Controlled concealment of exposed clearance and immunogenic domains by site-specific polyethylene glycol attachment to acetylcholinesterase hypolysine mutants.

Cholinesterases are efficient scavengers of organophosphates and are currently being developed as drugs for treatment against poisoning by such compounds. Recombinant ChE bioscavengers have very short circular longevity, a limitation that can be overcome by complex post-translation manipulations or by chemical modification such as polyethylene glycol conjugation. Series of multiple Lys-Ala mutants of human acetylcholinesterase were prepared allowing the generation of homogenous and well defined polyethylene-glycol conjugated AChEs with either one, two, three, four, or five appended polyethylene glycol (PEG) moieties/molecule. The rank order of circulatory longevity of these molecules was dependent on the number of PEG appendages up to a certain threshold: 5 = 4 > 3 > 2 > 1 > 0. Hypolysine acetylcholinesterases (AChEs) carrying the same number of PEGs, and therefore with identical masses, allowed us to demonstrate that circulatory longevity correlates with the predicted extent of concealment of the AChE surface. Furthermore, circulatory profiles of high number and low number PEG-AChEs differing in their sialic acid contents demonstrate a direct relationship between PEG loading and the effective seclusion of AChE from the hepatic asialoglycoprotein receptor clearance system. Finally, an inverse relationship is found between the extent of PEG loading and the ability of the human acetylcholinesterase to elicit specific anti-HuAChE antibodies. In conclusion, these findings suggest that for the extension of circulatory longevity, protein surface domain concealment exerted by polyethylene glycol attachment is at least as important as its effect on size enlargement and highlights the role of PEG attachment in masking interactions between biomolecules and their cognate receptors.

The role of AChE active site gorge in determining stereoselectivity of charged and noncharged VX enantiomers.

The reactivity of human acetylcholinesterase (HuAChE) toward the chemical warfare agent VX [O-ethyl S-[2-(diisopropylamino)ethyl] methyl-phosphonothioate] and its stereoselectivity toward the P(S)-enantiomer were investigated by examining the reactivity of HuAChE and its mutant derivatives toward purified enantiomers of VX and its noncharged isostere nc-VX [O-ethyl S-(3-isopropyl-4-methyl-pentyl) methylphosphonothioate]. Stereoselectivity of the wild-type HuAChE toward VX(S) is manifested by a 115-fold higher bimolecular rate constant (1.4 x 10(8) min(-1) M(-1)) as compared to that of VX(R). HuAChE was also 12,500-fold more reactive toward VX(S) than toward nc-VX(S), demonstrating the significance of the polar interactions of the ammonium substituent to their overall affinity toward VX. Indeed, substitution of the cation-binding subsite residue Trp86 by alanine resulted in a decrease of three orders of magnitude in HuAChE reactivity toward both VX enantiomers, with only a marginal effect on the reactivity toward the enantiomers of nc-VX. These results demonstrate that accommodation of the charged moieties of both VX enantiomers depends predominantly on interactions with the aromatic moiety of Trp86. Yet, these interactions seem to limit the stereoselectivity toward the P(S)-enantiomer, which for charged methylphosphonates is much lower than for the noncharged analogs, like sarin or soman. Marked decrease in stereoselectivity toward VX(S) was observed following replacements of Phe295 at the acyl pocket (F295A and F295A/F297A). Replacement of the peripheral anionic site (PAS) residue Asp74 by asparagine (D74N) practically abolished stereoselectivity toward VX(S) (a 130-fold decrease), while substitution which retained the negative charge at position 74 (D74E) had no effect. The results from kinetic studies and molecular simulations suggest that the differential reactivity toward the VX enantiomers originates predominantly from a different orientation of the charged leaving group with respect to residue Asp74. Such different orientations of the charged leaving group in the HuAChE adducts of the VX enantiomers seem to be a consequence of intramolecular interactions with the bulky phosphorus alkoxy group.

Stereoselectivity toward VX is determined by interactions with residues of the acyl pocket as well as of the peripheral anionic site of AChE.

The origins of human acetylcholinesterase (HuAChE) reactivity toward the lethal chemical warfare agent O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (VX) and its stereoselectivity toward the P(S)-VX enantiomer (VX(S)) were investigated by examining the reactivity of HuAChE and its mutant derivatives toward purified enantiomers of VX and its noncharged isostere O-ethyl S-(3-isopropyl-4-methylpentyl) methylphosphonothioate (nc-VX) as well as echothiophate and its noncharged analogue. Reactivity of wild-type HuAChE toward VX(S) was 115-fold higher than that toward VX(R), with bimolecular rate constants of 1.4 x 10(8) and 1.2 x 10(6) min(-1) M(-1). HuAChE was also 12500-fold more reactive toward VX(S) than toward nc-VX(S). Substitution of the cation binding subsite residue Trp86 with alanine resulted in a 3 order of magnitude decrease in HuAChE reactivity toward both VX enantiomers, while this replacement had an only marginal effect on the reactivity toward the enantiomers of nc-VX and the noncharged echothiophate. These results attest to the critical role played by Trp86 in accommodating the charged moieties of both VX enantiomers. A marked decrease in stereoselectivity toward VX(S) was observed following replacements of Phe295 at the acyl pocket (F295A and F295A/F297A). Replacement of the peripheral anionic site (PAS) residue Asp74 with asparagine (D74N) practically abolished stereoselectivity toward VX(S) (130-fold decrease), while a substitution which retains the negative charge at position 74 (D74E) had no effect. The results from kinetic studies and molecular simulations suggest that the differential reactivity toward the VX enantiomers is mainly a result of a different interaction of the charged leaving group with Asp74.