HEMOGLOBIN ALLOSTERY - RESONANCE RAMAN-SPECTROSCOPY OF KINETIC INTERMEDIATES

The end states, R and T, of the allosteric transition in hemoglobin (H b) are structurally well characterized, but there is little informatio n on intermediate structures along the allosteric pathway. These inter mediates were examined by means of time-resolved resonance Raman spect roscopy in the nanosecond-to-microsecond interval after HbCO photolysi s. Complementary spectra of the heme group and of the tyrosine and try ptophan residues were recorded during laser excitation at 436 and 230 nanometers. These spectra reveal a sequence of interleaved tertiary an d quaternary motions during the photocycle, motions involving the prox imal and distal helices, and the alpha(1) beta(2) subunit interface. T his sequence leads to a modified form of the T state, in which the alp ha(1) beta(2) interface is deformed as a result of two carbon monoxide molecules binding to the same dimer within the tetramer.