Pathway of Information Transmission from Heme to Protein upon Ligand Binding/Dissociation in Myoglobin Revealed by UV Resonance Raman Spectroscopy

Abstract

Gas sensory heme proteins respond to their environment by binding a specific gas molecule to heme and transmitting this primary binding signal to the protein. How the binding signal is transmitted from the heme to the protein remains to be clarified. Using UV resonance Raman (UVRR) spectroscopy, we investigated this pathway in sperm whale myoglobin as a model gas sensory heme protein. Based on the UVRR data and the effects of deleting one of three important pathways (His-93, 6-propionate, or 7-propionate), we determined the changes in the conformation of globin that occur upon binding of CO, nitric oxide (NO), or O(2) to heme and how they are transmitted from heme to globin. The UVRR results show that heme discriminates different ligands, resulting in different conformations in the globin protein. Specifically, NO induces changes in the spectrum of Trp residues in the A-helix that are significantly different from those induced by O(2) or CO binding. On the other hand, binding of O(2) to heme produces changes in the Tyr residues of the H-helix that are different from those induced by CO or NO binding. Furthermore, we found that cleavage of the Fe-His-93 covalent bond eliminates communication to the terminal region of the H-helix and that the 7-propionate hydrogen-bonding network is essential for transmitting the CO or NO binding signal to the N and C termini. Finally, the 6-propionate is important only for NO binding. Thus, the hydrogen-bonding network in the protein appears to be critical for intramolecular signal transduction in gas sensory heme proteins.