The of TcAChE consists of two binding subsites. First of them is the "catalytic anionic site" (CAS), which involves mentioned above catalytic triad (colored orange) and the conserved residues and also participating in ligands recognition. Another conserved residue (colored cyan) is situated at the second binding subsite, termed the "peripheral anionic site" (PAS), ~14 Å from CAS. Therefore, the ligands that will be able to interact with both these subsites, will be more potent AChE inhibitors in comparison to compounds interacting only with CAS. One of the ways to produce such ligands is to introduce two active substances into one compound. If it is spatially necessary these subunits could be divided by alkyl linker with suitable length.
According to the strategy of the use of a bivalent ligand, the (RS)-(±)-tacrine-(10)-hupyridone ((R)-3 or (S)-3) was designed and synthesized. It consists of mentioned in the page 'AChE inhibitors and substrates' (colored magenta), 10-carbon (yellow), and (red). The tacrine moiety of this inhibitor binds at the CAS, the linker spans the gorge, and the hupyridone moiety binds at the PAS.
The comparison of the (R)-3/TcAChE and tacrine/TcAChE complexes at the . A of the trigonal (R)-3/TcAChE structure (1zgb; (R)-3 colored cyan; TcAChE residues interacting with (R)-3 are colored sea-green) with the crystal structure of tacrine/TcAChE (1acj, tacrine colored magenta; residues interacting with tacrine are colored pink) reveals a similar binding mode for the tacrine moiety. In both structures the tacrine ring is situated at the CAS, between the aromatic residues Trp84 and Phe330. Steric clash with the 10-carbon linker could explain the tilt observed for the Phe330 (yellow and transparent in the tacrine/TcAChE). Water molecules are shown as red spheres.
The tacrine unit of (R)-3 N forms with His440 O (3.0 Å) similar to that of tacrine alone. Similarly to the tacrine/TcAChE structure the system of three water molecules at the CAS ((R)-3/TcAChE) binds the tacrine-linker N via hydrogen bonds to Ser81 O, Ser122 Oγ, and Asn85 Oδ1 (2.6-3.5 Å) .
The comparison of the (R)-3/TcAChE (1zgb) and bis-hupyridone/TcAChE complexes (1h22 and 1h23) at the . of the (R)-tacrine-(10)-hupyridone ((R)-3, cyan) and (S,S)-(-)-Bis(12)-hupyridone ((S,S)-(-)-4b, orange, i.e. 12-carbon-tether-linked hupyridone dimer) and (S,S)-(-)-Bis(10)-hupyridone ((S,S)-(-)-4a, plum) complexes demonstrates the binding mode of the hupyridone moiety. TcAChE residues of symmetry-related molecule are shown in magenta. X-ray structures of TcAChE complexed with these 10- and 12-carbon-tether-linked 2 (S,S)-(-)-4a and (S,S)-(-)-4b show one subunit bound at the , the linker spanning the gorge, and the other subunit bound at the .
There are two connecting the hupyridone O to Lys11 Nζ and hupyridone N to Gln185 Oε1 of a symmetry-related molecule at (R)-3/TcAChE complex. Water molecules are shown as red spheres. Another hydrogen bond connects the hupyridone O to a water molecule, which is bound to Ser286 N. Similarly, the hupyridone-PAS unit of both (-)-4a and (-)-4b forms direct and an indirect hydrogen bonds with the protein backbone in the PAS region.
The of (R)-3 (cyan) and (S)-3 (1zgc) bound to the TcAChE active site in the orthorhombic forms is shown. of (S,S)-(-)-4a (magenta) and (S)-3 (orange, orthorhombic TcAChE) demonstrates the similar mode of binding of the hupyridone unit at the PAS. The residues Trp279 (top) and Trp84 (bottom) represent the PAS and the CAS, respectively.
2d and 2f are bis(n)-tacrine derivatives with n=5 and 7 (number of carbons in the linkers), respectively. These compounds are more potent and selective AChE inhibitors than tacrine alone. The binding of the tacrine moiety of at the TcAChE catalytic anionic site (CAS) is similar to that of tacrine in the tacrine/TcAChE complex (1acj). The second tacrine moiety of the 2d interacts with the peripheral anionic site (PAS) near Trp279. The interaction of 2d at the CAS causes an increase of the between Ser200 Oγ and H440 Nε2 atoms, and, therefore, disruption of the catalytic triad (Ser200, H220, E327) as seen in the native structure (2ace). The binding of 2d results in in the Val281-Ser291 loop changing the surface of the active-site gorge from its native conformation (2ace). The tacrine moiety of the compound 2f (heptylene-linked bis-tacrine at the CAS, 2ckm) adopts similar as tacrine in the tacrine/TcAChE complex and the tacrine moiety of the 2d at the CAS. The second tacrine moiety of the 2f interacts with PAS near the Trp279, like 2d. The of 2f does not cause significant structural changes in TcAChE from its native structure. of the structures of 2d/TcAChE and 2f/TcAChE reveals different contacts between the tacrine moieties of these compounds at the PAS and TcAChE. There are two additional structures of tacrine-containing TcAChE complexes: compounds (1ut6) and (1odc). The tacrine moieties of these compounds adopt similar conformations and interactions with CAS as the tacrine in the tacrine/TcAChE, 2f/TcAChE and 2d/TcAChE. Inhibitors 6 and 7 are spanning the between the CAS and the PAS, but since compound 7 lacks the second tacrine moiety, Trp279 adopts a different conformation in this complex structure. In the three structures: native TcAChE (cyan), cmp 6/TcAChE complex (white), and cmp 7/TcAChE complex (crimson) , all the TcAChE active-site gorge residues have identical conformation except Trp279.
