Cholinergic Pharmacology Quiz

See Acetylcholine Life Cycle. Option C is correct because choline acetyltransferase (ChAT) is the enzyme that makes acetylcholine (ACh) in the nerve terminal. It takes choline, a small molecule that the terminal recovers from the synaptic cleft (the tiny gap between a nerve ending and its target), and transfers an acetyl group onto it from acetyl-CoA, the cell's carrier of acetyl groups. The enzyme's name describes exactly this job. Its activity is the rate-limiting step, meaning the slowest step, which sets the pace of the whole pathway. Because only cholinergic neurones (neurones that release ACh) contain ChAT, the enzyme is also used as a marker for them. Once made, ACh is packed into synaptic vesicles (small membrane bubbles) by the vesicular acetylcholine transporter (VAChT). An arriving action potential opens calcium channels, calcium triggers the vesicles to fuse with the membrane, and ACh is released into the cleft, where acetylcholinesterase (AChE) breaks it down within moments. Each enzyme or transporter has its own step, and the names tell you which: synthesis, storage, breakdown. The cycle in order:

• ChAT makes ACh from choline and acetyl-CoA in the terminal;
• VAChT packs ACh into vesicles for storage;
• calcium-triggered release sends ACh across the cleft;
• AChE splits ACh into acetate and choline, and choline is taken back up for reuse.

• Enzyme: choline acetyltransferase (ChAT) makes acetylcholine in the nerve terminal.
• Reaction: it transfers an acetyl group from acetyl-CoA onto choline.
• Rate-limiting step: ChAT activity sets the pace of acetylcholine synthesis.
• Marker: only cholinergic neurones contain ChAT.