Acetylcholine esterase mechanisms

In addition to hydrogen ions, other species can also affect the enzymatic catalytic activity. This phenomenon is called inhibition it may be specific, nonspecific, reversible, or irreversible. The inhibition reactions can also be used for the sensing of inhibitors. The best-known example is the sensor for detection of nerve gases. These compounds inhibit the hydrolysis of the acetylcholine ester which is catalyzed by the enzyme acetylcholine esterase. Acetylcholine ester is a key component in the neurotransmission mechanism. 

Both steps of the process are catalyzed by the basic form of the imidazole group of a histidine residue forming part of the active site. If the native conformation of the enzyme is disrupted by denaturation reagents such as urea, the unique seryl hydroxyl loses its characteristic reactivity in both the acylation and the deacylation process (15). This is easily understood if we realize that the reactive serine and the catalytically active histidine are extremely far from one another along the polypeptide chain, being separated by 137 amino acid residues (16) they are brought into the necessary juxtaposition only by the specific folding in the native enzyme structure. The mechanism by which the enzyme acetylcholine esterase catalyzes the hydrolysis of its substrate acetylcholine appears to be very similar (17). As we shall see, a number... 

Whereas acetylcholine is degraded by a membrane-anchored acetylcholine esterase (ACE) in the synaptic cleft (choline is afterwards taken up presynaptically), the biogenic amines adrenaline, noradrenaline, serotonin, and dopamine are taken up by the presynaptic membrane by transporters (Fig. 3) or by extraneuronal cells in which they are degraded by a catecholamine O-methyltransferase (COMT). These transporter have similar structure and contain 12 transmembrane regions. Once in the presynapse, the neurotransmitters are either degraded by monoamine oxidase (MAO) or taken up by synaptic vesicles. A proton pumping ATPase of the vesicle membrane (V-type ATPase as in plant vacuoles) causes an increase of hydrogen ion concentrations in the vesicles. Uptake of the neurotransmitter serotonin, adrenaline and noradrenaline could be partly achieved either via a diffusion of the free base into the vesicles where they become protonated and concentrated by an "ion trap" mechanism and via specific proton-coupled antiporters. The excitatory amino acids, acetylcholine and ATP cannot diffuse and enter the vesicles via specific transporters. 

Fig. 17.9 Mechanism of acetylcholine esterase and the binding of nerve agents...

An exceptionally reactive serine residue has been identified in a great number of hydrolase enzymes, e. g., trypsin, subtilisin, elastase, acetylcholine esterase and some lipases. These enzymes appear to hydrolyze their substrates by a mechanism analogous to that of chymotrypsin. Hydrolases such as papain, ficin and bromelain, which are distributed in plants, have a cysteine residue instead of an active serine residue in their active sites. Thus, the transient intermediates are thioesters. 

Overall, the present observations on acetylcholine support the notion that inclusion by CH3/aromatic-7T-electron interactions is an effective binding mechanism [14,15] but they provide little information on how reaction subsequent to binding may be enhanced in an enzyme such as acetylcholine esterase. The calixarene framework does, however, provide a means to bring other nucleophiles into proximity with an included species and to attach activating units such as metal ions, and we are continuing to attempt to develop a more sophisticated model of an acylating enzyme. A recent report [33] of the development of an active acylation catalyst on the basis of principles similar to those described above vividly illustrates the complexities which may be encountered in this task. 

The acyl group from an ester can be transferred to another alcohol group via the process of transesterilication. Aspirin is the most widely used dmg in which the ester group is partly responsible for its mechanism of action. In common with the acetylcholine esterase inhibitors discussed below, the target is a serine residue in the enzyme cydooxygenase 1 (COX-1). However, in the case of aspirin the covalent modification of the enzyme is irreversible. Acetylation of COX-1 prevents the production of prostaglandin inflammatory mediators. The salicylate portion of the molecule also possesses anti-inflammatory aaion (Fig. 5.46). 

Acetylcholine is a relatively small molecule that is responsible for nerve-impulse transmission in animals. As soon as it has interacted with its receptor and triggered the nerve response, it must be degraded and released before any further interaction at the receptor is possible. Degradation is achieved by hydrolysis to acetate and choline by the action of the enzyme acetylcholinesterase, which is located in the synaptic cleft. Acetylcholinesterase is a serine esterase that has a mechanism similar to that of chymotrypsin (see Box 13.5). 

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. 

The first is via hydrolytic destruction by acetylcholinesterase642-645 (pp. 634-637 Eq. 12-25). This esterase and the related butyrylcholinesterase646 are present in the synaptic membrane itself. The second mechanism is energy-dependent transport of acetylcholine into the neuron for reuse. Since much of the transmitter is hydrolyzed, new acetylcholine is synthesized by transfer of an acetyl group of acetyl-CoA to choline.647... 

Acetylcholinesterase (AChE) deesterifies the neurotransmitter acetylcholine (ACh). AChE belongs to a group of enzymes considered serine esterases and has a mechanism similar to that of chymotrypsin. AChE has an anionic binding site that attracts the positively charged quaternary ammonium group of ACh. The serine then attacks and cleaves the ester.910... 

The previous discussion of amino acid catabolic disorders indicates that catabolic processes are just as important for the proper functioning of cells and organisms as are anabolic processes. This is no less true for molecules that act as neurotransmitters. To maintain precise information transfer, neurotransmitters are usually quickly degraded or removed from the synaptic cleft. An extreme example of enzyme inhibition illustrates the importance of neurotransmitter degradation. Recall that acetylcholine is the neurotransmitter that initiates muscle contraction. Shortly afterwards, the action of acetylcholine is terminated by the enzyme acetylcholinesterase. (Acetylcholine must be destroyed rapidly so that muscle can relax before the next contraction.) Acetylcholinesterase is a serine esterase that hydrolyzes acetylcholine to acetate and choline. Serine esterases have catalytic mechanisms similar to those of the serine proteases (Section 6.4). Both types of enzymes are irreversibly inhibited by DFP (diisopropylfluorophosphate). Exposure to DFP causes muscle paralysis because acetylcholinesterase is irreversibly inhibited. With each nerve impulse, more acetylcholine molecules enter the neuromuscular synaptic cleft. The accumulating acetylcholine molecules repetitively bind to acetylcholine receptors. The overstimulated muscle cells soon become paralyzed (nonfunctional). Affected individuals suffocate because of paralyzed respiratory muscles. 

A different method for terminating the action of acetylcholine occurs in the hearts of bivalent molluscs which have no cholinesterases. The release of ACh from the nerve ending causes the muscle to excrete an ATP-like substance which lowers the sensitivity of the ACh receptors, apparently by an allosteric change. This mechanism persists in the hearts of higher animals, but is overshadowed by the more efficient ACh-esterase (Turpaev and Sakharov, 1973). 

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