STEREODYNAMIC PROPERTIES OF THE COOPERATIVE HOMODIMERIC SCAPHARCA INAEQUIVALVIS HEMOGLOBIN STUDIED THROUGH OPTICAL-ABSORPTION SPECTROSCOPY AND LIGAND REBINDING KINETICS

The study of the thermal evolution of the Soret band in heme proteins has proved to be a useful tool to understand their stereodynamic prope rties; moreover, it enables one to relate protein matrix fluctuations and functional behavior when carried out in combination with kinetic e xperiments on carbonmonoxide rebinding after flash photolysis. In this work, we report the thermal evolution of the Soret band of deoxy, car bonmonoxy, and nitric oxide derivatives of the cooperative homodimeric Scapharca inaequivalvis hemoglobin in the temperature range 10-300 K and the carbonmonoxide rebinding kinetics after flash photolysis in th e temperature range 60-200 K. The two sets of results indicate that Sc apharca hemoglobin has a very rigid protein structure compared with ot her hemeproteins. This feature is brought out i) by the absence of non harmonic contributions to the soft modes coupled to the Soret band in the liganded derivatives, and ii) by the almost ''in plane'' position of the iron atom in the photoproduct obtained approximate to 10(-8) s after dissociating the bound carbonmonoxide molecule at 15 K.