AChE inhibitors and substrates

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Revision as of 16:24, 1 December 2009

Dear readers, this page presents only a small part of the great world of the acetylcholinesterase inhibitors. So, please see also our pages AChE inhibitors and substrates (Part II), AChE bivalent inhibitors and AChE bivalent inhibitors (Part II).

AChE substrate

Solution of the three-dimensional (3D) structure of Torpedo californica acetylcholinesterase (TcAChE) in 1991 (Sussman et al. & Silman (1991)) opened up new horizons in research on an enzyme that had already been the subject of intensive investigation. The unanticipated structure of this extremely rapid enzyme, in which the active site was found to be buried at the bottom of a , lined by (colored dark magenta), led to a revision of the views then held concerning substrate traffic, recognition and hydrolysis (Botti et al. Sussman & Silman (1999)). This led to a series of theoretical and experimental studies, which took advantage of recent advances in theoretical techniques for treatment of proteins, such as molecular dynamics and electrostatics and to site-directed mutagenesis, utilizing suitable expression systems. Acetylcholinesterase hydrolysizes the neurotransmitter (ACh), producing group. directly binds (via its nucleophilic Oγ atom) within the (ACh/TcAChE structure 2ace). The residues are also important in the ligand recognition .

AChE monovalent inhibitors (Part I)

Alzheimer's disease (AD) is a disorder that attacks the central nervous system through progressive degeneration of its neurons. Patients with this disease develop dementia which becomes more severe as the disease progresses. It was suggested that symptoms of AD are caused by decrease of activity of cholinergic neocortical and hippocampal neurons. Treatment of AD by ACh precursors and cholinergic agonists was ineffective or caused severe side effects. ACh hydrolysis by AChE causes termination of cholinergic neurotransmission. Therefore, compounds which inhibit AChE might significantly increase the levels of ACh depleted in AD. Indeed, it was shown that AChE inhibitors improve the cognitive abilities of AD patients at early stages of the disease development. The first generation of AD drugs were AChE inhibitors: alcaloids like (-)-Huperzine A (HupA) and (-)-galanthamine (GAL, Reminyl); synthetic compounds tacrine (Cognex) and rivastigmine (Exelon).

1) (-)-Huperzine A. is extracted from a club moss that has been used in Chinese folk medicine. Its action is prescribed to its ability to strongly inhibit AChE (it has high binding affinity to this enzyme). The X-ray structure of the complex of TcAChE with optically pure HupA (colored blueviolet) at 2.5 Å resolution (1vot) reveals that this inhibitor binds to the TcAChE active site mentioned above (Ser200, His440, Glu327, Trp84, and Phe330; colored orange), but its observed orientation is almost orthogonal in comparison to ACh (transparent gray; 2ace). Ser200 (colored transparent yellow) is from ACh/TcAChE complex 2ace).

2) Tacrine. In the X-ray crystal structure of TcAChE/ complex which was determined at 2.8 Å resolution, the tacrine is seen (magenta) bound in the active site of TcAChE (1acj) . ACh (gray) is shown for comparison.

3) (HUPerzine A + tacRINE) is one of the most potent reversible AChE inhibitors. This synthetic hybrid consists of a carbobicyclic moiety resembling that of (−)- (colored blueviolet) and the 4-aminoquinoline substructure of (colored magenta). Both these compounds are known AChE inhibitors. (−)-Huperzine A and tacrine positions partially overlap each other at the TcAChE . TcAChE residues interacting with (−)-huperzine A (1vot) are colored orange and with tacrine (1acj) are colored cyan. The of the 4-aminoquinoline substructure of the huprine X in its complex with TcAChE (1e66, TcAChE interacting residues are in lime) is very similar to that of tacrine. The ring system of (−)-huperzine A is almost 180° relative to that of huprine X.

4) Galanthamine. (red) is an alkaloid from the flower snowdrop (Galanthus nivalis). The X-ray crystal structure of the TcAChE/GAL complex (1dx6) was determined at 2.3 Å resolution. The inhibitor binds at the base of the active site gorge of TcAChE, interacting with both the choline-binding site (Trp84) and the acyl-binding pocket (Phe288, Phe290). The tertiary amine appears to make a non-conventional hydrogen bond, via its N-methyl group, to Asp72. The hydroxyl group of the inhibitor makes a strong hydrogen bond (2.7 Å) with Glu199. ACh (gray) is shown for comparison.

