Acetylcholinesterase

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Revision as of 06:42, 19 November 2007

[Article by Clifford Felder, Structural Biology, Weizmann Institute, 18 November 2008.]

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The increasing longevity of people's lifespans, and the resulting increased prevelance of dementias such as Alzheimers Syndrome, led scientists to study the animal enzyme Acetylcholinesterase (AChE) as a possible cause. This enzyme rapidly degrades or hydrolizes the neurotransmitter acetylcholine in synapses (junctions between nerve cells) of cholinergic nerve pathways into acetic acid and choline, to turn off the chemical signal for the nerve to fire. Should something happen to deactivate or kill this vital enzyme, nervous paralysis of vital functions occurs, leading to rapid death. Although AChE is apparently not the cause of Alzheimers, it does seem to play a minor role, in that weak inibitory drugs such as Tacrine, E2020 (Aricept) and the natural Chinese natural produce Huperzine appear to delay symptoms. Furthermore, this enzyme is a key target of some very important nerve gasses and related insecticides. Furthermore, it is a pretty fascinating enzyme to study.

Because of the relative ease in obtaining purified protein in abundance, AceytlCholinesterase was first crystallized by Joel Sussman of the Weizmann Institute, Rehovot Israel, after being extracted from the electric organ of the Pacific Sting Ray, Torpedo Californica, order to determine its detailed 3-dimensional structure by X-ray crystallography (2ace). Subsequently its X-ray structure has been determined from over 20 species, ranging from the fruit fly Drosophila to human.

Acetylcholinesterase is a fairly large protein, consisting of at least 535 amino acid residues in a single peptide chain, that folds into a single protein domain without any apparent symmetry.

The active site region of this enzyme has two sites, a catalytic site and a peripheral site, which helps prebind the substrate and direct it toward the active site. When the 3-D structure was first determined, the big surprise was that the active site was deep inside the protein, at the end or base of a , lined with aromatic residues, with the peripheral site at the top or lip of this gorge. Amazingly, there were no acidic or negatively charged residues anywhere in these 2 sites or along this gorge, as would be expected to help attract and bind the basic, positively charged acetylcholine substrate, although are are some acidic residues nearby. Instead, bulky aromatic residues (These numbers are the sequential numbering of the residues, starting from the N-terminus, according to the Torpedo Californica form of the enzyme.)

It appears that the principal interaction between the aceylcholine and the enzyme is relatively newly discovered cation-pi interactions between the cationic moiety of the substrate and the many aromatic residues lining the catalytic gorge. Unlike most interatomic interactions in chemistry, cation-pi interactions are unusual in that their energy hardly changes as the cationic and aromatic ring centers vary between 4 and 7 Angstroms apart, and for a wide variety of relative orientations of the aromatic rings. This gives the substrate an energetically smooth ride down the gorge with few bumps or

Most acetylcholinesterases have a net negative charge and a large patch of negative potential around the entrance to the active site gorge, which may be usefull to attract the positively charged acetycholine substrate to the site. As one travels down the gorge, this potential becomes increasingly more and more negative, reaching a peak at the active site at the base. Because of this potential, the peripherial site is thought to act like a substrate trap, that forces practically molecule of substrate that reaches the peripheral site to travel down the gorge to the active site, that probably contributes greatly to the extremely rapid rate of degrading the substrate. This whole enzyme therefore acts like a brilliantly designed natural vacuum cleaner that clears the neurotransmitter out of the synapse extremely quickly. Yet to be solved, however, is how the products clear the active site rapidly, whether back through the gorge, or out a back door on the other side of the protein that quickly opens each catalytic cycle (Try 84 is actually near the surface of the 'underside' of the protein.)

Selected structures

• 2ace This is the original solved structure for Torpedo Californica
• 1ea5 This is one of the highest quality representative X-ray structures in the PDB.

PDBs containing acetylcholinesterase