Probing the anion binding promiscuity of the soluble nitrate sensor NreA from Staphylococcus carnosus.

Promiscuity, or selectivity on a spectrum, is an encoded feature in biomolecular anion recognition. To unravel the molecular drivers of promiscuous anion recognition, we have employed a comprehensive approach - spanning experiment and theory - with the Staphylococcus carnosus nitrate regulatory element A (ScNreA) as a model. Thermodynamic analysis reveals that ScNreA complexation with native nitrate and nitrite or non-native iodide is an exothermic process. Further deconvolution of the association and dissociation kinetics for each anion reveals that the release event can be limiting, in turn, giving rise to the observed selectivity: nitrate > iodide > nitrite. These conclusions are supplemented with molecular dynamics simulations that capture an entry and exit pathway coupled to subtle global protein motions unique to each anion. Taken together, our data point to how structural plasticity of the binding pocket controls the relative promiscuity of ScNreA to guarantee physiological nitrate sensing.