AChE bivalent inhibitors (Part II)

Galantamine derivative (compound 3)

Described in the page 'AChE inhibitors and substrates (Part II)' galanthamine (; colored red) is an AChE inhibitor and it is currently used in therapy of the Alzheimer's disease (AD). Conjugate of GAL through the (8 carbons, yellow) with a (blueviolet) called compound 3 has a larger affinity for AChE than that of GAL alone. This is similar to previously described cases of bivalent ligands. A comparison between /TcAChE (1w4l) and structure (1dx6) shows an identical binding mode of the GAL-moiety (transparent red) of compound 3 to that of GAL alone (blue) at the CAS. A PEG molecule (gray) is located at the active site of the galanthamine/TcAChE structure. The alkyl linker spans the active-site gorge and the phthalimido moiety of compound 3 is situated near Trp279 at the PAS. Compound 3 has higher affinity to TcAChE than GAL. This can be explained by the higher number of interactions between compound 3 (which interacts not only with residues within CAS but also within PAS) and TcAChE relative to GAL ^[1].

CPT-11

The drug (yellow) interacts with 13 residues of the from Trp84 at the bottom to Phe284 at the top (1u65). Nine of these residues are (Tyr70, Trp84, Tyr121, Trp279, Phe284, Phe330, Phe331, Tyr334, and His440; colored dark magenta). The contacts made by the drug at the bottom of the gorge involves with Trp84, Tyr121, Phe331, and His440 and, especially, a stacking interaction with Phe330. The carbamate moiety of CPT-11 is seen near residues . Carbon C9 (shown in magenta) of the carbamate linkage in CPT-11, is 9.3 Å from Oγ, the nucleophilic atom within the three catalytic residues Ser200, His440, and Glu327. The steric clashes between CPT-11 and TcAChE residues bar the positioning of CPT-11 near Ser200 Oγ (where hydrolysis could occur), therefore, TcAChE can not hydrolyze CPT-11 ^[2].

BW284C51

In a similar fashion to other AChE bivalent inhibitors, BW284C51 (BW) binds to TcAChE (1e3q) at both subsites of its - CAS and PAS. At the CAS, the BW makes a cation-aromatic interaction via its quaternary group to (colored orange). The BW phenyl ring forms an aromatic-aromatic interaction with His440. There is also an electrostatic interaction between the BW proximal quaternary group and Glu199. Near the PAS, BW via its distal quaternary group, interacts with (colored cyan) and forms an aromatic interaction with Tyr334. BW forms hydrogen bond with Tyr121 OH, and makes alkyl interactions with Phe331. The superposition of BW with two other AChE bivalent inhibitors (decamethonium, colored gray, 1acl) and (Aricept, colored blueviolet, 1eve) at the TcAChE active site gorge reveals similar mode of binding. These 3 inhibitors form cation-π and π-π interactions with active-site gorge aromatic residues (colored yellow). The superposition of reveals their similar trajectory along the active site gorge, but has a different one. This results in , which interacts with BW more strongly than with DECA and E2020. The conformations of the other important residues at the active site are similar in all these inhibitor complexes. It has been shown experimentally that BW and E2020 bind to TcAChE approximately 100-fold stronger than DECA. These findings have several explanations: i) E2020 and BW are less flexible than DECA; ii) the aromatic groups of E2020 and BW form favourable π-π interactions with TcAChE aromatic residues, in contrast to DECA; and iii) and have aromatic groups and, therefore, occupy more volume and better fit the active-site gorge, than . Mutations at the mouse or chicken AChE residues, corresponding to the TcAChE (colored red), cause significant increase of inhibition constant values for all these 3 inhibitors, supporting the notion that these residues are critical for inhibitor-AChE binding ^{[3] [4] [5]}.

is an untypical acetylcholinesterase inhibitor (1jjb). It consists of heptameric polyethylene glycol (PEG) with a thiol group (SH) at the terminus. The thiol group binds close to the and the second terminus binds to the . PEG-SH-350 interacts with Torpedo californica acetylcholinesterase via a system of (represented by oxygens colored red). Two out of the seven PEG-SH-350 ethylene glycol units are in trans (colored blue), while the others are in ±gauche .

Aricept. Among the most interesting drugs that have been designed to inhibit acetylcholinesterase are those that have two binding sites that bind both the peripheral and catatylic sites simultaneously. Such drugs bind strongly and with high specificly. A good example is (1eve). It appears that the principal interaction between the aceylcholine and the enzyme is the relatively newly discovered cation-pi interaction 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 distance vary between 4 and 7 Å, 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 barriers to impede passage down the gorge. Most acetylcholinesterases have a net negative charge and a large patch of negative potential around the entrance to the active site gorge. This may be useful 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 every molecule of substrate that reaches the peripheral site to travel down the gorge to the active site. This 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 (Trp84 is actually near the surface at the 'underside' of the protein). The X-ray structure of the E2020-TcAChE complex shows that E2020 has a along the active-site gorge, extending from the anionic subsite () of the active site, at the bottom, to the peripheral anionic site (), at the top, via aromatic stacking interactions with conserved aromatic acid residues. E2020 does not, however, interact directly with either the catalytic triad or the 'oxyanion hole' but only .