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Conformational changes associated to sensing mechanisms of heme-based protein sensors are a key molecular event that seems to modulate not only the protein activity but also the potential of the Fe III/II redox couple of the heme domain. In this work, midpoint potentials ( E m ) assigned to the Fe III/II redox couple of the heme domain of FixL from Rhizobium etli ( Re FixL) in the unliganded and liganded states were determined by spectroelectrochemistry in the presence of inorganic mediators. In comparison to the unliganded Re FixL protein (+ 19 mV), the binding to ligands that switch off the kinase activity induces a negative shift, i. e. E m = − 51, − 57 and − 156 mV for O 2 , imidazole and CN − , respectively. Upon binding to CO, which does not affect the kinase active, E m was observed at + 21 mV. The potential values observed for Fe III/II of the heme domain of Re FixL upon binding to CO and O 2 do not follow the expected trend based on thermodynamics, assuming that positive potential shift would be expected for ligands that bind to and therefore stabilize the Fe II state. Our results suggest that the conformational changes that switch off kinase activity upon O 2 binding have knock-on effects to the local environment of the heme, such as solvent rearrangement, destabilize the Fe II state and counterbalances the Fe II -stabilizing influence of the O 2 ligand. [ABSTRACT FROM AUTHOR]