Proton nuclear Overhauser effect investigation of the heme pockets in ligated hemoglobin: conformational differences...

Proton nuclear Overhauser effect (NOE) measurements have been used extensively to investigate the detailed conformations of peptides, proteins, and nucleic acids in the solution state. However, much of the published work has dealth with molecules of molecular weight less than 15 000. It is generally thought that specific NOEs cannot be observed in larger molecules (due to spin diffusion), so that NOE is of little use in conformational studies of such systems. By use of truncated-driven NOE with an irradiation time of 100 ms, specific NOEs are observed in a protein of the size of human normal adult hemoglobin (Hb A, 65 000 daltons). This technique has permitted us to assign several proton proton resonances arising from heme groups and from amino acid residues situated in the vicinity of the ligand binding site (such as E7 histidine and E11 valine) of the alpha and beta chains of Hb A. In addition, two-dimensional 1H[1H] J-correlated spectroscopy (COSY) experiments as well as theoretical ring-current calculations have confirmed the spectral assignments obtained by the one-dimensional NOE experiments. These new results not only have permitted us to map the heme pockets and to investigate the conformational differences in the heme pockets between oxy and carbonmonoxy forms of Hb A but also have demonstrated that the technique of truncated-driven NOE can be used to investigate the detailed conformations of selected regions in larger macromolecules in a way heretofore thought not to be feasible.