METASTABLE CO BINDING-SITES IN THE PHOTOPRODUCT OF A NOVEL COOPERATIVE DIMERIC HEMOGLOBIN

The infrared absorption spectrum of the CO-photoproduct from Scapharca inaequivalvis hemoglobin (HbI) at 10 K yields only a single line in t he ''B'' state region at 2132 cm-1. This is the same frequency as the B1 line observed in photodissociated vertebrate HbCO and MbCO. No evid ence was found for the B2 line detected in vertebrate hemoglobins and myoglobin in the 2118-2120 cm-1 region. These data demonstrate that th e protein does not have the same conformationally accessible ligand-bi nding sites as do vertebrate hemoglobins and myoglobins. The absence o f the B2 line indicates that only a single weak site is accessible to the photolyzed CO molecule. These results are in accord with geminate rebinding experiments and ligand escape pathway calculations which hav e shown that the distal properties of HbI are distinct from those of t etrameric hemoglobins and vertebrate myoglobins.