The explosive-degrading cytochrome P450 XplA: biochemistry, structural features and prospects for bioremediation.

XplA is a cytochrome P450 that mediates the microbial metabolism of the military explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). It has an unusual structural organisation comprising a heme domain that is fused to its flavodoxin redox partner. XplA along with its partnering reductase XplB are plasmid encoded and the gene xplA has now been found in divergent genera across the globe with near sequence identity. Importantly, it has only been detected at explosives contaminated sites suggesting rapid dissemination of this novel catabolic activity, possibly within the 50-year period since the introduction of RDX into the environment. The X-ray structure of XplA-heme has been solved, providing fundamental information on the heme binding site. Interestingly, oxygen is not required for the degradation of RDX, but its presence determines the final degradation products, demonstrating that the degradation chemistry is flexible with both anaerobic and aerobic pathways resulting in the release of nitrite from the substrate. Transgenic plants expressing xplA are able to remove saturating levels of RDX from soil leachate and may provide a low cost sustainable remediation strategy for contaminated military sites.