RIGIDITY OF THE HEME POCKET IN THE COOPERATIVE SCAPHARCA HEMOGLOBIN HOMODIMER AND RELATION TO THE DIRECT COMMUNICATION BETWEEN HEMES

The Soret spectra of the dimeric hemoglobin from Scapharca and of hors e myoglobin reconstituted with protoporphyrin IX and Zn-protoporphyrin IX have been measured over the range 290-80 K. With increase in tempe rature the Soret band broadens and shifts to a different extent depend ing on the protein and the presence of the metal. In the Zn-protoporph yrin IX derivatives the spectral changes are more marked in myoglobin than in the dimeric hemoglobin. In the protoporphyrin IX derivatives, in which the spectral changes are significantly reduced, the opposite is true, The data have been analyzed in terms of coupling of the prote in low-frequency vibrational motions to the porphyrin electronic trans ition (V. Srajer et al., 1986, Phys. Rev. Lett. 57, 1267-1270; A. Di P ace et al., 1992, Biophys. J. 63, 475-484). The analysis indicates tha t the heme pocket of the dimeric hemoglobin is characterized by an unu sual rigidity and that the metal plays a different role in the transmi ssion of the protein motions to the heme moiety in the dimeric hemoglo bin and in myoglobin. Static and dynamic fluorescence measurements car ried out at room temperature are in line with these conclusions. (C) 1 995 Academic Press, Inc.