Resonance Raman investigation of redox-induced structural changes of protein and heme in the sensor domain of Ec DOS protein

The direct oxygen-sensor protein from Escherichia coli (Ec DOS) is a heme-based signal transducer protein responsible for phosphodiesterase (PDE) activity. We investigated the redox-dependent conformational changes of the sensor domain (Ec DOSH) of Ec DOS by using ultraviolet resonance Raman (UVRR) spectroscopy excited at 229 and 244 nm in combination with site-directed mutagenesis. The UVRR-difference spectra (reduced-oxidized) of the wild-type (WT) revealed several features for the Trp and Tyr bands, indicating that these residues undergo environmental changes. Some of these features were assigned to Trp53, Tyr126, and Tyr80, which are located near the 2-vinyl, 4-vinyl, and propionate side chains of heme, respectively. Other changes of UVRR spectra were assigned to Tyr55, Trp110, and Tyr125, which are located near the surface of Ec DOSH, implying that this protein undergoes global conformational changes upon redox change of heme. Furthermore, the mutation of Tyr55 in the full-length protein (Ec DOS), which is located in heme proximal side near Trp53, perturbed the PDE activity compared with WT, demonstrating the importance of Tyr55 for the catalytic reaction. Finally, visible RR spectra of Ec DOSH showed significant changes in vibrations of heme peripheral groups upon a redox change. Thus, our results strongly suggest that heme redox changes induce structural changes of the heme side chains, and then are communicated to the surface of the sensor domain through Trp53, Tyr126, and Tyr80, resulting in global changes of the protein conformation.