Determination of Conformational Entropy of Fully and Partially Folded Conformations of Holo- and Apomyoglobin

Holo- and apomyoglobin can be stabilized in native folded, partially folded molten globules (MGs) and denatured states depending on the solvent composition. Although the protein has been studied as a model system in the field of protein folding, little is known about the internal dynamics of the different structural conformations on the picosecond time scale. In a comparative experimental study we investigated the correlation between protein folding and dynamics on the picosecond time scale using incoherent quasielastic neutron scattering (QENS). The measured mean square displacements (MSDs) of conformational motions depend significantly on the secondary structure content of the protein, whereas the correlation times of the observed internal dynamics were found to be similar irrespective of the degree of folding. The conformational entropy difference ΔSconf between the folded conformations and the acid denatured state could be determined from the measured MSDs and was compared to the entropy difference ΔS obtained from thermodynamic parameters reported in the literature. The observed difference between ΔS and ΔSconf was attributed to the entropy difference ΔShydr of dynamically disordered water molecules of the hydration shell. The entropy content of the hydration water is significantly larger in the native folded proteins than in the partially folded MGs. We demonstrate the potential of incoherent neutron scattering for the investigation of the role of conformational dynamics in protein folding.