The Ferric-superoxo Intermediate of the TxtE Nitration Pathway Resists Reduction, Facilitating Its Reaction with Nitric Oxide

TxtE is a cytochrome P450 (CYP) homolog that mediates the nitric oxide (NO)-dependent direct nitration of L-tryptophan (Trp) to form 4-nitro-L-tryptophan (4-NO2-Trp). A recent report showed evidence that TxtE activity requires NO to react with a ferric-superoxo intermediate. Given this minimal mechanism, it is not clear how TxtE avoids Trp hydroxylation, a mechanism that also traverses the ferric-superoxo intermediate. Our combined results suggest 1) autoxidation is the sole TxtE uncoupling pathway and 2) the TxtE ferric-superoxo intermediate cannot be reduced by these electron transfer partners. We conclude that resistance of the ferric-superoxo intermediate to reduction is a key feature of TxtE that increases the lifetime of the intermediate and enables its reaction with NO and efficient nitration activity.