AChE is a hydrolase that hydrolyzes choline esters. It has a very high catalytic activity—each molecule of AChE degrades about 25,000 molecules of acetylcholine per second, approaching the limit allowed by diffusion of the substrate. The active site of AChE comprises 2 subsites—the anionic site and the esteratic subsite. The structure and mechanism of action of AChE have been elucidated from the crystal structure of the enzyme. The anionic subsite accommodates the positive quaternary amine of acetylcholine as well as other cationic substrates and inhibitors. The cationic substrates are not bound by a negatively charged amino acid in the anionic site, but by interaction of 14 aromatic residues that line the gorge leading to the active site. All 14 amino acids in the aromatic gorge are highly conserved across different species. Among the aromatic amino acids, tryptophan 84 is critical and its substitution with alanine results in a 3000-fold decrease in reactivity. The gorge penetrates halfway through the enzyme and is approximately 20 angstroms long. The active site is located 4 angstroms from the bottom of the molecule. The esteratic subsite, where acetylcholine is hydrolyzed to acetate and choline, contains the catalytic triad of three amino acids: serine 200, histidine 440 and glutamate 327. These three amino acids are similar to the triad in other serine proteases except that the glutamate is the third member rather than aspartate. Moreover, the triad is of opposite chirality to that of other proteases. The hydrolysis reaction of the carboxyl ester leads to the formation of an acyl-enzyme and free choline. Then, the acyl-enzyme undergoes nucleophilic attack by a water molecule, assisted by the histidine 440 group, liberating acetic acid and regenerating the free enzyme.
Biological function
During neurotransmission, ACh is released from the presynaptic neuron into the synaptic cleft and binds to ACh receptors on the post-synaptic membrane, relaying the signal from the nerve. AChE, also located on the post-synaptic membrane, terminates the signal transmission by hydrolyzing ACh. The liberated choline is taken up again by the pre-synaptic neuron and ACh is synthesized by combining with acetyl-CoA through the action of choline acetyltransferase. A cholinomimetic drug disrupts this process by acting as a cholinergic neurotransmitter that is impervious to acetylcholinesterase's lysing action.
Disease relevance
For a cholinergic neuron to receive another impulse, ACh must be released from the ACh receptor. This occurs only when the concentration of ACh in the synaptic cleft is very low. Inhibition of AChE leads to accumulation of ACh in the synaptic cleft and results in impeded neurotransmission. Irreversible inhibitors of AChE may lead to muscular paralysis, convulsions, bronchial constriction, and death by asphyxiation. Organophosphates, esters of phosphoric acid, are a class of irreversible AChE inhibitors. Cleavage of OP by AChE leaves a phosphoryl group in the esteratic site, which is slow to be hydrolyzed and can become covalently bound. Irreversible AChE inhibitors have been used in insecticides and nerve gases for chemical warfare. Carbamates, esters of N-methyl carbamic acid, are AChE inhibitors that hydrolyze in hours and have been used for medical purposes. Reversible inhibitors occupy the esteratic site for short periods of time and are used to treat of a range of central nervous system diseases. Tetrahydroaminoacridine and donepezil are FDA-approved to improve cognitive function in Alzheimer's disease. Rivastigmine is also used to treat Alzheimer's and Lewy body dementia, and pyridostigmine bromide is used to treat myasthenia gravis. An endogenous inhibitor of AChE in neurons is Mir-132 microRNA, which may limit inflammation in the brain by silencing the expression of this protein and allowing ACh to act in an anti-inflammatory capacity. It has also been shown that the main active ingredient in cannabis, tetrahydrocannabinol, is a competitive inhibitor of acetylcholinesterase.
Distribution
AChE is found in many types of conducting tissue: nerve and muscle, central and peripheral tissues, motor and sensory fibers, and cholinergic and noncholinergic fibers. The activity of AChE is higher in motor neurons than in sensory neurons. Acetylcholinesterase is also found on the red blood cellmembranes, where different forms constitute the Yt blood group antigens. Acetylcholinesterase exists in multiple molecular forms, which possess similar catalytic properties, but differ in their oligomeric assembly and mode of attachment to the cell surface.
AChE gene
In mammals, acetylcholinesterase is encoded by a single AChE gene while some invertebrates have multiple acetylcholinesterase genes. Note higher vertebrates also encode a closely related paralog BCHE with 50% amino acid identity to ACHE. Diversity in the transcribed products from the sole mammalian gene arises from alternative mRNA splicing and post-translational associations of catalytic and structural subunits. There are three known forms: T, R, and H.
AChET
The major form of acetylcholinesterase found in brain, muscle, and other tissues, known as is the hydrophilic species, which forms disulfide-linked oligomers with collagenous, or lipid-containing structural subunits. In the neuromuscular junctions AChE expresses in asymmetric form which associates with ColQ or subunit. In the central nervous system it is associated with PRiMA which stands for Proline Rich Membrane anchor to form symmetric form. In either case, the ColQ or PRiMA anchor serves to maintain the enzyme in the intercellular junction, ColQ for the neuromuscular junction and PRiMA for synapses.
The third type has, so far, only been found in Torpedo sp. and mice although it is hypothesized in other species. It is thought to be involved in the stress response and, possibly, inflammation.
Nomenclature
The nomenclatural variations of ACHE and of cholinesterases generally are discussed at Cholinesterase § Types and nomenclature.
Inhibitors
For acetylcholine esterase, reversible inhibitors are those that do not irreversibly bond to and deactivate AChE. Drugs that reversibly inhibit acetylcholine esterase are being explored as treatments for Alzheimer's disease and myasthenia gravis, among others. Examples include tacrine and donepezil.
