
(3) Anti-competitive inhibitors only bind to enzyme-substrate complexes, reducing the amount of intermediate products.
The significance of studying enzyme inhibitors
- Helps to study the catalytic mechanism of enzymes and the design and development of inhibitor-type drugs,such as anticancer drugs;
- Artificially regulate the metabolic pathways of the organism;
- It is helpful to study the relationship between the structure and function of enzymes.
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