The hemoglobins of the bullfrog Rana catesbeiana. The structure of the beta chain of component C and the role of the alpha chain in the formation of intermolecular disulfide bonds.

The adult bullfrog Rana catesbeiana has two major hemoglobin components, B and C. Component C polymerizes by disulfide bond formation between tetramers but component B does not. The amino acid sequence of the first 112 residues of the beta chain of component C has been reported (Baldwin, T. O., and Riggs, A. (1974) J. Biol. Chem. 249, 6110-6118). We have completed the sequence of the beta chain of component C by determining the last 28 residues. This segment contains the 2 cysteinyl residues of the chain. Examination of models indicates that neither of these is in a readily accessible position for the formation of intertetramer disulfide bonds. Reactive sulfhydryl groups of the alpha chains are shown to be responsible for the initial formation of disulfide bonds between tetramers. The beta chains within the tetramers form disulfide bonds only when the hemoglobin molecules are subjected to prolonged incubation at 37 degrees C under oxygen. The beta chains of components B and C appear to be identical; the alpha chains are clearly quite different. This suggests that the alpha B and alpha C subunits interact in the association of the deoxygenated tetramers B and C to form what appears to be a BC2 molecule.