Structural dynamics of the heme pocket and intersubunit coupling in the dimeric hemoglobin from Scapharca inaequivalvis.

This article focuses on the structural dynamics and cooperativity of hemoglobin from *Scapharca inaequivalvis* (HbI) following carbon monoxide (CO) dissociation, investigated using time-resolved resonance Raman (RR) spectroscopy. The study reveals that ligand dissociation induces synchronous structural rearrangements in the Fe–proximal histidine bond, heme positioning within the pocket, and hydrogen bonding involving heme propionates and interfacial water, which correlate with subunit rotation and quaternary structural changes. Differences in the kinetics and spectral changes between singly and doubly dissociated HbI species highlight intersubunit coupling underlying HbI's positive cooperativity, contrasting with the multi-step transitions observed in human adult hemoglobin (HbA) due to differences in subunit assembly and interface interactions. These findings advance understanding of allosteric mechanisms in homodimeric hemoglobins and their protein dynamics regulating cooperative ligand binding.