The hemoglobins of the bullfrog, Rana catesbeiana. Deoxygenation-linked association of tetrameric components B and C to form the trimer BC2: sedimentation analysis and oxygen equilibria.

Hemolysates from the adult bullfrog, Rana catesbeiana, show an unusually high degree of cooperativity of oxygen binding with Hill coefficients greater than 4. The principal components of the tetrameric hemoglobin, B and C, do not show this high cooperativity when isolated, but it reappears when the components are mixed. Sedimentation velocity measurements show that the unusual behavior results from the mixed association of components B and C to form complexes larger than tetramers. Computer simulation of the sedimentation behavior of mixtures of deoxygenated B and C components shows that the gradient profiles can be satisfactorily described in terms of an equilibrium between the B and C tetramers and a BC2 trimer. The simplest model consistent with the results is the mixed association: B + C<-->BC and BC + C<-->BC2, with the second binding constant being higher than the first, indicating significant cooperativity. The extent of association is highest at low pH and low temperature. The dissociation of the B.C complex with low oxygen affinity to higher affinity B and C molecules during oxygenation results in greatly increased cooperativity of oxygen binding with higher Hill coefficients than possessed by either component alone in equilibria measured between 5 and 25 degrees C and between pH 6 and 8.