Evidence for fast conformational change upon ligand dissociation in the HemAT class of bacterial oxygen sensors

Here we report the results of transient absorption and photoacoustic calorimetry studies of CO photodissociation from the heme domain of the bacterial oxygen sensor HemAT-Bs. The results indicate that CO photolysis is accompanied by an overall ΔH of −19 kcal mol−1 and ΔV of +4 ml mol−1 as well as a red-shifted kinetic difference spectrum all occurring in <50 ns. Analysis of the ΔH/ΔV reveals that a conformational change takes place with a ΔHconf of −40 kcal mol−1 and ΔVconf of −22 ml mol−1. These thermodynamic changes are consistent with an increase in the solvent accessible surface area of the protein upon ligand dissociation, as observed in the X-ray structure of the ferric CN-bound and CN free forms of HemAT-Bs.