Enzyme Inhibition Definition: The decline in the velocity of an enzymatic reaction is called enzyme inhibition.
In other words, enzyme inhibition refers to a reduction in enzyme-related processes, enzyme activity, or enzyme’s productivity.
A variety of small molecules can reduce the rate of enzymatic reaction. Hence, these molecules are called enzyme inhibitors.
Enzyme inhibition is mainly due to physical factors, substrate limitation, accumulation of end product or the presence of chemical agents (inhibitors) which compete with the substrate for the same active site of an enzyme.
An inhibition in an enzymatic reaction often refers to the decrease in velocity by the chemical compounds are called chemical inhibitors. Many drugs and poisonous compounds also act as enzyme inhibitors. Chemicals-mediated enzyme inhibition can be divided into two distinctive types such as covalent (irreversible) and non-covalent (reversible) inhibition.
Different types of enzyme inhibition
Covalent inhibition is permanent and irreversible, which means if an enzyme inhibited by an inhibitor then it cannot be reactivated again. Upon binding of inhibitors to enzymes, the enzymes undergo covalent modification in their structure, resulting in changes in their active site. So, these enzymes no longer bind to the substrate molecules. Moreover, this covalent inhibition continues until all the enzyme molecules get inhibited.
Examples for covalent inhibition
- Cyclooxygenase (COX) is a prostaglandin and thromboxane biosynthetic enzyme, which is irreversibly inhibited by non-steroidal anti-inflammatory drugs, serine and aspirin cause the acetylation of COX active site.
- Cyanide and azide are the most poisonous molecules that enter the body through respiration and inhibit an enzyme, cytochrome oxidase.
- Insecticides, parathion, malathion and neurotoxic gas, sarin can inhibit an enzyme, acetylcholinesterase (AChE).
- Iodoacetate and iodoacetamide can inhibit an enzyme, glyceraldehyde 3-phosphate dehydrogenase which contains the thiol functional group in its active site.
- Inhibition of enzymes including elastase, a serine protease, trypsin and chymotrypsin by diisopropyl fluorophosphate (DIPF) is the best example for covalent inhibition.
- Some metal ions (e.g., Hg, Hg2+ and pb2+) also inhibit the functions of several enzymes covalently.
Non-covalent inhibition is temporary and reversible, which means once an enzyme inhibited later it can get reactivation. In this inhibition, inhibitory molecule forms very week non-covalent interactions with the active site of enzymes. An inhibitor from the enzyme’s active site can easily be removed by using simple techniques including dialysis and dilution. This inhibition continues until an equilibrium is reached, after which no changes occur in the degree of inhibition.
Depending on the effect of inhibitory molecules on enzyme kinetic parameters, the non-covalent inhibition has been divided into four distinct types.
1. Competitive inhibition
Competitive inhibition occurs when an inhibitory molecule has a structure that sufficiently resembles that of the normal substrate to be able to fit into the active site. A competitive inhibitor can bind only to a free enzyme but not to an enzyme-substrate complex. The real substrate and inhibitor, therefore, compete for the same position in an active site.
The binding of competitive inhibitor and substrate is mutually exclusive; when the substrate binds to an enzyme then the inhibitor cannot bind and similarly when an inhibitor binds an active site then the substrate cannot bind.
The main characteristic feature of competitive inhibition is that when the substrate concentration increases, then the rate of reaction increases, while if the inhibitor concentration is more than the substrate concentration, then, the rate of inhibition becomes significant.
In the presence of competitive inhibitor, the Km increase but the Vmax (Maximal Velocity) remain same. However, Vmax can be achieved by increasing the substrate concentration.
Competitive inhibition has been considered in drug development for treating harmful diseases.
Examples of competitive inhibitors
- sulfa drugs are structural analogues of Para-aminobenzoic acid, a substrate for folic acid. This drug used in the inhibition of folic acid biosynthesis in bacteria.
- Aminopterin and methotrexate are other types of folate analogues used in the treatment of cancer to inhibit the dihydroxy folate reductase.
- inhibition of succinate dehydrogenase by malonate and inhibition of PEP carboxylase by malate.
- The inhibitory reaction requires the transfer of an amino group of glutamine by glutamine analogue such as Azaserine.
- Inhibition of HMG Co.A reductase by lovastatin and provostatin, cholesterol-lowering drugs.
2. Non-competitive inhibition
Non-competitive reversible inhibition
In non-competitive reversible inhibition, an inhibitor has no structural similarity to the substrate and combines with the enzyme at a position other than the active site. A non-competitive inhibitor can bind either to a free enzyme or an enzyme-substrate complex.
The bindings of the substrate and an inhibitor to the enzymes are independent events. It does not affect the ability of the substrate to bind with an enzyme, but it makes an enzyme impossible for its catalysis process to takes place. In which, the rate of reaction decreases with increasing an inhibitor concentration. When the inhibitor reaches saturation point, then the rate of enzymatic reaction will be almost zero.
It is a characteristics feature of this type of inhibition that if increases in substrate concentration there will be no changes in the overall rate of reaction, which is unlike with competitive inhibition. For example, cyanide kills an animal by inhibiting an enzyme, cytochrome oxidase.
Non-competitive irreversible inhibition
A variety of chemicals involved in irreversible non-competitive inhibition of the enzymatic reaction are reported. A very small amount of chemical reagents can permanently inhibit the activity of enzymes.
Example: The compounds containing heavy metal ions like mercury, silver and arsenic or certain iodine could inhibit enzyme activity at their lower concentrations.
3. Uncompetitive inhibition
In uncompetitive inhibition, an inhibitor binds only to the enzyme-substrate complex but not to the free enzyme. This is because the inhibitor binding site will be concealed in a free enzyme, whereas the binding of the substrate leads to conformational changes in the enzyme structure resulting in the opening of the inhibitor binding site.
These kinds of inhibitor produce a more amount of enzyme-substrate complex and therefore decreasing the Km for the substrate. The formed enzyme-substrate-inhibitor (ESI) complex cannot produce the required product, therefore the Vmax will be decreased.
4. Mixed type enzyme inhibition
Mixed type inhibition is similar to competitive and non-competitive inhibitions. In this inhibition, the inhibitor can bind either to the free enzyme or enzyme-substrate complex. This kind of inhibition is often a characteristic feature of multi-substrate reactions having an obligatory sequential binding of substrate molecules and when the binding fails to follow the sequential order then it can result in mixed inhibition.