Allosteric Regulation of Redox Potential and Enzyme Activity of Tryptophan Dioxygenase

Human tryptophan dioxygenase (hTDO) is a tetrameric heme-containing enzyme that is responsible for metabolizing 95% of our dietary Trp, the least abundant essential amino acid. It catalyzes the first and rate-limiting step of the kynurenine pathway by dioxygenation of Trp to N-formylkynurenine. hTDO is mostly expressed in the liver. In addition to be crucial for Trp homeostasis, hTDO has been implicated in a variety of human diseases including cancer, neurological and immunological diseases. hTDO is hence considered as an important drug target. Here we show that hTDO contains two Trp binding sites: an active site, where the dioxygenation reaction takes place, and an exo site, where Trp, or other effector molecules, binds. Mutagenesis, biochemical and biophysical studies indicate that the L-Trp affinity towards the exo site is 15-fold higher than that towards the active site; in addition, the occupation of the exo site by L-Trp changes the conformation of the enzyme, thereby perturbing the redox potential of the heme as well as the activity of the enzyme. The data will be discussed in the context of the structural and functional relationships of the enzyme.