Functional evaluation of heme vinyl groups in myoglobin with symmetric protoheme isomers

We replaced protoheme-IX in native myoglobin with the symmetric protohemes-III and -XIII, in order to investigate the role of heme vinyl-globin contacts on Mb function. The UV-visible spectra and the resonance Raman spectra in the high-frequency region (containing oxidation, spin, and coordination state marker lines) of the two reconstituted Mbs were very similar. However, the signal intensity of the Soret band in the CD spectra and the resonance Raman lines for vinyl bending modes in the low-frequency region notably differed, thereby reflecting altered heme peripheral contacts. The redox potentials, formal heterogeneous electron-transfer rates, and thermal denaturation temperatures of the two reconstituted Mbs were also indistinguishable. In addition, the oxygen binding properties of the ferrous deoxy Mbs were comparable. These results demonstrate that altered heme vinyl-globin interactions only slightly affect the physical properties of Mb. It is therefore likely that the orientation of protoheme-IX about the alpha,gamma-axis in the heme pocket is not necessarily a crucial factor for oxygen binding to native Mb.