title: Multiple internal reflectance infrared spectra of variably hydrated hemoglobin and myoglobin films: effects of globin hydration on ligand conformer dynamics and reactivity at the heme.
abstract: Multiple internal reflectance infrared (IR) spectra are reported for variably hydrated films (1.2-0.1 g of H2O/g of protein) of the carbon monoxy and oxy forms of human Hb and sperm whale Mb. The spectra show that even the limited removal of liquid and icelike hydration constraints at the globin surface is sufficient to cause a dramatic, but completely reversible, shift toward a normally minute population of sterically unhindered, linear-perpendicular, Fe-CO conformer modes (nu CO = 1968-1967 cm-1), and the destabilization of distally hindered, tilted (or bent), Fe-CO modes (nu CO = 1951, 1944-1933 cm-1). Corroborative evidence from IR band broadening trends [delta delta nu 1/2 (1968, 1967 cm-1) approximately 2-4 cm-1], corresponding changes in the visible, and H-D exchange kinetics confirm that the shift toward 1968-1967 cm-1 results in a more open distal heme pocket configuration and that it is also accompanied by a buildup of deoxy-like steric hindrance proximal to the heme. Denaturation effects are eliminated as a potential cause of the shifts, as are specific protein-protein, ion-protein, intersubunit, and MIR crystal-film surface interactions. The hydration effect exhibits globin-dependent and ligand-dependent differences, which highlight the intrinsic importance of distal steric effects within the heme pocket and their dynamic coupling with exterior solvent constraints. CO-photodissociation and O2-exchange experiments conducted on rapidly interconverting (coupled and fully hydrated) and noninterconverting (uncoupled and partially hydrated) Fe-CO conformers also suggest that the open linear-perpendicular mode corresponds to a more tightly bound form of CO than the axially distorted Fe-CO species; similar differences are not evident in Fe-O2, which already prefers a bent end-on geometry within the heme pocket. Control IR spectra aimed at monitoring the progressive effects of various denaturants on HbCO further indicate that this same open mode serves as a common precursor to any of a number of more highly disordered folding modes. The overall properties of the 1968-1967-cm-1 conformer are discussed in terms of (1) the possibility of its corresponding to an available relaxation mode capable of facilitating the dynamics of ligand entry-release events and (2) its potential additional significance as a native folding mode that exhibits a marked tendency to be destabilized by hydration.
mesh_terms: D002263:Carboxyhemoglobin / Q000378:metabolism; D005914:Globins; D006418:Heme; D006454:Hemoglobins / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D009211:Myoglobin / Q000378:metabolism*; D010108:Oxyhemoglobins / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation
chemical_list: D006454:Hemoglobins; D009211:Myoglobin; D010108:Oxyhemoglobins; D006418:Heme; D005914:Globins; D002263:Carboxyhemoglobin