R-STATE AND T-STATE OF FLUOROMETHEMOGLOBIN PROBED BY ULTRAVIOLET RESONANCE RAMAN-SPECTROSCOPY

Ultraviolet resonance Raman spectra are reported for the fluoride complex of methemoglobin (metHbF), with and without added inositol hexaphosphate (IHP), an allosteric effector known to stabilize the T quaternary structure. Environmental changes in Trp and Tyr residues give rise to the difference features, which are similar to those observed in the difference spectrum between deoxyHb and the CO adduct, consistent with T-state formation in metHbF in the presence of IHP. There are, however, important differences. The intensities of the difference signals are attenuated by about one-third, indicating a lower T-state population in the IHP-bound metHbF than in native deoxyHb. And a new signal is seen which arises from the interior tryptophans, probably reflecting a change in their H-bond status associated with the presence of fluoride as the sixth heme ligand in the T state. Implications of these results for the nature of the molecular forces opposing ligation in the T state are discussed.