Resonance Raman spectra of cobalt-substituted hemoglobin: cooperativity and displacement of the cobalt atom upon oxygenation.

The resonance Raman spectra of oxy and deoxy cobalt-substituted hemoglobin (CoHb) are reported. Comparison of these spectra to those of hemoglobin, methemoglobin, cytochrome c, and model cobalt porphyrin complexes suggests that the displacement of the cobalt atom upon oxygenation of CoHb is no greater than the out-of-plane distance in five-coordinate Co(II) porphyrins, 0.15 A. Combining this distance with the expected contraction of the cobalt-histidine bond, Ibers has estimated a maximum displacement of 0.37 A for the proximal histidine with respect to the heme plane upon oxygenation, about one-third the corresponding distance estimated for iron hemoglobin. The free energy of cooperativity for cobalt hemoglobin is also estimated to be one-third that of iron hemoglobin. These results are therefore consistent with Hopfield's distributed energy model, which predicts proportionality between proximal histidine displacement and the free energy of cooperativity. By implication they support Perutz's trigger mechanism for cooperativity.