EFFECT OF THE VINYL-GLOBIN INTERACTIONS ON THE TEMPERATURE-DEPENDENT BROADENING OF THE SORET SPECTRA - A STUDY WITH HORSE MYOGLOBIN AND SCAPHARCA DIMERIC HEMOGLOBIN RECONSTITUTED WITH UNNATURAL 2,4-HEME DERIVATIVES

The temperature dependence of the Soret absorption spectra has been me asured over the range 80 to 300 K on deoxygenated and carbonmonoxy hor se heart myoglobin and Scapharca inaequivalvis dimeric hemoglobin reco nstituted with proto- or with meso- and deutero-heme, in which the vin yl groups have been replaced with ethyl groups or hydrogen atoms, resp ectively, In the meso- and deutero-derivatives of both proteins the li newidth of the absorption spectra is narrower and less sensitive to th ermal broadening effects than in the proto-derivatives. Moreover, the broadening effects are larger in the deoxygenated proteins with respec t to the liganded adducts. The quantitative analysis of these effects shows that the change in linewidth is due to a marked decrease in the extent of coupling between the heme vibronic transitions and the prote in low-frequency motions. The relevance of the vinyl groups in the dyn amics of the heme-globin interaction is highlighted by this experiment al approach which shows that the protein is capable of transmitting st ructural information to the heme by coupling the ensemble of the low-f requency modes to the stereochemistry of the vinyl itself. This mechan ism, which entails adjustment of the equilibrium between vinyl torsion al conformers, represents an additional pathway for the control of the heme reactivity in addition to the iron-histidine link. (C) 1997 Acad emic Press.