Photoacoustic Calorimetry Studies of O-Sensing FixL and (R200, I209) Variants from Reveal Conformational Changes Coupled to Ligand Photodissociation from the Heme-PAS Domain.

FixL is an oxygen-sensing heme-PAS protein that regulates nitrogen fixation in the root nodules of plants. In this paper, we present the first photothermal studies of the full-length wild-type FixL protein from and the first thermodynamic profile of a full-length heme-PAS protein. Photoacoustic calorimetry studies reveal a quadriphasic relaxation for FixL*WT and the five variant proteins (FixL*R200H, FixL*R200Q, FixL*R200E, FixL*R200A, and FixL*I209M) with four intermediates from <20 ns to ∼1.