## Mechanism of Acetylcholinesterase Action **Key Point:** Acetylcholinesterase (AChE) is a serine esterase that catalyzes the hydrolysis of acetylcholine (ACh) into choline and acetate, rapidly terminating neuromuscular transmission. ### Enzymatic Kinetics AChE is one of the fastest enzymes known, with a turnover rate of ~5,000 molecules per second. This rapid hydrolysis ensures: - Immediate cessation of postsynaptic receptor activation - Prevention of sustained depolarization - Restoration of muscle fiber excitability for the next action potential ### Location and Function - **Site:** Synaptic cleft, bound to the basal lamina of the neuromuscular junction - **Substrate:** Acetylcholine (released from presynaptic vesicles) - **Products:** Choline (recycled back to presynaptic terminal) + Acetate (diffuses away) **High-Yield:** The rate of ACh hydrolysis (~1 ms) is far faster than reuptake mechanisms, making enzymatic degradation the dominant termination mechanism at the NMJ. ### Clinical Correlation - **Anticholinesterase drugs** (e.g., neostigmine, edrophonium) inhibit AChE, prolonging ACh action—used diagnostically in myasthenia gravis and therapeutically in organophosphate poisoning - **Organophosphates** irreversibly phosphorylate the serine residue in AChE's active site, causing ACh accumulation and cholinergic crisis
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