Quaternary transformation induced changes at the heme in deoxyhemoglobins

Abstract

Quaternary-structure-induced differences in the high- and low-frequency regions of the resonance Raman spectrum of the heme were detected in a variety of Hb. These differences may be the result of changes in the amino acid sequence, induced by genetic and chemical modifications, and alterations in the quaternary structure. For samples in solution in low ionic strength buffers, differences in the 1357-cm-1 line (an electron-density-sensitive vibrational mode) correlated with differences in the 216-cm-1 line (the Fe-histidine stretching mode). Changes in the Fe-histidine bond and changes in the .pi.-electron density of the porphyrin depended upon a common heme-globin interaction. The quaternary-structure-induced changes in the vibrational modes associated with the heme demonstrated that there was extensive communication between the heme and the globin and impact on models for the energetics of cooperativity. The local interactions of the Fe-histidine mode were energetically small and destabilized the deoxy heme in the T structure with respect to the R structure. These interactions must be larger in the ligated protein than in the deoxy protein to obtain a negative free energy of cooperativity. The deprotonation of the proximal histidine does not play a major role in the energetics of cooperativity. Models for cooperativity that require conformational changes in the Fe-histidine bond or direct interaction between the porphyrin and the protein were qualitatively consistent with the observed variation of heme electronic structure in concert with protein quaternary structure.