Enzymatic inhibition by drugs, Part 1

Summary

This lesson explains the concept of enzymatic inhibition and its significance in medications. Enzymes are proteins that speed up reactions in the body, and inhibitors can either temporarily or permanently block their activity. Reversible inhibitors, like competitive inhibitors, can be reversed by increasing substrate levels, while irreversible inhibitors, such as proton pump inhibitors and nerve gases, bind permanently to enzymes. The lesson also discusses the unique case of aspirin, which irreversibly inhibits cyclooxygenase in platelets, leading to prolonged effects. The difference in enzyme regeneration in platelets versus other cells is also explored.

Takeaways

• 🧬 Enzymes are proteins that catalyze reactions, significantly speeding up biochemical processes in the body.
• 🔴 Substrates are the molecules upon which enzymes act, forming enzyme-substrate complexes that produce specific products.
• ⚖️ Enzymatic inhibition is a key mechanism by which many medications exert their effects.
• 🟢 Reversible inhibitors are the most common type and are divided into competitive and non-competitive inhibitors.
• 🏁 Competitive inhibitors bind to the enzyme’s active site, competing directly with the substrate, and can be overcome by increasing substrate concentration.
• 🔄 Non-competitive inhibitors bind to an allosteric site, changing the enzyme’s shape and preventing substrate binding without competing at the active site.
• ⛔ Irreversible inhibitors bind covalently to enzymes, permanently inactivating them until new enzymes are synthesized, making their effects longer-lasting than reversible inhibitors.
• 💊 Examples of irreversible inhibitors include proton pump inhibitors (Omeprazole), aspirin (in platelets), penicillin (bacterial enzymes), and certain nerve gases.
• 🩸 Platelets lack a nucleus, so they cannot synthesize new enzymes; aspirin’s inhibition of platelet cyclooxygenase lasts the full lifespan of the platelet (~9 days).
• 🏭 Other cells can degrade drugs and synthesize new enzymes, so the effect of irreversible inhibitors in those cells is shorter compared to platelets.
• ⚡ Enzymes like acetylcholine esterase can degrade thousands of substrate molecules per second, highlighting why their inhibition can have significant physiological effects.
• ⏳ Irreversible inhibitors are time-dependent; their clinical effect may persist well beyond the drug’s half-life, which influences dosing schedules.

Q & A

What are enzymes, and why are they important in the body?

Enzymes are proteins that catalyze or speed up chemical reactions in the body. They play a crucial role in various biological processes by making reactions occur much faster than they would without the enzyme, often reducing reaction time from hours to milliseconds.

What is a substrate in the context of enzymatic reactions?

A substrate is the substance upon which an enzyme acts. It binds to the enzyme's active site, allowing the reaction to occur and eventually breaking down into products.

What are the two major types of enzymatic inhibitors?

The two major types of enzymatic inhibitors are reversible inhibitors and irreversible inhibitors.

What are competitive inhibitors, and how do they work?

Competitive inhibitors are a type of reversible inhibitor that competes with the substrate for binding to the enzyme's active site. The inhibitor prevents the substrate from binding, thereby reducing the enzyme's activity.

How can increasing the amount of substrate influence competitive inhibition?

Adding more substrate can overcome competitive inhibition because the substrate and the inhibitor are in competition for the enzyme's active site. Increasing the substrate concentration allows more substrate molecules to bind to the enzyme, reducing the effect of the inhibitor.

What is a non-competitive inhibitor, and how does it differ from a competitive inhibitor?

A non-competitive inhibitor binds to a different part of the enzyme, called the allosteric site, not the active site. This changes the enzyme's shape, preventing the substrate from binding, unlike competitive inhibitors that directly block the active site.

What are irreversible inhibitors, and how do they differ from reversible inhibitors?

Irreversible inhibitors bind to the enzyme, usually at the active site, and inactivate it permanently. Unlike reversible inhibitors, which can be displaced or metabolized, irreversible inhibitors cause a lasting effect on enzyme function.

How do proton pump inhibitors like Omeprazole function as irreversible inhibitors?

Proton pump inhibitors such as Omeprazole irreversibly bind to and inhibit the enzyme that produces stomach acid, reducing acid production. Despite their short half-life, they only need to be taken once a day due to their irreversible action.

Why do irreversible inhibitors have a longer-lasting effect than reversible inhibitors?

Irreversible inhibitors inactivate the enzyme permanently, and their effects last until the enzyme is replaced or metabolized. In contrast, reversible inhibitors can be displaced or broken down relatively quickly, so their effects are shorter-lived.

Why does aspirin have a longer-lasting effect on platelets compared to other tissues?

Aspirin irreversibly inhibits the enzyme cyclooxygenase (COX) in platelets, but since platelets lack a nucleus, they cannot produce new COX enzymes. This results in a prolonged effect, as the platelets cannot regenerate the enzyme, unlike other cells that can replace inhibited enzymes.