The human heme enzyme tryptophan 2,3-dioxygenase (hTDO) catalyzes the insertion of dioxygen into its cognate substrate, L-tryptophan (L-Trp). Its active site structure is highly dynamic, and the mechanism of enzyme substrate ligand complex formation and the ensuing enzymatic reaction is not yet understood. Here we have studied complex formation in hTDO by using time-resolved optical and infrared spectroscopy with carbon monoxide (CO) as a ligand. We have observed that both substrate-free and substrate-bound hTDO coexist in two discrete conformations with greatly different ligand binding rates. In the fast rebinding hTDO conformation, there is facile ligand access to the heme iron, but it is greatly hindered in the slowly rebinding conformation. Spectroscopic evidence implicates active site solvation as playing a crucial role for the observed kinetic differences. Substrate binding shifts the conformational equilibrium markedly toward the fast species and thus primes the active site for subsequent ligand binding, ensuring that formation of the ternary complex occurs predominantly by first binding L-Trp and then the ligand. Consequently, the efficiency of catalysis is enhanced because 02 binding prior to substrate binding, resulting in nonproductive oxidation of the heme iron, is greatly suppressed.