Mössbauer spectroscopic evidence on the heme binding to the proximal histidine in unfolded carbonmonoxy myoglobin by guanidine hydrochloride

The unfolded heme structure in myoglobin is controversial because of no chance of direct X-ray structure analyses. The unfolding of carbonmonoxy myoglobin (MbCO) by guanidine hydrochloride (GdnHCl) was studied by the Mössbauer spectroscopy. The spectra show the presence of a sort of spectrum in the unfolded MbCO, independent on the concentration of GdnHCl from 1 to 6 M and the increase of the fraction of unfolded MbCO, depending on the GdnHCl concentration. The isomer shift of the iron of heme in the unfolded MbCO was identified to be different from that of the native MbCO as the globin structure in Mb collapses under the unfolded conditions. This result and the existing related Mössbauer data proved that the heme in the unfolded MbCO may remain coordinated to the proximal histidine.