Any decline in the velocity of an enzymatic reaction is called enzyme inhibition. A variety of small molecules can reduce the rate of enzyme catalyzed reaction. they are termed as enzyme inhibitors.
Enzyme inhibition mainly due to physical factors, substrate limitation, accumulation of end product or due to presence of chemical agents which compete the substrate for the enzyme called as inhibitors.
The term inhibition in enzymatic reaction often refers to the decrease in velocity is called chemical inhibitors. many drugs and poisonous compounds also act as enzyme inhibitors. chemical inhibion can be divided in to two distinctive types such as covalent and (irreversable) and non-covalent (reversable) inhibition.
1. Covalent inhibition:
This inhibition is permanent and irreversible, this means enzyme once inhibited further it cannot reactivated. If the inhibitor binds to enzyme, the enzymes makes covalent modification in their structure, it resulted into changes in active site, so that the enzyme no longer binds to the substrate molecule. The inhibition continues until all the enzyme molecules get inhibited.
Examples for covalent inhibition
- Cyclooxigenase (COX) is the prostaglandin and thrombaxane biosynthetic enzyme, inhibited irreversably by non steroidal anti inflomatory drugs serine and aspirine causes the acetylation of active site in COX.
- Cyanide and Azide are the poisonous molecules enters the body through respiration and they can inhibit the enzyme cytochrome oxidase.
- insectisides parathion and malathion as well as neurotoxic gas sarine can inhibit the enzyme acetyl choline esterase.
- Iodo aceate and iodo acetamide can inhibit the enzyme containing the thiol functional group in its active site such enzyme is Glyceraldehyde 3-phosphate dehydrogenase.
- inhibition of enzymes elastase, serine protease, tripsin and chimotripsin by Di idopropyle phasphofloridate (DIPF).
- Some metal ions also can inhibit the enzymes, such ions are Hg, Hg2+ and pb2+.
2. Non covalent inhibition
This inhibition is temporary and reversible, this means once the enzyme inhibited further it can get reactivation. in this inhibition, inhibitory molecule forms very week non-covalent interactions with active site of enzymes. the inhibitor from active site can be easily removed by performing simple techniques such as dialysis and dilution. This inhibition continues until an equilibrium is reached, after which no change is occur in the degree of inhibition.
Depending on the effect of inhibitory molecule on the kinetic parameters non-covalent inhibition has been divided into four distinct types.
3. Competitive inhibition
This occurs when a inhibitory molecule has a structure sufficiently resembles to that of the normal substrate to be able to fit into the active site. A competitive inhibitor can bind only to free enzyme, it cannot bind to enzyme-substrate complex. the genuine substrate and inhibitor therefore compete for a position in the active site. the binding of competitive inhibitor and substrate mutually exclusive; when substrate binds to the enzyme the inhibitor cannot bind and similarly when the inhibitor bins the active site the substrate cannot bind. A characteristic feature of competitive inhibition is that when the substrate concentration is increased, the rate of reaction increases, as well as if the inhibitor concentration more than the substrate the inhibition becomes significant. in the presence of competitive inhibitor the Km increase but the V max remain same. Vmax can be achieved by increasing the substrate concentration.
Competitive inhibition has been considered in the development of pharmaceutical drugs for treating harmful diseases.
Examples for competitive inhibitors
- sulfa drugs are structural analogues of Para amino benzoic acid, substrate for folic acid. This drug used in the inhibition of folic acid biosynthesis in bacteria.
- Aminopterin and methotrexate are another type of folate analogues used in the treatment of cancer to inhibit the dihydroxy folate reductase.
- inhibition of succinate dehydrogenase by melonate and inhibition of PEP carboxylase by malate.
- inhibitory reaction requires the transfer of amino group of glutamine by glutamine analogue such as Azaserine.
- Inhibition of HMG Co.A reductase by lovastatin and provostatin , cholesterol lowering drugs.
- Non-competitive inhibition
(i) Non-competitive reversible inhibition
Inhibitor has no structural similarity to the substrate and combines with the enzyme at a position other than its active site. A non-competitive inhibitor can bind either to free enzyme or enzyme-substrate complex. The bindings of the substrate and the inhibitor to the enzymes are independent events. It does not affect the ability of the substrate to bind with the enzyme, but it makes it impossible for catalysis to take place. The rate of reaction decreases with increasing inhibitor concentration. When inhibitor reaches to saturation point, the rate of the enzymatic reaction will be almost zero. It is a characteristics feature of this type of inhibition that if increases in substrate concentration does not affect the overall rate of reaction, unlike with competitive inhibition. For example cyanide kills an animal by inhibiting cytochrome oxidase.
(ii) Non-competitive irreversible inhibition
A verity of chemicals cause irreversible non-competitive inhibition of enzymes. Very small concentration of chemical reagents Permanently inhibit activity of some enzymes.
Example: Heavy metal ions such as mercury, silver and arsinic or certain iodine-containing compounds.
4. Uncompetitive inhibition
In uncompetitive inhibition the inhibitor binds only to enzyme-substrate complex but not to the free enzyme. This is because the inhibitor binding site will be concealed in free enzyme, where as the binding of substrate leads to conformational changes in enzyme structure resulting in the opening of inhibitor binding site. this kind of inhibitor produces the more amount of enzyme-substrate complex and therefore decreasing the Km for substrate. The formed Enzyme-substrate-inhibitor (ESI) complex cannot produce the required product, therefore the Vmax are decreased.
5. Mixed type inhibition
Mixed type inhibition is similar to competitive and non-competitive inhibition. In this inhibition, inhibitor can bind either to free enzyme or enzyme-substrate complex. This kind of inhibition is often characteristic feature of multi-substrate reaction having obligatory sequential binding of substrate molecules, when the binding fails to follow the sequential order it can resulting into mixed inhibition.