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

Abstract

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 Δ H of −19 kcal mol −1 and Δ V of +4 ml mol −1 as well as a red-shifted kinetic difference spectrum all occurring in <50 ns. Analysis of the Δ H/Δ V reveals that a conformational change takes place with a Δ H conf of −40 kcal mol −1 and Δ V conf 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.