Ligand binding to the dimeric hemoglobin from Scapharca inaequivalvis, a hemoglobin with a novel mechanism for cooperativity.

The homodimeric hemoglobin from Scapharca inaequivalvis has an unusual spatial arrangement of the subunits (Royer, W.E., Jr., Love, W.E., and Fenderson, F.F. (1985) Nature 316, 277-280). The time course of oxygen and nitric oxide rebinding to this protein following flash photolysis has been measured on a nanosecond time scale. A large amplitude is observed with a half-time of 20 ns (NO). With oxygen the half-time decreases from 70 ns at low fractional photolysis to 30 ns at large breakdown. The second order rate of NO binding is 1.6 x 10(7)/MS, and is the same as that for oxygen. Analysis of the geminate data suggests that oxygen and nitric oxide react more rapidly with the heme than in myoglobin, but also escape much more rapidly from its vicinity.