Structural, functional, and spectroscopic characterization of the substrate scope of the novel nitrating cytochrome P450 TxtE

A novel cytochrome P450 enzyme, TxtE, was recently shown to catalyze the direct aromatic nitration of L-tryptophan. This unique chemistry inspired us to ask whether TxtE could serve as a platform for engineering new nitration biocatalysts to replace current harsh synthetic methods. As a first step toward this goal, and to better understand the wild-type enzyme, we obtained high-resolution structures of TxtE in its substrate-free and substrate-bound forms. We also screened a library of substrate analogues for spectroscopic indicators of binding and for production of nitrated products. From these results, we found that the wild-type enzyme accepts moderate decoration of the indole ring, but the amino acid moiety is crucial for binding and correct positioning of the substrate and therefore less amenable to modification. A nitrogen atom is essential for catalysis, and a carbonyl must be present to recruit the αB'1 helix of the protein to seal the binding pocket.