TDO2 is a ferrous heme enzyme that catalyzes the first and rate-limiting step in the kynurenine pathway which is the major pathway of tryptophan metabolism. TDO2 integrates oxygen into the indole moiety of tryptophan. TDO2 has broad specificity towards tryptamine and derivatives including D- and L-tryptophan, 5-hydroxytryptophan and serotonin.