5) Galanthamine iminium derivative. The X-ray structure of TcAChE in complex with galanthamine iminium derivative (compound 5) was determined at 2.05 Å resolution (1w6r). The binding mode of this (cyan) with TcAChE is virtually identical to that of galanthamine (red) itself (1dx6). The TcAChE residues which interact with galanthamine in the galanthamine/TcAChE complex are colored pink, while those of compound 5/TcAChE are in lime. The main structural change is the side-chain movement of Phe330 in compound 5/TcAChE complex, in comparison to that of galanthamine. Compound 5 differs from galanthamine by the presence of (N⁺; blue) instead N of galanthamine. This chemical difference causes the structural change in TcAChE and the slight decrease in affinity of compound 5 to TcAChE in comparison to galanthamine.

6) Organophosphorus acid anhydride (OP) nerve agents are potent inhibitors which rapidly phosphonylate AChE and then may undergo an internal dealkylation reaction (called "aging") to produce an OP-enzyme conjugate that cannot be reactivated.

Reaction between Ser200OG and Soman, assuming an in-line attack by the OG, followed by spontaneous dealkylation of the O-pinacolyl group.

As was mentioned above (AChE inhibitors and substrates), AChE hydrolysizes the neurotransmitter , producing group. directly binds catalytic (via its nucleophilic Oγ atom). , O-(1,2,2-trimethylpropyl) methylphosphonofluoridate (fluorine atom is colored violet and phosphorus atom is colored darkmagenta), is one of the most toxic OPs. Soman inhibits AChE by to catalytic Ser200, . This process implicates nucleophilic attack of the Ser200 nucleophilic Oγ atom on the phosphorus atom of soman, with concomitant departure of its fluoride atom. After that AChE catalyzes the of the soman or other OP. This causes irreversible inhibition of AChE, "aged" soman/AChE conjugate can not be reactivated. However, before “aging”, at the step of , AChE can be by nucleophiles, such as pralidoxime (2-PAM), resulting in of the phosphonyl adduct from Ser200 Oγ. At the (2wfz) the catalytic His440 forms hydrogen bonds with Ser200 Oγ and Glu327 Oε1 via its Nε2 and Nδ1 nitrogens, respectively. The O2 atom of soman is within hydrogen bonding distance of His440 Nε2. Soman O1 mimicks carbonyl oxygen of ACh. A water molecule 1001 interacting with soman O2 is represented as a red ball. The active site residues of the nonaged soman/TcAChE are colored yellow. The O2 atom of the (2wg0) forms a salt bridge with His440 Nε2. The active site residues of the aged soman/TcAChE are colored pink. of the structures of the nonaged (2wfz) and aged (2wg0) conjugates reveals a small, but important, change within the active site - the imidazole ring of His440 is tilted back to a native-like conformation after dealkylation. The water molecule 1001, which interacts with soman O2 in the nonaged crystal structure, is not within hydrogen bonding distance of O2 in the aged crystal structure. 2-PAM binds poorly to the nonaged phosphonylated enzyme (its electron density was not found) and binds in an after soman aging to TcAChE (2wg1).

To understand the basis for irreversible inhibition, the structure of the aged conjugate obtained by reaction of Torpedo californica AChE (TcAChE) with soman was solved by X-ray crystallography to 2.2Å resolution (1som). The highest positive difference density peak corresponded to the OP phosphorus and was located within covalent bonding distance of the active-site serine (S200). The are within hydrogen-bonding distance of four potential donors from catalytic subsites of the enzyme, suggesting that electrostatic forces significantly stabilize the aged enzyme. The methyl group of soman occupies the , bounded by Trp233, Phe288, and Phe290. The active sites of aged sarin-TcAChE (1cfj) and soman-TcAChE were essentially identical and provided structural models for the negatively charged, tetrahedral intermediate that occurs during deacylation with the natural substrate, acetylcholine.

For information about additional AChE monovalent inhibitors please see AChE inhibitors and substrates (Part II).

AChE bivalent inhibitors

Please see pages AChE bivalent inhibitors and AChE bivalent inhibitors (Part II)

Selected 3D Structures of AChE

• 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.
• 1eve The E2020 (Aricept) complex.
• 1ax9 Endrophonium complex.
• 1vot Complex with Huperzine, a Chinese folk medicine.
• 1fss Complex with snake venum toxin Fasciculin-II.
• 1acj Complex with tacrine.
• 1e66 Complex with huprine X.
• 1dx6 Complex with galanthamine.
• 1qti Complex with galanthamine.
• 1w6r Complex with galanthamine iminium derivative.
• 2ack Complex with edrophonium.
• 1vzj Structure of the tetramerization domain of acetylcholinesterase.
• 1gqr Complex with rivastigmine.
• 1gqs Complex with NAP alone.
• 1vxr Complex with VX.
• 2vja Complex with OTMA.
• 1som Complex with soman.

