enzyme inhibition

M.Prasad NaiduMSc Medical Biochemistry,

Ph.D.Research Scholar

Definition: Any substance that decreases the velocity of an enzyme catalysed reaction.

Types of Inhibition:1. Competitive inhibition2. Non competitive inhibition3. Allosteric inhibition4. Suicide inhibition

Malonate

Statins

Statins inhibit HMG CoA reductase

Folate reductaseFolic acid Dihydrofolate

O Dihydrofolate reductase

Tetra hydrofolate

Methotrexate analogue of FH2 inhibits DHFR enzyme. Used in treatment of cancers.

Sulfonamide

PABA analoguePABA required for folate production in bacteriaSulfonamide prevent bacterial folate synthesis & prevent growth of bacteria.

Ethanol:analogue of methanol, used for the

treatment of methanol intoxication.

CH3OH HCHO (formaldehyde)

AD

HCOOH (Formic acid)HCHO Causes retinal damage & blindnessHCOOH produces severe acidosis & death

CH3CH2OH CH3CHO CH3COOH

NAD NADH

CH3CHO Less toxic

Ethanol competes for alcohol dehydrogenase &

prevents methanol toxicity.

Non Competitive Inhibition (Irreversible Inhibition)1.Heavy metals (mercury, lead, silver etc) bind covalently to – SH groups in the active site of the enzyme. They bind irreversibly they are highly toxic

E – SH + Hg2+ ES --- Hg2++ H+

Eg: delta ALA Porphobilinogen ALA dehydratase – Lead

Proto porphyrin IX Heme Fe2+

Ferrochelatase - Lead

2. Cyto chrome C oxidase inhibited by cyanide poisoning

Allosteric Inhibition:- 1.In glycolysis

HexokinaseGlucose glucose-6phosphate

ATP ADPGlucose-6phosphate acts as allosteric inhibitor for

hexokinase

Phospho fructo kinase2.Fructose-6phosphate fructose-

1,6bisphosphate

ATP ADP

ATP is negative allosteric modulator for PFK1

3. In TCA cycle ICD

Isocitrate alpha ketoglutarate

NAD NADH CO2

ATP is negative allosteric modulator for ICD

CO2

4. Acetyl CoA Malanoyl CoAAcetyl CoA carboxylase

Palmitate

Palmitate acts as negative allosteric modulator for acetyl CoA caboxylase

Suicide inhibition

It is a type of irreversible inhibition.

Also known as mechanism based inactivation.

Here , the structural analogue is converted into a

more effective inhibitor with the help of the

enzyme to be inhibted.

The substrate like compound initially binds with the

enzyme and the first few steps of the pathway

are catalysed.

This new product irreversibly binds to the enzyme and inhibits further reactions.

Substrate analogue of Hypoxanthine1.Hypoxanthine

XO (xanthine oxidase)

xanthine

xanthine oxidase

Uric acid

Allopurinol xo

xoAlloxanthine Uric acid

xanthine oxidase inhibits

1. Allopurinol alloxanthine

2. 5fluorouracil is used in the treatment of cancer

5Fluorouracil 5fluoro-deoxyuridylate

dUMP TMPThymidylate synthase

1. Compartmentalization of Pathways2. Induction & Repression of Enzymes3. Degradation of Enzymes4. Covalent Regulation5. Allosteric Regulation6. Iso enzymes

1. Compartmentalization

Enzymes for fatty acid synthesis are present in cytosolEnzymes for oxidation of fatty acids are present in mitochondria.

2. Induction & Repression Induction is increased synthesis of enzymes at

gene level through hormones or other substances.

Eg: Insulin induces the synthesis of glucokinase /glycogen synthase.

Repression is decreased synthesis of enzymes at gene level.Eg: Ala synthase is repressed by heme.

3. Enzyme degradationThe regulatory /key enzymes are degraded if not needed and they are rapidly synthesized when required.

Half lives of enzymes vary from one another.

4.Covalent Regulation

1.Irriversible by hydrolysis of chemical bonds - proenzymes.eg: Inactive enzymes Active enzymes

Zymogen / Proenzyme

HCLPepsinogen Pepsin

2. Reversible by phosphorylation and dephospharylation of enzymes

eg: Glycogen phosphorylase ( a & b)

5. Allosteric Regulation