Strongly Coupled Redox-Linked Conformational Switching at the Active Site of the Non-Heme Iron-Dependent Dioxygenase, TauD

2-Oxoglutarate (2OG)-dependent dioxygenases catalyze C-H activation while performing a wide range of chemical transformations. In contrast to their heme analogues, non-heme iron centers afford greater structural flexibility with important implications for their diverse catalytic mechanisms. We characterize an in situ structural model of the putative transient ferric intermediate of 2OG:taurine dioxygenase (TauD) by using a combination of spectroelectrochemical and semi-empirical computational methods, demonstrating that the Fe (III/II) transition involves a substantial, fully reversible, redox-linked conformational change at the active site. This rearrangement alters the apparent redox potential of the active site between -127 mV for reduction of the ferric state and 171 mV for oxidation of the ferrous state of the 2OG-Fe-TauD complex. Structural perturbations exhibit limited sensitivity to mediator concentrations and potential pulse duration. Similar changes were observed in the Fe-TauD and taurine-2OG-Fe-TauD complexes, thus attributing the reorganization to the protein moiety rather than the cosubstrates. Redox difference infrared spectra indicate a reorganization of the protein backbone in addition to the involvement of carboxylate and histidine ligands. Quantitative modeling of the transient redox response using two alternative reaction schemes across a variety of experimental conditions strongly supports the proposal for intrinsic protein reorganization as the origin of the experimental observations

Ligand Dynamics in the Binuclear Site in Cytochrome Oxidase

The dioxygen-reduction mechanism in cytochrome oxidase relies on proton control of the electron-transfer events that drive the process. Recent work on proton delivery and efflux channels in the protein that are relevant to substrate reduction and proton pumping is considered, and the current status of this area is summarized. Carbon monoxide photo dissociation and the ligand dynamics that occur subsequent to photolysis have been valuable tools in probing possible coupling schemes for linking exergonic electron-transfer chemistry to endergonic proton translocation. Our picosecond-time-resolved Raman results show that the heme a3- proximal histidine bond remains intact following CO photo dissociation but that the local environment around the heme a3 center in the photoproduct is in a nonequilibrium state. This photoproduct relaxes to its equilibrium configuration on the same time scale as ligand release occurs from CUB' which suggests a coupling between the two events and a potential signaling pathway between the site of O2 binding and reduction and the putative element, CUB' that links the redox chemistry to the proton pump