Indoleamine 2,3-dioxygenase (IDO) is a heme enzyme which catalyzes dioxygenation of l-Trp (tryptophan), yielding N-formylkynurenine. IDO thus plays a key role in l-Trp catabolism in mammals. In the present study, resonance Raman (RR) spectra of the reduced carbon monoxide- (CO-) bound form of IDO were measured in order to gain insights into the active site environment of O(2). Binding of CO to l-Trp-bound IDO causes a significant change in the electronic and RR spectra of the heme, indicating that the π* orbitals of the carbon atom of CO interact with π orbitals of Fe and the porphyrin. On the other hand, binding of CO to d-Trp-bound IDO does not induce the same change. This is also the case with substrate-free IDO. Based on the distinct absorption spectra and RR bands of the vibrational signature of CO (ν(CO), δ(FeCO), and ν(Fe-CO)) of the l-Trp-bound species relative to the other two species, it is confirmed that sterically constrained geometry of the Fe-O-O unit exists as previously reported (Terentis, A. C., et al. (2002) J. Biol. Chem. 277, 15788-15794). In contrast, binding of d-Trp does not induce such constraint. The comparable values of V(max) reported for l-Trp and d-Trp are interpreted as a result of a change in the rate-limiting step in the reaction cycle of the enzyme induced by the d-enantiomer relative to the l-enantiomer. Enhancements of the overtone and the combination Raman modes of the Fe-CO stretching vibration are evident. The anharmonicity of the Fe-CO stretching oscillator is significantly higher than those of oxygen carrier proteins. This is a specific character of IDO and might be responsible for the unique reactivity of this enzyme.