Ferric Heme as a CO/NO Sensor in the Nuclear Receptor Reverbβ by Coupling Gas binding to Electron Transfer

Rev-Erb{beta} is a nuclear receptor that couples circadian rhythm, metabolism, and inflammation.1-7 Heme binding to the protein modulates its function as a repressor, its stability, its ability to bind other proteins, and its activity in gas sensing.8-11 Rev-Erb{beta} binds Fe3+-heme tighter than Fe2+-heme, suggesting its activities may be regulated by the heme redox state.9 Yet, this critical role of heme redox chemistry in defining the proteins resting state and function is unknown. We demonstrate by electrochemical and whole-cell electron paramagnetic resonance experiments that Rev-Erb{beta} exists in the Fe3+ form within the cell essentially allowing the protein to be heme-replete even at low concentrations of labile heme in the nucleus. However, being in the Fe3+ redox state contradicts Rev-Erbs known function as a gas sensor, which dogma asserts must be a Fe2+ protein This paper explains why the resting Fe3+-state is congruent both with heme-binding and cellular gas sensing. We show that the binding of CO/NO elicits a striking increase in the redox potential of the Fe3+/Fe2+ couple, characteristic of an EC mechanism in which the unfavorable Electrochemical reduction of heme is coupled to the highly favorable Chemical reaction of gas binding, making the reduction spontaneous. Thus, Fe3+-Rev-Erb{beta} remains heme-loaded, crucial for its repressor activity, and only undergoes reduction when diatomic gases are present. This work has broad implications for hemoproteins where ligand-triggered redox changes cause conformational changes influencing proteins function or inter-protein interactions, like NCoR1 for Rev-Erb{beta}. This study opens up the possibility of CO/NO-mediated regulation of the circadian rhythm through redox changes in Rev-Erb{beta}.