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How does acetylcholinesterase break down acetylcholine?
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Enzyme
In the induced-fit theory of enzyme-substrate binding, a substrate approaches the surface of an enzyme (step 1 in box A, B, C) and causes a change in the enzyme shape that results in the correct alignment of the catalytic groups (triangles A and B; circles C and D represent substrate-binding groups on the enzyme that are essential for catalytic activity). The catalytic groups react with the substrate to form products (step 2). The products then separate from the enzyme, freeing it to repeat the sequence (step 3). Boxes D and E represent examples of molecules that are too large or too small for proper catalytic alignment. Boxes F and G demonstrate binding of an inhibitor molecule (I and I′) to an allosteric site, thereby preventing interaction of the enzyme with the substrate. Box H illustrates binding of an allosteric activator (X), a nonsubstrate molecule capable of reacting with the enzyme.
Acetylcholinesterase breaks down acetylcholine through a process known as a double displacement reaction, which involves the formation of a temporary covalent intermediate.1
The mechanism proceeds in these steps:
- Binding and Acylation: Acetylcholine binds to the enzyme's active site. A chemical bond forms between the acetyl group of the acetylcholine and a serine amino acid residue on the enzyme, creating an acyl–serine bond. This releases the first product, choline.1
- Deacylation: A water molecule then reacts with the acyl–serine bond. This reaction releases the second product, acetic acid.1
- Regeneration: Once the acetic acid dissociates, the enzyme is regenerated to its original state, allowing it to catalyze the breakdown of another acetylcholine molecule.1
By forming this covalent intermediate, the enzyme significantly accelerates the reaction rate while remaining chemically unchanged at the end of the process.1