Human tryptophan dioxygenase: A comparison to indoleamine 2,3-dioxygenase

In contrast to the diverse superfamily of monooxygenases, there are only two classes of heme-containing dioxygenases in humans. One is tryptophan 2,3 dioxygenase (hTDO), and the other is indoleamine 2,3-dioxygenase (hIDO), both of which catalyze the oxidative degradation of Trp to N-formyl kynurenine. Although hTDO and hIDO catalyze the same reaction, they engage in distinct physiological functions. The molecular properties of hTDO, unlike hIDO, have never been explored in the past. Here, we report the first structural and functional characterization of hTDO with resonance Raman and optical absorption spectroscopies. We show that the proximal Fe-His stretching frequency of hTDO is 229 cm^-1, 7 cm^-1 lower than that of hIDO, indicating its weaker imidazolate character as compared to hIDO. In the CO derivative of the L-Trp-bound enzyme, the Fe-CO stretching and C-O stretching frequencies are 488 and 1972 cm^-1, respectively, suggesting that L-Trp binds to the distal pocket with its C₂-C₃ double bond facing the heme-bound ligand, in contrast to hIDO, in which the indole NH group forms an H-bond with the heme-bound ligand. Moreover, the K_m values of hTDO for D-Trp and L-Trp are similar, but the k_cat value for D-Trp is 10-fold lower than that for L-Trp. In contrast, in hIDO, the K_m value for D-Trp is 700-fold higher than L-Trp, whereas the k_cat values are comparable for the two stereoisomers. Taken together, the data indicate that the initial deprotonation reaction of the indole NH group in hTDO is carried out by the evolutionarily conserved distal His, whereas that in hIDO is performed by the heme-bound dioxygen; in addition, the stereospecificity of hTDO is determined by the efficiency of the dioxygen chemistry, whereas that in hIDO is controlled by the substrate affinity.