Ligand-dependent polymerization of tetrameric hemoglobin from the blood clam Anadara broughtonii

Hemoglobin (Hb II) of the blood clam Anadara broughtonii has a alpha 2 beta 2 sub-unit structure in athe oxy form with a sedimentation constant of 4.8 S. When deoxygenated, Hb II polymerizes with a major component, S20,w = 11.5 (above 150 microM in heme). Deoxy polymerization was not observed in a highly diluted protein below 20 microM (in heme). Gel filtration of Hb II in the deoxygenated state indicated that the major component has an apparent molecular weight of 195 000, which corresponds to a dodecamer. However, the sedimentation pattern and the elution profile of gel filtration showed the polymerization to be somewhat asymmetric. These results suggest that deoxy Hb II may polymerize with different polymerization states. We examined oxygen equilibria of Hb II in a range of 3--180 microM (in heme). Influences of the polymerization on its oxygen affinity and cooperativity were found to be very small. We have also found that the deoxy polymerization was completely prevented when all the sulfhydryl groups of the hemoglobin molecule were modified with p-chloromercuribenzoate.