The Influence of ATP on the Association and Unfolding of the Tyrosine Repressor Ligand Response Domain of Haemophilus influenzae

The secondary structure of the ligand response domain of the Haemophilus influenzae tyrosine repressor, TyrRlrd, was investigated using CD spectroscopy which revealed 42.5% α-helix, 17.6% β-sheet, and 39.9% loops. Quaternary structure analysis by fluorescence anisotropy showed that TyrRlrd is monomeric at a concentration of 100 nM to 2 μM but that the protein readily dimerizes in the presence of its natural ligand ATP. Equilibrium unfolding studies of TyrRlrd using guanidinium hydrochloride suggested a two-state model with no detectable stable intermediates. The unfolding transition monitored by CD spectroscopy was responsive to tyrosine and ATP resulting in a shift to higher denaturant concentrations in the presence of these ligands. Differential scanning calorimetry yielded melting temperatures, Tm, of 51.15 and 58.07°C for the unliganded and for the ATP-liganded protein, respectively. ATP is thus proposed to be a major structural cofactor for the molecular architecture of TyrRlrd.