Probing the ligand recognition and discrimination environment of the globin-coupled oxygen sensor protein YddV by FTIR and time-resolved step-scan FTIR spectroscopy

YddV is a newly discovered signal transducer heme protein that recognizes O-2 and CO. Structural differences in the ligand-bound heme complex in YddV reflect variations in catalytic regulation by O2 and CO. Time-resolved step-scan (TRS2) FTIR studies of the wild type and of the important in oxygen recognition and stability of the heme Fe(II)-O-2 complex L65M, L65T, Y43A, Y43F and Y43W mutants were performed to determine the site-specific protein dynamics following carbon monoxide (CO) photodissociation. These mutations were designed to perturb the electrostatic field near the iron-bound gaseous ligand (CO) and also to allow us to investigate the communication pathway between the distal residues of the protein and heme. TRS2-FTIR spectra of YddV-heme-CO show that the heme propionates are in protonated and deprotonated states. Moreover, the rate of decay of the vibrations of amide I is on a time scale that coincides with the rate of rebinding of CO, which suggests that there is coupling between ligation dynamics in the distal heme environment and (i) relaxation of the protein backbone and (ii) the environment sensed by the heme propionates. The fast recombination rates in L65M, L65T and Y43W imply a significant role of L65 and Y43 in controlling the ligand dynamics. The implications of these results with respect to the role of the heme propionates and the charged or proton-donating residues in the distal pocket, which are crucial for stabilizing bound gaseous ligands, are discussed.