title: Oxygen binding and aggregation of bullfrog hemoglobin.
abstract: The hemoglobin of the bullfrog, Rana catesbeiana, forms aggregates larger than tetramers in two ways. The first, which results from intermolecular disulfide bonds, can be prevented by treatment with iodoacetamide. The second way results from the association of the deoxygenated forms of the two major components, B and C, to form reversibly an aggregate which is believed to be a trimer, BC2. The sedimentation velocity data show that the stoichiometry of the aggregate cannot be 1:1. The electrophoretic pattern of the deoxygenated B/C mixture suggests that the association is not indefinite. No significant aggregation of the separate deoxygenated tetramers of the components nor of the oxygenated components or mixture occurs. Gel chromatography of the oxygenated forms of components B and C and of mixtures indicates that the B and C tetramers both form dimers upon dilution with a dissociation constant of 2-3 micron. The oxygen-binding data indicate that the B/C aggregate has a much lower oxygen affinity than its constituent tetramers. Dissociation of the low affinity B/C aggregate to higher affinity B and C tetramers with increasing oxygenation gives rise to enhanced cooperativity as measured by the Hill coefficient which is maximal near 75-80% oxygenation and is as high as 4.1 at a heme concentration of 15 mM.
mesh_terms: D000818:Animals; D006454:Hemoglobins / Q000378:metabolism*; D007460:Iodoacetamide / Q000494:pharmacology; D007700:Kinetics; D046911:Macromolecular Substances; D010100:Oxygen / Q000097:blood*; D010108:Oxyhemoglobins / Q000302:isolation & purification / Q000378:metabolism*; D011892:Rana catesbeiana
chemical_list: D006454:Hemoglobins; D046911:Macromolecular Substances; D010108:Oxyhemoglobins; D010100:Oxygen; D007460:Iodoacetamide