COOPERATIVE OXYGEN-BINDING TO SCAPHARCA-INAEQUIVALVIS HEMOGLOBIN IN THE CRYSTAL

Oxygen binding to homodimeric Scapharca inaequivalvis hemoglobin (HbI) crystals has been investigated by single-crystal polarized absorption microspectrophotometry. The saturation curve, characterized by a Hill coefficient n(H) = 1.45 and an oxygen pressure at half saturation p(5 0) = 4.8 torr, at 15 degrees C, shows that HbI in the crystalline stat e retains positive cooperativity in ligand binding. This finding will permit the correlation of the oxygen-linked conformational changes in the crystal with the expression of cooperativity. Polarized absorption spectra of deoxy-HbI, oxy-HbI, and oxidized HbI crystals indicate tha t oxygenation does not induce heme reorientation, whereas oxidation do es. Lattice interactions prevent the dissociation of oxidized dimers t hat occurs in solution and stabilize an equilibrium distribution of pe ntacoordinate and hexacoordinate high spin species.