5.13
Indirect-Acting Cholinergic Agonists: Mechanism of Action
Indirect-acting cholinergic agonists work by interacting with an enzyme called acetylcholinesterase (AChE) in the synaptic cleft. They can be reversib…
Recall that indirect-acting cholinergic agonists inhibit AChE enzymes. This prevents hydrolysis of acetylcholine in the synapse and potentiates cholinergic activity at post-synaptic neurons or neuromuscular junctions.
The active site of the enzyme has a choline-binding anionic and a catalytic esteratic subsite.
Reversible inhibitors such as simple alcohols bearing quaternary ammonium groups interact electrostatically with the enzyme. As the enzyme–inhibitor complex is less stable and reversible, such agents have a brief duration of action.
Reversible inhibitors that are carbamoyl esters transfer their carbamoyl group to the serine hydroxyl group of the esteratic subsite. As the carbamoylated enzyme is more stable and undergoes slow hydrolysis, these drugs have a medium duration of action.
Organophosphates, however, interact covalently and phosphorylate the serine residue. This forms an irreversible complex resulting in a long-lasting effect of the drug.
The phosphorylated enzyme's stability is enhanced through the process called aging. Here, one of the alkyl group is lost which further strengthens the phosphorus–enzyme bond.
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Q1: How do indirect-acting cholinergic agonists increase cholinergic activity?
Indirect-acting cholinergic agonists inhibit acetylcholinesterase (AChE), the enzyme that breaks down acetylcholine in the synapse. By preventing hydrolysis of acetylcholine, these drugs allow the neurotransmitter to accumulate and potentiate cholinergic activity at post-synaptic neurons or neuromuscular junctions. This amplifies natural cholinergic signaling.
Q2: What is the difference between reversible and irreversible AChE inhibitors?
Reversible inhibitors form noncovalent, temporary bonds with AChE, creating unstable enzyme-inhibitor complexes with brief duration. Irreversible inhibitors form covalent bonds that permanently phosphorylate the serine residue, creating long-lasting effects. Irreversible inhibitors like organophosphates are much more potent and persistent than reversible agents.
Q3: How do carbamoyl ester inhibitors differ from simple quaternary ammonium compounds?
Simple quaternary ammonium compounds interact electrostatically with the anionic site of AChE, producing brief effects. Carbamoyl esters transfer their carbamoyl group to the esteratic subsite, forming a carbamoylated enzyme that undergoes slow hydrolysis. This creates a more stable complex with medium duration of action compared to simple reversible inhibitors.
Q4: What is enzyme aging and how does it affect organophosphate toxicity?
Enzyme aging is a process where one alkyl group is lost from the phosphorylated enzyme-inhibitor complex, further strengthening the phosphorus-enzyme bond. This makes the irreversible complex even more stable and resistant to reversal. Aging increases the persistence of organophosphate effects, making poisoning more difficult to treat once aging occurs.
Q5: Where do reversible inhibitors bind on the acetylcholinesterase enzyme?
Reversible inhibitors bind to specific subsites on acetylcholinesterase. Simple quaternary ammonium compounds interact with the choline-binding anionic site through electrostatic interactions. Carbamoyl esters bind to the esteratic subsite, where they transfer their carbamoyl group to the serine hydroxyl residue of the enzyme.
Q6: Why do organophosphates produce longer-lasting effects than other indirect-acting agonists?
Organophosphates form irreversible covalent bonds by phosphorylating the serine residue at the esteratic site of acetylcholinesterase. Unlike reversible inhibitors that dissociate quickly, these covalent complexes persist indefinitely. The subsequent aging process further stabilizes the phosphorus-enzyme bond, making the inhibition essentially permanent.
Q7: What role does the esteratic subsite play in indirect-acting cholinergic agonist activity?
The esteratic subsite is the catalytic region of acetylcholinesterase where both carbamoyl esters and organophosphates bind and form stable complexes. Carbamoyl esters transfer their carbamoyl group to this site, while organophosphates phosphorylate the serine hydroxyl group. This subsite determines the duration and reversibility of enzyme inhibition.