RESONANCE RAMAN STUDIES OF THE HEME ACTIVE-SITE OF THE HOMODIMERIC MYOGLOBIN FROM NASSA-MUTABILIS - A PECULIAR CASE

A spectroscopic investigation by resonance Raman has been carried out at pH 7.0 in 0.1 M phosphate buffer on the cooperative homodimeric myo globin from Nassa mutabilis. The study has been performed on the unlig ated ferrous form, as well as on the ligated species MbO(2) and MbCO, and on the ferric form met-Mb. Two nu(C=C) vinyl stretching modes have been observed in all the investigated forms, reflecting different deg rees of vinyl conjugation with the porphyrin ring, as a consequence of a strongly asymmetric environment for the two side groups of the heme . Furthermore, the ferric form displays a hexacoordinate low-spin heme , which suggests the presence of an endogenous ligand bound to the Fe atom. The frequency of the nu(Fe-Im) stretching mode of Mb from Nassa mutabils shifts down by 4 cm(-1) as compared with that of horse heart myoglobin, reflecting a protein-induced proximal strain as a result of heme-heme interaction due to the close proximity of the two hemes in the dimer. The lower frequency of the nu(Fe-Im) stretching mode agrees well with the lower affinity for oxygen binding found for Nassa mutab ilis Mb and with the slight heme core expansion with respect to horse heart Mb, suggesting a critical role for the Fe-His bond on the heme's function and structure.