Neurotransmitters acetyl-coenzyme

The neurotransmitter must be present in presynaptic nerve terminals and the precursors and enzymes necessary for its synthesis must be present in the neuron. For example, ACh is stored in vesicles specifically in cholinergic nerve terminals. It is synthesized from choline and acetyl-coenzyme A (acetyl-CoA) by the enzyme, choline acetyltransferase. Choline is taken up by a high affinity transporter specific to cholinergic nerve terminals. Choline uptake appears to be the rate-limiting step in ACh synthesis, and is regulated to keep pace with demands for the neurotransmitter. Dopamine [51 -61-6] (2) is synthesized from tyrosine by tyrosine hydroxylase, which converts tyrosine to L-dopa (3,4-dihydroxy-L-phenylalanine) (3), and dopa decarboxylase, which converts L-dopa to dopamine. 

Acetylcholine synthesis. Acetylcholine (ACh) is a prominent neurotransmitter, which is formed in cholinergic neurons from two precursors, choline and acetyl coenzyme A (AcCoA) (Fig. 12—8). Choline is derived from dietary and intraneuronal sources, and AcCoA is synthesized from glucose in the mitochondria of the neuron. These two substrates interact with the synthetic enzyme choline acetyltransferase to produce the neurotransmitter ACh. 

Apart from the relatively small amounts that are required for synthesis of the neurotransmitter serotonin (5-hydroxytryptamine), and for net new protein synthesis, essentially the whole of the dietary intake of tryptophan is metabolized by way of the oxidative pathway shown in Figures 8.4 and 9.4, which provides both a mechanism for total catabolism by way of acetyl coenzyme A and a pathway for synthesis of the nicotinamide nucleotide coenzymes (Section 8.3). 

Acetyl coenzyme A (acetyl CoA) can serve as a donor of acetate groups in biochemical reactions. One such reaction is the formation of acetylcholine, an important neurotransmitter involved in nerve signal transmission at neuromuscular junctions. The structure of choline is shown below. Draw the structure of acetylcholine. 

Acetylcholine is a neurotransmitter that functions at the neuromuscular junction, carrying signals from the nerve to the muscle. It is synthesized in a reaction between the quaternary ammonium ion choline and acetyl coenzyme A (Figure 16.10). When it is released from the nerve cell, acetylcholine binds to receptors on the surface of muscle cells. This binding stimulates the muscle cell to contract. Acetylcholine is then broken down to choline and acetate ion. 

Acetylcholine is a neurotransmitter formed in nerve cells by the enzyme-catalyzed reaction of choline with acetyl coenzyme A. 

---