United States Army Medical Research and Materiel Command

Organophosphate nerve agents inhibit the enzyme acetylcholinesterase (AChE), which is involved in nerve signal transduction, by forming covalent adducts with its catalytic serine residue. AChE adducts with soman and sarin nerve agents undergo dealkylation, a process known as aging, within a few minutes and a few hours, resp. This transformation is detrimental because it precludes reactivation of AChE with known oxime-based antidotes. Here, we designed a β-aminoalc. mol. for aged AChE reactivation, using a multi-tiered computational approach. This approach includes high-quality quantum mech./mol. mech. calculations, providing reliable reactivation steps and energetics. The calculations suggest that the designed β-aminoalc. can selectively reactivate aged sarin-/soman-inhibited AChE. Furthermore, unlike existing antidotes, the designed β-aminoalc. lacks a permanent charge, making it potentially active in the central nervous system. The mechanistic insights of this study can help guide the development of new AChE reactivators with improved access to the central nervous system.

The CapD enzyme of Bacillus anthracis is a γ-glutamyl transpeptidase from the N-terminal nucleophile hydrolase superfamily that covalently anchors the poly-γ-D-glutamic acid (pDGA) capsule to the peptidoglycan. The capsule hinders phagocytosis of B. anthracis by host cells and is essential for virulence. The role CapD plays in capsule anchoring and remodeling makes the enzyme a promising target for anthrax medical countermeasures. Although the structure of CapD is known, and a covalent inhibitor, capsidin, has been identified, the mechanisms of CapD catalysis and inhibition are poorly understood. Here, we used a computational approach to map out the reaction steps involved in CapD catalysis and inhibition. We found that the rate-limiting step of either CapD catalysis or inhibition was a concerted asynchronous formation of the tetrahedral intermediate with a barrier of 22-23 kcal/mol. However, the mechanisms of these reactions differed for the two amides. The formation of the tetrahedral intermediate with pDGA was substrate-assisted with two proton transfers. In contrast, capsidin formed the tetrahedral intermediate in a conventional way with one proton transfer. Interestingly, capsidin coupled a conformational change in the catalytic residue of the tetrahedral intermediate to stretching of the scissile amide bond. Furthermore, capsidin took advantage of iminol-amide tautomerism of its diacetamide moiety to convert the tetrahedral intermediate to the acetylated CapD. As evidence of the promiscuous nature of CapD, the enzyme cleaved the amide bond of capsidin by attacking it on the opposite side compared to pDGA.

Since it was identified in 1963 as the antileukemic agent in guinea pig serum, L-asparaginase (ASNase) has become an integral component of chemotherapy protocols to treat patients with acute lymphoblastic leukemia (ALL). Escherichia coli and Erwinia chrysanthemi provide the sources of ASNase used clinically today. From the time ASNase was first introduced into treatment protocols, the 5-year survival rate has increased significantly, particularly in children and adolescents. E. coli-derived ASNase was approved in 1978 to be used as part of a multiagent chemotherapy to treat ALL. However, the development of hypersensitivity in 10-30% of patients often leads to treatment discontinuation. E. chrysanthemi-derived ASNase (referred to herein as ASNase Erwinia chrysanthemi) is immunologically distinct from E. coli ASNase and therefore does not cross-react with the E. coli enzyme. In 2011, ASNase Erwinia chrysanthemi was approved in the United States for patients who develop hypersensitivity to E. coli-derived ASNase. When indicated, a switch from ASNase E. coli to ASNase E. chrysanthemi allows patients to continue to receive treatment and maintain therapeutic levels of ASNase activity. Therapeutic drug monitoring may help ensure that therapeutic enzyme levels are maintained. Pegylated recombinant ASNase Erwinia chrysanthemi is currently being developed to improve pharmacokinetic properties and reduce immunogenicity.