Enzyme Inhibition PDF

3. Enzyme Inhibition  Prof / Ayman El Baz Professor of Medical Biochemistry & Molecular Biology Dr/ Sara El Derbaly Assistant Professor of Medical Biochemistry& Molecular Biology  Learning Outcomes At the end of the lecture, the students should be...

3. Enzyme Inhibition  Prof / Ayman El Baz Professor of Medical Biochemistry & Molecular Biology Dr/ Sara El Derbaly Assistant Professor of Medical Biochemistry& Molecular Biology  Learning Outcomes At the end of the lecture, the students should be able to: Define enzyme inhibition. Describe competitive inhibition of enzymes. Identify the different examples for non-competitive inhibition of enzymes.  Case Scenario Marian was diagnosed with acute gouty arthritis involving her right great toe. The presence of insoluble urate crystals within the joint space confirmed the diagnosis. Several weeks after her acute gout attack subsided, Marian was started on allopurinol therapy.  Enzyme inhibition Enzyme inhibition is one way of regulating enzyme activity. Most therapeutic drugs function by inhibition of a specific enzyme. Enzyme inhibitors are reversible or Irreversible ❑Enzyme Inhibition means decreasing or cessation of the enzyme activity. ❑The inhibitor is the substance that decreases or abolishes the rate of enzyme action. ❑According to the similarity between the inhibitor and the substrate, enzyme inhibition is either: ❖ Competitive inhibition: Reversible ❖ Non-competitive inhibition: Irreversible  I- Competitive Inhibition There is structural similarity between the inhibitor and substrate. The inhibitor and the substrate compete with each other to bind to the same catalytic site of the enzyme. The inhibition is reversible. It can be relieved by increasing substrate concentration. It does not affect Vmax but It increases Km.  Competitive inhibition 1. Inhibitor competes with substrate 2. Structurally is similar to the substrate 3. Inhibition is reversible 4. Inhibitor does not bind with E-S complex 5. Inhibition is relieved in excess substrate concentration 6. Inhibitor with enzyme forms enzyme inhibitor complex (E-I) Enzyme behavior in presence of competitive inhibitor: Vmax is unchanged. Km is increased, more substrate is needed to reach the same half maximum velocity Vmax - inhibitor Reaction Rate + competitive Vmax Inhibitor 2 K m Km −𝟏\𝐊𝐦 [Substrate]  Examples and Clinical Use Enzymatic process Substrate Competitive Inhibitor (drug) 1. Folic acid synthesis Para aminobenzoic Sulfonilamide in bacteria (dihydropteroate acid (PABA) (antibiotic) synthase (DHPS)) 2. Prothrombin synthesis Vitamin K Dicumarol (vitamin K epoxide reductase) (anticoagulant) 3. Xanthine oxidase Xanthine converted Allopurinol (zyloric) to uric acid (Gout) 4. Alcohol dehydrogenase Methanol converted Ethanol to formaldehyde (converted to (Optic neuropathy acetaldehyde) and blindness)  II- Non-competitive inhibition There is no structural similarity between the inhibitor and the substrate. The inhibitor does not bind to the catalytic site as the substrate but it binds to another site. It can bind enzyme to form (EI) complex or to enzyme substrate (ES) to form (ESI)complex. The inhibition is irreversible. It cannot be relieved by increasing substrate concentration. It decreases Vmax. It does not affect Km.  Non-competitive inhibition Not resemble substrate Binds to site other than active site Can bind with E or ES complex Usually, It is irreversible Increasing substrate concentration does not abolish inhibition Irreversible Non-competitive inhibitors Enzyme behavior in presence of non-competitive inhibitor: Vmax is decreased Km remains unchanged.. V ma x - Inhibitor Reaction Rate V max,app 1 V + Inhibitor 2 m ax 1 2 V max,app K m Km,app [Substrate] Non-competitive inhibition may be caused by: Inhibition of sulphahydryl group Inhibition of cofactors Inhibition of specific ion activator A) Inhibition of Sulphahydryl (-SH) group Many enzymes depend on free SH group for its activity. SH group can be inhibited by heavy metals poisoning As mercury (Hg++) & lead (Pb++) block SH group of enzymes. B) Inhibition of cofactors ✓ Coenzyme inhibition Cyanide hydrazine inhibits the action of coenzyme pyridoxal phosphate (PLP) PLP is coenzyme for many reactions as: transamination, decarboxylation, and deamination ✓ Prosthetic group inhibition Carbon monoxide (CO) and cyanide block the iron in the heme which is the prosthetic group of cytochrome oxidase enzyme. C) Inhibition of metal ion activator Chelation of calcium ions by oxalate prevents blood coagulation. As Ca++ is needed to activate thrombokinase enzyme which converts the inactivate prothrombin to active thrombin that initiates blood clotting. Chelation of Mg ions by fluoride inhibits glycolysis As Mg ions are essential for activity of enolase enzyme of glycolysis Application??? Competitive & Non-competitive Inhibition  Case Report and Clinical Correlates Sarah is being treated with allopurinol for gout, which is caused by an accumulation of sodium urate crystals in joints and joint fluid, particularly in the ankle and great toe. Allopurinol is an inhibitor of the enzyme xanthine oxidase, which is involved in formation of uric acid (urate). Hypoxanthine level increases in the presence of allopurinol & it is excreted in the urine. Brainstorming question (Case Scenario) Ahmed was diagnosed with ischemic heart disease. The presence of repeated attacks of acute chest pain and ECG changes confirmed the diagnosis. Ahmed was started on dicumarol therapy. How dicumarol could help him? Enzyme Inhibition means decreasing or stop in the enzyme activity Enzyme inhibition: competitive and non-competitive. Summary Competitive Inhibition: There is structural similarity between & the inhibitor and substrate. Wrap up Non-Competitive Inhibition: There is No structural similarity between the inhibitor and substrate. Non-Competitive Inhibition: inhibition of SH group, cofactors, specific ion activator.  References Vasudevan’s Textbook of Biochemistry For Medical Students, 7th Edition. Chatterjea’s Textbook of Medical Biochemistry, 8th edition. BRS Biochemistry, Molecular Biology, and Genetics, 5th Edition.