The 1.5-angstrom Structure of XplA-heme, an Unusual Cytochrome P450 Heme Domain That Catalyzes Reductive Biotransformation of Royal Demolition Explosive

Federico Sabbadin, Rosamond Jackson, Kamran Haider, Girish Tampi, Johan P. Turkenburg, Sam Hart, Neil C. Bruce, Gideon Grogan

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XplA is a cytochrome P450 of unique structural organization, consisting of a heme- domain that is C-terminally fused to its native flavodoxin redox partner. XplA, along with flavodoxin reductase XplB, has been shown to catalyze the breakdown of the nitramine explosive and pollutant hexahydro-1,3,5-trinitro-1,3,5-triazine (royal demolition explosive) by reductive denitration. The structure of the heme domain of XplA (XplA-heme) has been solved in two crystal forms: as a dimer in space group P2(1) to a resolution of 1.9 angstrom and as a monomer in space group P2(1)2(1)2 to a resolution of 1.5 angstrom, with the ligand imidazole bound at the heme iron. Although it shares the overall fold of cytochromes P450 of known structure, XplA-heme is unusual in that the kinked I-helix that traverses the distal face of the heme is broken by Met-394 and Ala-395 in place of the well conserved Asp/Glu plus Thr/Ser, important in oxidative P450s for the scission of the dioxygen bond prior to substrate oxygenation. The heme environment of XplA-heme is hydrophobic, featuring a cluster of three methionines above the heme, including Met-394. Imidazole was observed bound to the heme iron and is in close proximity to the side chain of Gln-438, which is situated over the distal face of the heme. Imidazole is also hydrogen-bonded to a water molecule that sits in place of the threonine side-chain hydroxyl exemplified by Thr-252 in Cyt-P450cam. Both Gln-438 -> Ala and Ala-395 -> Thr mutants of XplA-heme displayed markedly reduced activity compared with the wild type for royal demolition explosive degradation when combined with surrogate electron donors.

Original languageEnglish
Pages (from-to)28467-28475
Number of pages9
JournalJournal of Biological Chemistry
Issue number41
Publication statusPublished - 9 Oct 2009


  • P450

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