Different effects of subunit association upon absorption and circular dichroism spectra of methemoglobin

Abstract

Absorption and circular dichroism (CD) spectra of the Soret band, assigned as a π-π∗ transition of the porphyrin π-electron system, showed a great difference between α and β subunits in the ferric state ( α +, β +). The nonequivalence of the spectra between α + and β + subunits partly originates from the difference in the strength of the bond between heme iron and the proximal histidine. The peak positions for absorption and CD spectra of the ferric derivatives associated with F −, H 2O, N 3 − and CN − of the isolated subunits qualitatively correlate with the spin state of the ferric heme. The Soret absorption spectra obtained by simple addition of those for α + and β + subunits are very similar to those for methemoglobin A (metHB A). On the other hand, the arithmetic means for the Soret CD spectra of α + and β + subunits are different from those for metHb A. These differences were not observed between the Soret CD spectra of α 1 β 1 dimer, which is predominantly present in metHb Hirose (β37Trp-Ser), and those of tetrameric metHb A. Therefore, the interaction between α 1 and β 1 subunits to make the α 1 β 1 dimer may strongly affect the CD spectral properties of α + and β + subunits. The effect of the interaction between two homogeneous dimers, α 1 β 1 and α 2 β 2, forming a tetramer, on the Soret CD spectral properties, if any, is very small compared with that between α 1 and β 1 subunits.