The {FeO2}8 Intermediate of the TxtE Nitration Pathway Resists Reduction, Facilitating Its Reaction with Nitric Oxide

TxtE is a cytochome P450 (CYP) homolog that mediates a nitric oxide (NO)-dependent direct nitration of l-tryptophan (l-Trp) to form 4-nitrotryptophan (4-NO₂-l-Trp). This nitrated product is a precursor for thaxtomin A, a virulence factor produced by plant-pathogenic bacteria that causes the disease potato scab. A recent study provided the first characterization of intermediates along the TxtE nitration pathway.¹ The authors’ accumulated evidence supported a mechanism in which O₂ binds to Fe^II TxtE to form an {FeO₂}⁸ intermediate, which subsequently reacted with NO to ultimately form Fe^III TxtE and 4-NO₂-l-Trp. Typical CYP mechanisms reduce and protonate the {FeO₂}⁸ intermediate to form a ferric-hydroperoxo species (Fe^III–OOH) en route to formation of the active oxidant compound I. The previously reported lack of hydroxylated tryptophan resulting from TxtE turnover suggests that the TxtE cycle must stall at the {FeO₂}⁸ intermediate to avoid hydroxylation. Here we present LC-MS experiments showing suggesting that TxtE can hydroxylate l-Trp by the peroxide shunt but not via reduction of the {FeO₂}⁸ intermediate. Comparison of stopped-flow time courses in the presence and absence of excess reducing equivalents and common CYP electron transfer partners shown no spectral or kinetic evidence for reduction of the {FeO₂}⁸ intermediate. Furthermore, the electron coupling efficiency of TB14—a self-sufficient TxtE variant with C-terminal reductase domain—to form 4-NO₂-l-Trp exhibits a 3% electron coupling efficiency when it is loaded with one reducing equivalent. This efficiency increases by 2-fold when TB14 is loaded with two or four reducing equivalents. This observation provides further evidence for our key conclusion that the TxtE {FeO₂}⁸ intermediate resists reduction. The resistance of the {FeO₂}⁸ intermediate to reduction is a key feature of TxtE, enabling reaction with NO and efficient nitration turnover.