Acetylcholine active sites

Acetylcholine Active Site Allosteric Enzymes Amino Acid Antibiotics Artificial Sweeteners Base Pairing Bioluminescence Caffeine Carbohydrates Cellulose... 

Hexamethonium (Fig. 16.28) is a quaternary ammonium agent. It was the first successful antihypertensive treatment. However, its side effects led to discontinuation of its use. It is thought to exert its action by blocking the ion channel rather than the acetylcholine active site. Blocking the ganglionic nicotinic receptor will stop the... 

FIGURE 5.46 Interaction of the serine hydroxyl residue in the catalytically active site of acetylcholinesterase enzyme with esters of organophosphates or carbamates. The interaction leads to binding of the chemical with the enzyme, inhibition of the enzyme, inhibition of acetylcholine hydrolysis, and thus accumulation of acetylcholine in the synapses. 

CBs, like OPs, act as inhibitors of ChE. They are treated as substrates by the enzyme and carbamylate the serine of the active site (Figure 10.8). Speaking generally, car-bamylated AChE reactivates more rapidly than phosphorylated AChE. After aging has occurred, phosphorylation of the enzyme is effectively irreversible (see Section 10.2.4). Carbamylated AChE reactivates when preparations are diluted with water, a process that is accelerated in the presence of acetylcholine, which competes as a substrate. Thus, the measurement of AChE inhibition is complicated by the fact that reactivation occurs during the course of the assay. Carbamylated AChE is not reactivated by PAM and related compounds that are used as antidotes to OP poisoning (see Box 10.1). 

We may now consider in a little more detail the interaction of true (or a-) cholinesterase with acetylcholine. Wilson and Berg mann1 suggest that there are two active sites in the enzyme, known as anionic site and esteratic site respectively. These sites (represented diagrammatically in fig. II)2 are not to be considered independent. The mode of attachment will be seen to depend upon (a) the quaternary nitrogen atom (N+< ) and... 

Figure 4.14 Gated ion channel (ligand activated, acetylcholine) at neuromuscular junction, (a) receptor sites unoccupied and gate closed (b) acetylcholine receptor sites occupied and gate is open allowing influx of ion...

Fig. 2. The acetylcholinesterase gorge showing regions where binding takes place and interaction of acetylcholine with active site.

Reversible cholinesterase inhibitors form a transition state complex with the enzyme, just as acetylcholine does. These compounds are in competition with acetylcholine in binding with the active sites of the enzyme. The chemical stracture of classic, reversible inhibitors physostigmine and neostigmine shows their similarity to acetylcholine. Edrophonium is also a reversible inhibitor. These compounds have a high affinity with the enzyme, and their inhibitory action is reversible. These inhibitors differ from acetylcholine in that they are not easily broken down by enzymes. Enzymes are reactivated much slower than it takes for subsequent hydrolysis of acetylcholine to happen. Therefore, the pharmacological effect caused by these compounds is reversible. 

Organophosphates form stable phosphoesters with the active site serine of acetylcholinesterase, the enzyme responsible for hydrolysis and inactivation of acetylcholine at cholinergic synapses. 

Neuromuscular transmission involves the events leading from the liberation of acetylcholine (ACh) at the motor nerve terminal to the generation of end plate currents (EPCs) at the postjunctional site. Release of ACh is initiated by membrane depolarization and influx of Ca++ at the nerve terminal (Fig. 28.1). This leads to a complex process involving docking and fusion of synaptic vesicles with active sites at the presynaptic membrane. Because ACh is released by exocytosis, functional transmitter release takes place in a quantal fashion. Each quantum corresponds to the contents of one synaptic vesicle (about 10,000 ACh molecules), and about 200 quanta are released with each nerve action potential. 

Neuromuscular transmission. Transmitter release at the motor nerve terminal occurs by exocytosis of synaptic vesicles that contain acetylcholine (ACh). The process is enhanced by an action potential that depolarizes the membrane and allows Ca++ entry through channels at the active sites. ACh may be hydrolyzed by acetylcholinesterase (AChE) or bind to receptors (AChRs) located at the peaks of the subsynaptic folds. Simultaneous activation of many AChRs produces an end plate current, which generates an action potential in the adjacent muscle membrane. 

The actions of acetylcholine released from autonomic and somatic motor nerves are terminated by enzymatic hydrolysis of the molecule. Hydrolysis is accomplished by the action of acetylcholinesterase, which is present in high concentrations in cholinergic synapses. The indirect-acting cholinomimetics have their primary effect at the active site of this enzyme, although some also have direct actions at nicotinic receptors. The chief differences between members of the group are chemical and pharmacokinetic—their pharmacodynamic properties are almost identical. 

Acetylcholinesterase is the primary target of these drugs, but butyrylcholinesterase is also inhibited. Acetylcholinesterase is an extremely active enzyme. In the initial catalytic step, acetylcholine binds to the enzyme s active site and is hydrolyzed, yielding free choline and the acetylated enzyme. In the second step, the covalent acetyl-enzyme bond is split, with the addition of water (hydration). The entire process occurs in approximately 150 microseconds. 

Edrophonium Alcohol, binds briefly to active site of acetylcholinesterase (AChE) and prevents access of acetylcholine (ACh) Amplifies all actions of ACh increases parasympathetic activity and somatic neuromuscular transmission Diagnosis and acute treatment of myasthenia gravis Parenteral quaternary amine does not enter CNS Toxicity Parasympathomimetic excess Interactions Additive with parasympathomimetics... 

Diazinon toxicity results predominantly from the inhibition of acetylcholinesterase in the central and peripheral nervous system. The enzyme is responsible for terminating the action of the neurotransmitter, acetylcholine, in the synapse of the pre- and post-synaptic nerve endings or in the neuromuscular junction. However, the action of acetylcholine does not persist long as it is hydrolyzed by the enzyme, acetylcholinesterase, and rapidly removed. As an anticholinesterase organophosphate, diazinon inhibits acetylcholinesterase by reacting with the active site to form a stable phosphorylated complex which is incapable of destroying acetylcholine at the synaptic gutter between the pre- and post-synaptic nerve... 

The mechanism of action of anticholinesterases is to form a stable covalent complex with the Achase enzyme. Achase is one of several enzymes known as serine esterases. Other examples include the intestinal enzymes trypsin and chymotrypsin as well as the blood clotting agent thrombin. During the course of the catalysis the alcohol -OH of a serine side chain in the active site of the enzyme forms an ester complex, called the acyl-enzyme, with the substrate. So, acetylcholine will go through similar chemical reactions with Achase. 

Inhibition of the cholinesterase enzymes depends on blockade of the active site of the enzyme, specifically the site that binds the ester portion of acetylcholine (Fig. 7.48). The organophosphorus compound is thus a pseudosubstrate. However, in the case of some compounds such as the phosphorothionates (parathion and malathion, for example), metabolism is necessary to produce the inhibitor. 

---