ENZYME INHIBITION

1. ENZYME INHIBITION M.Prasad Naidu MSc Medical Biochemistry, Ph.D.Research Scholar

2. Enzyme Inhibition Definition: Any substance that decreases the velocity of an enzyme catalysed reaction. Types of Inhibition: 1. Competitive inhibition 2. Non competitive inhibition 3. Allosteric inhibition 4. Suicide inhibition

4. Malonate

5. Statins inhibit HMG CoA reductase Statins

6. Methotrexate Folate reductase Folic acid Dihydrofolate O Dihydrofolate reductase Tetra hydrofolate

7. Methotrexate analogue of FH2 inhibits DHFR enzyme. Used in treatment of cancers. Sulfonamide PABA analogue PABA required for folate production in bacteria Sulfonamide prevent bacterial folate synthesis & prevent growth of bacteria.

10. Ethanol: analogue of methanol, used for the treatment of methanol intoxication. CH3OH HCHO (formaldehyde) AD HCOOH (Formic acid) HCHO Causes retinal damage & blindness HCOOH produces severe acidosis & death

11. CH3CH2OH CH3CHO CH3COOH NAD NADH CH3CHO Less toxic • Ethanol competes for alcohol dehydrogenase & prevents methanol toxicity.

13. 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

15. Allosteric Inhibition:- 1.In glycolysis Hexokinase Glucose glucose-6phosphate ATP ADP Glucose-6phosphate acts as allosteric inhibitor for hexokinase Phospho fructo kinase 2.Fructose-6phosphate fructose- 1,6bisphosphate ATP ADP

16. 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

17. CO2 4. Acetyl CoA Malanoyl CoA Acetyl CoA carboxylase Palmitate Palmitate acts as negative allosteric modulator for acetyl CoA caboxylase

20. 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.

21. 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.

22. Allopurinol Substrate analogue of Hypoxanthine 1.Hypoxanthine XO (xanthine oxidase) xanthine xanthine oxidase Uric acid

23. Allopurinol xo xo Alloxanthine Uric acid

24. xanthine oxidase inhibits 1. Allopurinol alloxanthine 2. 5fluorouracil is used in the treatment of cancer 5Fluorouracil 5fluoro-deoxyuridylate dUMP TMP Thymidylate synthase

25. Regulation of Enzyme Activity • Compartmentalization of Pathways • Induction & Repression of Enzymes • Degradation of Enzymes • Covalent Regulation • Allosteric Regulation • Iso enzymes

26. Compartmentalization Enzymes for fatty acid synthesis are present in cytosol Enzymes for oxidation of fatty acids are present in mitochondria.

27. 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.

29. 3. Enzyme degradation The regulatory /key enzymes are degraded if not needed and they are rapidly synthesized when required. Half lives of enzymes vary from one another.

30. 4.Covalent Regulation 1.Irriversible by hydrolysis of chemical bonds - proenzymes. eg: Inactive enzymes Active enzymes Zymogen / Proenzyme HCL Pepsinogen Pepsin 2. Reversible by phosphorylation and dephospharylation of enzymes eg: Glycogen phosphorylase ( a & b)

32. 5. Allosteric Regulation