A conformational change in the peripheral anionic site ofTorpedo californicaacetylcholinesterase induced by a bis-imidazolium oxime

Abstract

As part of ongoing efforts to design improved nerve agent antidotes, two X-ray crystal structures of Torpedo californica acetylcholinesterase (TcAChE) bound to the bis-pyridinium oxime, Ortho-7, or its experimental bis-imidazolium analogue, 2BIM-7, were determined. Bis-oximes contain two oxime groups connected by a hydrophobic linker. One oxime group of Ortho-7 binds at the entrance to the active-site gorge near Trp279, and the second binds at the bottom near Trp84 and Phe330. In the Ortho-7-TcAChE complex the oxime at the bottom of the gorge was directed towards the nucleophilic Ser200. In contrast, the oxime group of 2BIM-7 was rotated away from Ser200 and the oxime at the entrance induced a significant conformational change in the peripheral anionic site (PAS) residue Trp279. The conformational change alters the surface of the PAS and positions the imidazolium oxime of 2BIM-7 further from Ser200. The relatively weaker binding and poorer reactivation of VX-inhibited, tabun-inhibited or sarin-inhibited human acetylcholinesterase by 2BIM-7 compared with Ortho-7 may in part be owing to the unproductively bound states caught in crystallo. Overall, the reactivation efficiency of 2BIM-7 was comparable to that of 2-pyridine aldoxime methyl chloride (2-PAM), but unlike 2-PAM the bis-imidazolium oxime lacks a fixed charge, which may affect its membrane permeability.

Cholinesterase reactivators and bioscavengers for pre‐ and post‐exposure treatments of organophosphorus poisoning

“…; Legler et al . ). Despite some success in structure‐based high throughput screening of chemical libraries (Berg et al .…”

Productive reorientation of a bound oxime reactivator revealed in room temperature X-ray structures of native and VX-inhibited human acetylcholinesterase

“…Finally, it is instructive to compare the overall binding of the RS-170B reactivator in the current structures to that observed previously for other oximes that have similar variable-length alkyl linkers flanked by two heterocyclic rings. Binding of such reactivators was demonstrated to induce a conformational change of Trp-286 (Trp-279 for TcAChE) at the PAS relative to its position in the apo-enzyme, allowingsandwich formation between the nonreactive pyridinium heterocycle of HI-6 (4, 6, 10, 11), HLo-7 (9), ortho-7, or obidoxime (6,9,14) and the side chains of Trp-286 and Tyr-72, or Trp-286 and Tyr-124. The sandwiched heterocycle was often characterized by welldefined electron density, most likely signifying tight binding of the ligand within thesandwich.…”

“…Many X-ray structures of AChE:oxime complexes, OP-AChE conjugates, and complexes of OP-AChE with reactivator oxime molecules have been published with mouse (mAChE) (6 -11) and electric ray (Torpedo californica, TcAChE) (12)(13)(14)(15)(16) and only a few with human AChE (hAChE) (17)(18)(19)(20)(21). However, this wealth of structural information has yet to yield improved reactivation efficacy of oxime antidotes (22).…”

Potent 3‐Hydroxy‐2‐Pyridine Aldoxime Reactivators of Organophosphate‐Inhibited Cholinesterases with Predicted Blood–Brain Barrier Penetration

“…Superposition on the crystal structures of the sarin-hAChE (PDB ID:5 FPQ) [23] and VX-TcAChE (PDB ID: 1VXR) [24] conjugates shows that 17 could bind to phosphonylated hAChE in the same active orientation as seen in the crystal structure of the 17-TcAChE complex. In vivo, in the absence of PEG, the oxime could oscillate between variousc onformations, as observed in other crystal structures of complexes of oximes with AChE (e.g.,b is-imidazolium oxime;P DB ID: 5BWB, [25] bis-pyridinium oxime ortho-7;P DB ID:5 BWC). [25] In complexes of other oximes with AChE, ac onformational change in the side-chain of Trp279 (Trp286 in hAChE) was reported.…”

“…[25] In complexes of other oximes with AChE, ac onformational change in the side-chain of Trp279 (Trp286 in hAChE) was reported. [25] However,i nt he 17-TcAChE complex, the Trp279 side-chain retains its apo conformation. The specific bindingo ft he morpholine portion of 17 to the peripherals ite confers on it an advantage compared with 2-PAM, which binds to the catalytic site only stabilised by p-p interactions with Trp84 (Trp86 in hAChE) with the oxime pointing away from the catalytic serine in both the apo and the phosphonylatede nzymes (PDB ID:2 VQ6;P DB ID:2 WG1).…”