Acetylcholine
Acetylcholine at the Wikipedia

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Acetylcholinesterase (e.g. from Torpedo californica) hydrolysizes the neurotransmitter acetylcholine , producing group. directly binds (via its nucleophilic Oγ atom) within the catalytic triad of (ACh/TcAChE structure 2ace). The residues are also important in the ligand recognition.