Structure of nerve agent with its target unraveled!

Structure of nerve agent with its target unraveled!

Nerve gas sarin is a colorless, odorless liquid fatal even at very low concentrations. Serious sarin poisoning causes visual disturbance, vomiting, breathing difficulties and, finally, death.

Nerve agents destroy the function of a very important protein in the nervous system called acetylcholinesterase. As long as the nerve agent is bound to the protein, the breakdown of an important signal substance is prevented. The antidote HI-6 removes the nerve agent and restores the function of the nervous system. Drugs against nerve agent poisoning have been used for a long time, still it has been unclear how they actually work.

After years of hard work, chemists from FOI and Umeå University are now presenting a three-dimensional structure that depicts the HI-6 moments before the bond between the nerve agent and the protein is broken. The structure gives a high-resolution image that, in detail, describes the individual positions of atoms and provides an understanding of how the bond breaks.

The scientific breakthrough was enabled by combining three-dimensional structural depictions with advanced calculations and biochemical experiments. The calculations supported the theory that the weak signal in the X-ray crystallography data actually came from HI-6 and sarin. Important knowledge also fell into place after experiments where the system was disturbed by mutating the protein or by introducing isotopes.

The analyses reveal previously unknown conformations of the system and suggest that the cleavage of the covalent enzyme–sarin bond is preceded by a conformational change in the sarin adduct itself. Together with data from the reactivation kinetics, this alternate conformation suggests a key interaction between Glu202 and the O-isopropyl moiety of sarin.

Moreover, solvent kinetic isotope effect experiments using deuterium oxide reveal that the reactivation mechanism features an isotope-sensitive step. These findings provide insights into the reactivation mechanism and provide a starting point for the development of improved antidotes.

"After seven years of work using many different techniques, we have finally been able to bring this to a successful close and can show a uniform picture of how HI-6 approaches sarin. It opens up for new opportunities in finding antidotes to sarin and other nerve agents by structure-based molecular design," says the author.