Cooperative Protein Dynamics of Heterotetrameric Hemoglobin from Scapharca inaequivalvis

Hemoglobins achieve cooperative oxygen binding by diverse strategies based on different assemblies of globin subunits. Heterotetrameric hemoglobin from Scapharca inaequivalvis (HbII) consists of two AB-dimers, whose structure closely resembles that of homodimeric hemoglobin from the same organism (HbI). Herein, we investigated the structural dynamics of HbII following carbon monoxide (CO) dissociation using time-resolved resonance Raman (RR) spectroscopy. The observed spectra showed that the heme structure of the transient dissociated form of HbII was similar to that of HbI; however, the transition from the transient dissociated form to the equilibrium unligated form was faster for HbII than for HbI. Furthermore, the dependence of the time-resolved spectra on the yield of CO dissociation revealed that the transition became faster as the number of dissociated ligands increased from one to four. The positive correlation between the rate constants and number of dissociated ligands indicates that the structural transition of HbII following CO dissociation is cooperative.