Abstract
The first step of the kynurenine pathway for l-tryptophan (l-Trp) degradation is catalyzed by heme-dependent dioxygenases, tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase. In this work, we employed stopped-flow optical absorption spectroscopy to study the kinetic behavior of the Michaelis complex of Cupriavidus metallidurans TDO (cmTDO) to improve our understanding of oxygen activation and initial oxidation of l-Trp. On the basis of the stopped-flow results, rapid freeze-quench (RFQ) experiments were performed to capture and characterize this intermediate by Mössbauer spectroscopy. By incorporating the chlorite dismutase-chlorite system to produce high concentrations of solubilized O(2), we were able to capture the Michaelis complex of cmTDO in a nearly quantitative yield. The RFQ-Mössbauer results confirmed the identity of the Michaelis complex as an O(2)-bound ferrous species. They revealed remarkable similarities between the electronic properties of the Michaelis complex and those of the O(2) adduct of myoglobin. We also found that the decay of this reactive intermediate is the rate-limiting step of the catalytic reaction. An inverse α-secondary substrate kinetic isotope effect was observed with a k(H)/k(D) of 0.87 ± 0.03 when (indole-d(5))-l-Trp was employed as the substrate. This work provides an important piece of spectroscopic evidence of the chemical identity of the Michaelis complex of bacterial TDO.