During a clinical biochemistry practical, a student observes enzyme kinetics data from a patient with a metabolic disorder. All of the following are true about irreversible enzyme inhibition EXCEPT:

Explanation

## Irreversible vs. Reversible Enzyme Inhibition ### Fundamental Mechanism of Irreversible Inhibition **Key Point:** Irreversible inhibitors form **covalent bonds** with the enzyme (usually with active site residues), permanently destroying catalytic activity. Once the inhibitor is bound, no amount of substrate can displace it. **High-Yield:** The critical distinction: - **Reversible inhibitors** (competitive, non-competitive, uncompetitive, allosteric) form **non-covalent interactions** (H-bonds, van der Waals, electrostatic) that can be overcome by changing conditions or substrate concentration. - **Irreversible inhibitors** form **covalent bonds** that cannot be broken by substrate competition. ### Comparison Table | Feature | Reversible Inhibition | Irreversible Inhibition | | --- | --- | --- | | **Bond type** | Non-covalent (H-bonds, van der Waals) | Covalent | | **Overcome by ↑[S]?** | Yes (competitive); No (non-competitive) | **NO** — never | | **Recovery** | Spontaneous dissociation | Requires new enzyme synthesis | | **Kinetics** | Follows Michaelis-Menten | Time-dependent, pseudo-first-order | | **Examples** | Statins (competitive), cyanide (non-competitive) | Penicillin, aspirin, organophosphates | ### Clinical Examples of Irreversible Inhibition 1. **Penicillin:** Covalently acylates serine-405 in bacterial transpeptidase → cross-linking of peptidoglycan is blocked → cell wall rupture. 2. **Aspirin:** Acetylates serine-529 in COX-1 and COX-2 → irreversible inhibition of prostaglandin and thromboxane synthesis. 3. **Organophosphate pesticides:** Phosphorylate serine residue in acetylcholinesterase active site → acetylcholine accumulates → cholinergic crisis. 4. **Cyanide:** Binds irreversibly to Fe³⁺ in cytochrome c oxidase → blocks electron transport chain → cellular hypoxia. **Warning:** Do NOT confuse cyanide (irreversible, non-competitive) with competitive inhibitors — cyanide cannot be overcome by any amount of substrate or oxygen. ### Why Option 2 Is Incorrect Irreversible inhibition, by definition, **cannot be overcome by increasing substrate concentration**. Once a covalent bond forms between inhibitor and enzyme, substrate molecules cannot displace the inhibitor. This is the fundamental difference from competitive inhibition. ### Verification of Other Options 1. **Option 0 (True):** Irreversible inhibitors form covalent bonds, permanently inactivating the enzyme. 2. **Option 1 (True):** Penicillin acetylates serine in transpeptidase, a classic example of irreversible inhibition. 3. **Option 3 (True):** Aspirin acetylates serine-529 in COX, irreversibly blocking prostaglandin synthesis.