Assignment of proton resonances in the NMR spectrum of carbonmonoxy hemoglobin beta subunit tetramers.

Isolated beta chains from human adult hemoglobin at millimolar concentration are mainly associated to form beta 4 tetramers. We were able to obtain relevant two-dimensional proton nuclear magnetic resonance (NMR) spectra of such supermolecular complexes (Mr approximately 66,000) in the carboxylated state. Analysis of the spectra enabled us to assign the major part of the proton resonances corresponding to the heme substituents. We also report assignments of proton resonances originating from 12 amino acid side chains mainly situated in the heme pocket. These results provide a basis for a comparative analysis of the tertiary heme structure in isolated beta(CO) chains in solution and in beta(CO) subunits of hemoglobin crystals. The two structures are generally similar. A significantly different position, closer to the heme center, is predicted by the NMR for Leu-141 (H19) in isolated beta chains. Comparison of the assigned resonances of conserved amino acids in alpha chains, beta chains and sperm whale myoglobin indicates a close similarity of the tertiary heme pocket structure in the three homologous proteins. Significant differences were noted on the distal heme side, at the position of Val-E11, and on Leu-H19 and Phe-G5 position on the proximal side.