More structures can be obtained by searching for AChE

References

• Raves ML, Harel M, Pang YP, Silman I, Kozikowski AP, Sussman JL. Structure of acetylcholinesterase complexed with the nootropic alkaloid, (-)-huperzine A. Nat Struct Biol. 1997 Jan;4(1):57-63. PMID:8989325
• Harel M, Schalk I, Ehret-Sabatier L, Bouet F, Goeldner M, Hirth C, Axelsen PH, Silman I, Sussman JL. Quaternary ligand binding to aromatic residues in the active-site gorge of acetylcholinesterase. Proc Natl Acad Sci U S A. 1993 Oct 1;90(19):9031-5. PMID:8415649
• Greenblatt HM, Guillou C, Guenard D, Argaman A, Botti S, Badet B, Thal C, Silman I, Sussman JL. The complex of a bivalent derivative of galanthamine with torpedo acetylcholinesterase displays drastic deformation of the active-site gorge: implications for structure-based drug design. J Am Chem Soc. 2004 Dec 1;126(47):15405-11. PMID:15563167 doi:http://dx.doi.org/10.1021/ja0466154
• Ravelli RB, Raves ML, Ren Z, Bourgeois D, Roth M, Kroon J, Silman I, Sussman JL. Static Laue diffraction studies on acetylcholinesterase. Acta Crystallogr D Biol Crystallogr. 1998 Nov 1;54(Pt 6 Pt 2):1359-66. PMID:10089512
• Haviv H, Wong DM, Greenblatt HM, Carlier PR, Pang YP, Silman I, Sussman JL. Crystal packing mediates enantioselective ligand recognition at the peripheral site of acetylcholinesterase. J Am Chem Soc. 2005 Aug 10;127(31):11029-36. PMID:16076210 doi:http://dx.doi.org/10.1021/ja051765f
• Carlier PR, Du DM, Han Y, Liu J, Pang YP. Potent, easily synthesized huperzine A-tacrine hybrid acetylcholinesterase inhibitors. Bioorg Med Chem Lett. 1999 Aug 16;9(16):2335-8. PMID:10476864
• Wong DM, Greenblatt HM, Dvir H, Carlier PR, Han YF, Pang YP, Silman I, Sussman JL. Acetylcholinesterase complexed with bivalent ligands related to huperzine a: experimental evidence for species-dependent protein-ligand complementarity. J Am Chem Soc. 2003 Jan 15;125(2):363-73. PMID:12517147 doi:http://dx.doi.org/10.1021/ja021111w
• Sussman JL, Harel M, Frolow F, Oefner C, Goldman A, Toker L, Silman I. Atomic structure of acetylcholinesterase from Torpedo californica: a prototypic acetylcholine-binding protein. Science. 1991 Aug 23;253(5022):872-9. PMID:1678899
• Greenblatt HM, Kryger G, Lewis T, Silman I, Sussman JL. Structure of acetylcholinesterase complexed with (-)-galanthamine at 2.3 A resolution. FEBS Lett. 1999 Dec 17;463(3):321-6. PMID:10606746
• Bar-On P, Millard CB, Harel M, Dvir H, Enz A, Sussman JL, Silman I. Kinetic and structural studies on the interaction of cholinesterases with the anti-Alzheimer drug rivastigmine. Biochemistry. 2002 Mar 19;41(11):3555-64. PMID:11888271
• Colletier JP, Bourgeois D, Sanson B, Fournier D, Sussman JL, Silman I, Weik M. Shoot-and-Trap: use of specific x-ray damage to study structural protein dynamics by temperature-controlled cryo-crystallography. Proc Natl Acad Sci U S A. 2008 Aug 19;105(33):11742-7. Epub 2008 Aug 13. PMID:18701720
• Dvir H, Wong DM, Harel M, Barril X, Orozco M, Luque FJ, Munoz-Torrero D, Camps P, Rosenberry TL, Silman I, Sussman JL. 3D structure of Torpedo californica acetylcholinesterase complexed with huprine X at 2.1 A resolution: kinetic and molecular dynamic correlates. Biochemistry. 2002 Mar 5;41(9):2970-81. PMID:11863435