Binding of nitric oxide to reduced L-tryptophan-2,3-dioxygenase as studied by electron paramagnetic resonance

Ferrous L-tryptophan-2,3-dioxygenase reacts with nitric oxide both in the presence and in the absence of L-tryptophan. Electron paramagnetic resonance studies suggest that the proximal ligand of the heme is a nitrogen atom, probably from an histidyl residue. The interaction of the protein with substrate changes both the symmetry of the paramagnetic center and the mode of interaction of the iron atom with its two axial ligands, NO and the proximal nitrogen atom. Optical absorption and EPR spectra suggest that the affinity of NO for tryptophan dioxygenase increases in the order: tryptophan dioxygenase, tryptophan dioxygenase + alpha-methyltryptophan, tryptophan diogenase " 5-hydroxytryptophan, tryptophan dioxygenase + L-tryptophan. A possible correlation between the number of superhyperfine lines in the EPR spectrum and the affinity of the enzyme for NO is discussed.