Question

Acetylcholine is the substrate for the enzyme acetylcholinesterase.

요 H39 CH3 + CH3 Acetylcholine 1. Suggest what sort of binding interactions could be involved in holding acetylcholine to the active site. 2. The ester bond of acetylcholine is hydrolysed by acetylcholinesterase. Suggest a mechanism by which the enzyme catalyses this reaction. 3. Suggest how binding interactions might make acetylcholine more susceptible to hydrolysis. 4. Structure is an agonist which binds to the cholinergic receptor and mimics the action of the natural ligand acetylcholine. Structure II, however, shows no activity and does not bind to the receptor. Suggest why this might be the case. 요 H, NO (1) N(CH3)2 Ń(CH3) CH2CH2 (11)

1. Suggest what sort of binding interactions could be involved in holding acetylcholine to the active site.

2. The ester bond of acetylcholine is hydrolysed by acetylcholinesterase. Suggest a mechanism by which the enzyme catalyses this reaction.

3. Suggest how binding interactions might make acetylcholine more susceptible to hydrolysis.

4. Structure I is an agonist which binds to the cholinergic receptor and mimics the action of the natural ligand acetylcholine. Structure II, however, shows no activity and does not bind to the receptor. Suggest why this might be the case.

Answer 1

Acetylcholinesterase hydrolyse the ester bond of acetylcholine. Acetylcholine also has a cationic part.

1.

So active site must have an anionic part that will take part in ionic interaction with cationic quternery amine, tyrosine or aspartic acid or glutamic acid take part for this kind of interaction. For the ester part (carbonyl group) there must be a hydrogen bonding interaction with oxygen that will enhance electrophilicity of carbonyl group, serine or tyrosine can take part in this kind of interaction.

2.

For breaking of ester bond there must be a nucleophilic attack on the carbonyl group and for this kind of reaction serine is the best nucleophile. To deprotonate serine there must be hystidine residue which further associated with a glutamic acid/aspartic acid residue.

glu/asp will abstract H from Hys that will further abstract H from Ser so the anionic Ser attacks carbonyl group and breaking of ester result in acylserine which will be further hydrolysed by residual water as shown by the mechanism below.

His Asp/Glu ОН Ser Asp/Glu Tyr His to ASANGI ОН HN N Asp/GI Ser" Asp/Glu Туг .. His Но HN ОН Asp/Glu о Ser Туг Asp/Glu НО. His High Assign Asp/Glu ОН Ser Asp/Glu Туг НО His ОН ОН N Asp/Glu NH ОН Ser Asp/Glu Tyr

3.

If carbonyl group is hydrogen bonded to another residue or ester oxygen is hydogen bonded to another residue such as Hys/Tyr then that will make acetylcholine more succeptible to hydrolyse.

4.

structure (I) is an amine substitution of methyl that is structurally similar to natural ligand and also amine can interact with active side Asp/Glu amino acid. But structure (II) is ethyl substitution of methyl group that increases hydrophobicity of that part of ligand and the space in active cannot accomodate the enhanced length of the substitution So sructure (II) does not bind to receptor.