Ultrafast ligand rebinding in the heme domain of the oxygen sensors FixL and Dos: general regulatory implications for heme-based sensors

International audienceHeme-based sensors carry out crucial roles in biological signaling by responding to the physiological messengers NO, CO and O2. Binding of these gaseous ligands to a heme sensor domain initiates chemistry, eventually resulting in the organism's response to changes in ligand availability. To get insight into the dynamical aspects underlying these features and the ligand specificity of the signal transduction from the heme domain, we used ultrafast spectroscopy to study ligand dynamics in the heme domains of the oxygen sensors FixL [1] from Bradyrhizobium japonicum (FixLH) and Dos [2] from Escherichia coli (DosH). The transient spectra after ligand photodissociation are distorted compared to the ground state difference spectra. With O2 as a ligand, this distortion is particularly strong, indicating dramatic differences in the heme environment with respect to the unliganded state. Moreover, heme-O2 recombination occurs with an efficiency unprecedented for heme proteins, in f5 ps for f90% of the dissociated O2. For DosH-O2, where the signal transduction mechanism presumably is quite different, a similarly fast recombination was found with an even higher efficiency. Altogether, these results indicate that the heme pocket in these sensors acts as a ligand-specific trap [3]. The general implications for the functioning of heme-based ligand sensors are discussed in the light of recent studies on heme-based NO [4] and CO [5] sensors