Patrick: An Introduction to Medicinal Chemistry 6e Chapter 7 ENZYMES AS DRUG TARGETS Modified

1. Overall Process of Enzyme Catalysis E + P S E ES P E EP Notes Binding interactions must be strong enough to hold the substrate sufficiently long for the reaction to occur Interactions must be weak enough to allow the product to depart Implies a fine balance Designing molecules with stronger binding interactions results in enzyme inhibitors which block the active site

2. Reversible Inhibitors Notes Inhibitor binds reversibly to the active site Intermolecular bonds are involved in binding The inhibitor undergoes no reaction Inhibition depends on the strength of inhibitor binding and inhibitor concentration Substrate is blocked from the active site Increasing substrate concentration reverses inhibition Inhibitor likely to be similar in structure to substrate, product or cofactor

2. Reversible Inhibitors ACE Inhibitors Examples Diuretics Protease inhibitors Statins Sulphonamides Kinase inhibitors modified Antidepressants

3. Irreversible Inhibitors OH X OH X O Covalent Bond Irreversible inhibition Notes Inhibitor binds irreversibly to the active site Covalent bond formed between the drug and the enzyme Substrate is blocked from the active site Increasing substrate concentration does not reverse inhibition Inhibitor likely to be similar in structure to the substrate

3. Irreversible Inhibitors Examples Nerve gases Penicillins Treatment of alcoholism Cephalosporins Proton pump inhibitors Anti-obesity modified

3. Irreversible Inhibitors Examples - orlistat Orlistat Pancreatic lipase Ser Pancreatic lipase Ser Pancreatic lipase Ser Middle and right hand structures refreshed Notes Orlistat is an anti-obesity drug that inhibits pancreatic lipase The enzyme is blocked from digesting fats in the intestine Fatty acids and glycerol are less absorbed as a result Leads to reduced biosynthesis of fat in the body

4. Allosteric Inhibitors (open) ENZYME Enzyme Induced fit Active site unrecognisable Active site ACTIVE SITE (open) ENZYME Enzyme Allosteric binding site Allosteric inhibitor Notes Inhibitor binds reversibly to the allosteric site Intermolecular bonds are formed Induced fit alters the shape of the enzyme Active site is distorted and is not recognised by the substrate Increasing substrate concentration does not reverse inhibition Inhibitor is not similar in structure to the substrate

4. Allosteric Inhibitors Example: 6-Mercaptopurine Notes Inhibits the first enzyme in the biosynthesis of purines Blocks the biosynthesis of purines and DNA Used in the treatment of leukaemia

5. Transition-state Inhibitors Notes Drugs designed to mimic the transition state of an enzyme-catalysed reaction Transition-state inhibitors are likely to bind more strongly than drugs mimicking the substrate or product Transition states are high energy, transient species and cannot be isolated or synthesised Drug design can be based on reaction intermediates which are closer in character to transition states than substrates or products Design a drug that mimics the stereochemistry and binding properties of the reaction intermediate, but is stable.

5. Transition-state Inhibitors Example: Renin inhibitors Renin Angiotensinogen Angiotensin I Angiotensin II Angiotensin converting enzyme (ACE) Notes Renin inhibitors block synthesis of angiotensin I and II Angiotensin II constricts blood vessels and raises blood pressure Renin inhibitors act as antihypertensives (lower blood pressure)

5. Transition-state Inhibitors Example: Renin inhibitors Reaction mechanism O A s p Renin H O H A s p Renin + O H A s p Renin Water substrate intermediate and products refreshed Notes Two aspartyl residues involved in enzyme-catalysed reaction Tetrahedral intermediate involved

5. Transition-state Inhibitors Example: Renin inhibitors Aliskiren Reaction intermediate Hydroxyethylene transition-state mimic Structures refreshed Notes Aliskiren contains a hydroxyethylene transition-state mimic Mimics the tetrahedral geometry of the reaction intermediate Mimics one of the hydroxyl groups (binding group) Stable - no leaving group present

5. Transition-state Inhibitors Other examples ACE inhibitors Statins Protease inhibitors Modifed All structures refreshed

6. Suicide Substrates Notes Agents which are converted to irreversible inhibitors by the enzyme-catalysed reaction React with the target enzyme once formed Example Trifluoroalanine as a suicide substrate of alanine transaminase Inhibition refreshed Transaminase enzyme

6. Suicide Substrates Reaction mechanism - alanine transaminase Base Enzyme Condensation + Hydrolysis Last two structrues redrawn as well as first Pyridoxal phosphate refreshed

6. Suicide Substrates Inhibition mechanism - alanine transaminase Base Enzyme Condensation Nu Enzyme Pyridoxal phosphate refreshed H+

6. Suicide Substrates Tienilic acid Notes Marketed as a diuretic Withdrawn due to interaction with cytochrome P450 enzymes Agent acts as a suicide substrate on cytochrome P450 enzymes

Enzyme alkylated and inhibited 6. Suicide Substrates Tienilic acid Tienilic acid Oxidation Cyt P450 NADPH O2 Suicide substrate Alkylation -H2O Hidden caption at the end Alltwo structures refreshed Enzyme alkylated and inhibited

7. Enzyme targets for useful medications 7.1 Antibacterial agents Dihydropteroate synthetase, transpeptidase 7.2 Antiviral agents HIV reverse transcriptase, HIV protease, viral DNA polymerase 7.3 Anti-inflammatory agents Cyclooxygenase 7.4 Cholesterol lowering agents HMG-CoA reductase 7.5 Antidepressants Monoamine oxidase 7.6 Anticancer agents Tyrosine kinase, dihydrofolate reductase, thymidylate synthase, aromatase etc

7. Enzyme targets for useful medications 7.7 Antihypertensive agents Renin, angiotensin converting enzyme 7.8 Treatment of male erectile dysfunction Phosphodiesterase 7.9 Anti-gout agents Xanthine oxidase 7.10 Anti-ulcer agents Proton pump 7.11 Alzheimers disease Cholinesterases 7.12 Diuretics Carbonic anhydrase