Chapter 7 Irreversible Inhibitors PDF

Irreversible inhibitors Chapter 7  Contents Introduction. Agents that work as irreversible inhibitors. – Heavy metals – Organophosphates – Aspirin – Penicillin – Fluorouracil – Cyanide – Lopinavir & Tipranavir Natural irreversible inhibi...

Irreversible inhibitors Chapter 7  Contents Introduction. Agents that work as irreversible inhibitors. – Heavy metals – Organophosphates – Aspirin – Penicillin – Fluorouracil – Cyanide – Lopinavir & Tipranavir Natural irreversible inhibitors (SERPINS). – Alpha 1 protease inhibitor – Antithrombin  LEARNING OBJECTIVES On completion of this section, the student will be able to: Explain irreversible inhibition. List 6 irreversible inhibitors. In each case, describe their mechanism of action and their clinical significance. Describe the serine protease inhibitors (serpins) giving examples. How are you feeling about these learning objectives ⓘ Start presenting to display the poll results on this slide.  Irreversible inhibitors Shown on p 37 of text E+I EI Modify enzyme structure permanently through covalent bonds. Irreversible inhibitors = Inactivators – High Affinity (Low dissociation constants). Free enzyme is not regenerated, no constants are calculated, and no classical kinetics is applied. Exhibit time dependency, the degree of inhibition increasing with the amount of time the enzyme is in contact with the inhibitor (covalent bonds are slower to form).  Please download and install the Slido app on all computers you use Irreversible inhibitors of enzyme activity reduce reaction rates because they ⓘ Start presenting to display the poll results on this slide. Irreversible inhibitors of medical interest Heavy metals Organophosphates Aspirin Penicillin Fluorouracil Cyanide  Irreversible inhibition by covalent modification Figure 7.1 on p 37 of text Heavy metals (mercury) Active site of glyceraldehyde 3-phosphate DH (glycolysis) contains a thiol group  Nerve gas: organophosphates Organophosphates irreversibly inhibit AChE and paralyze diaphragm and intercostal muscles. ACh accumulates and receptors are desensitized. Aspirin inhibits the pathway of prostaglandin synthesis Aspirin irreversibly inhibits cyclooxygenase. – Reduces pain and inflammation. COX – 1 is constitutively expressed (immediate prostaglandin synthesis) and COX – 2 is rapidly inducible (inflammation). Acetylates a serine residue at enzyme active site in both enzymes (Ser 530 in COX-1 and Ser 516 in COX-2). Described on p 37  Aspirin inhibits COX isoenzymes Several isoenzymes of the cyclooxygenase have been identified, though COX-1 and COX-2 are the most widely known. Aspirin is approx. 170x more potent at inhibiting COX-1. – At low doses, aspirin selectively and irreversibly inhibits COX-1 – antiplatelet, antithrombotic activity. – At higher doses, aspirin also irreversibly inhibits COX-2. Penicillin irreversibly inhibits glycopeptide transpeptidase  Glycopeptide transpeptidase Catalyzes the final step in bacterial cell wall synthesis Penicillin resembles D-ala-D-ala moiety. It is an irreversible competitive inhibitor  Penicillin forms a stable complex with the enzyme glycopeptide transpeptidase Penicillin-enzyme complex is stable and cannot react further  Fluorouracil Fluorouracil is a thymine analog that is converted in vivo into fluorodeoxyuridylate. This agent first acts as a normal substrate for the enzyme thymidylate synthase before forming a covalent complex with the enzyme. The enzyme is thus irreversibly inhibited reducing thymidylate synthesis that is an essential precursor of DNA synthesis. Structure of some anticancer agents Figure 7.2 on p 38 of text Cyanide - irreversible inhibition of the respiratory chain Note cyanide’s irreversibly inhibition of cytochrome oxidase by binding to heme groups.  HIV infects cells Note role of viral protease: a target for therapy HIV protease Cleaves viral polyproteins. Essential for viral replication/maturation. Dimer of identical chains. Active site located between chains. Aspartate pair at active site. Aspartic protease (like pepsin)  Potent symmetric protease inhibitor Ritonavir & Saquinavir – 1st generation Lopinavir & Tipranavir – 2nd generation More in pipeline. Initially suffered from short t1/2. Problem with viral resistance to drug Protease inhibitors now used in combination with AZT (that inhibits viral reverse transcriptase). In theory combination therapy – “drug cocktail” – Enables lower doses to be used. – Once daily admin. See figure 7.3 p 36 Natural irreversible inhibitors (serpins) 1-protease inhibitor Inhibits leukocyte elastase. 1-protease inhibitor deficiency predisposes to emphysema. Antithrombin Inhibits thrombin and additional serine proteases in the clotting cascade (IXa, Xa, XIa and XIIa). Deficiency leads to reoccurring thrombosis. Activity increased more than 1000-times by heparin.  Please download and install the Slido app on all computers you use Which drug works by irreversible acetylation of serine residues at the active site of the target enzyme? ⓘ Start presenting to display the poll results on this slide.  Please download and install the Slido app on all computers you use Which anti-cancer drug works as an irreversible inhibitor of DNA synthesis? ⓘ Start presenting to display the poll results on this slide.  Summary Irreversible inhibitors bind to the enzyme molecule with high affinity to reduce enzyme activity. No kinetic studies can be performed due to the covalent nature of the enzyme-inhibitor interaction. Heavy metals, organophosphates, aspirin, penicillin, fluorouracil, Lopinavir and Tipranavir are all medically relevant irreversible inhibitors of enzyme action. Natural serpins, 1 protease inhibitor and antithrombin function as irreversible enzyme inhibitors in vivo.  Please download and install the Slido app on all computers you use Irreversible inhibitors of enzyme activity work because they ⓘ Start presenting to display the poll results on this slide.