Hemoglobins of the tadpole of the bullfrog, Rana catesbeiana. Amino acid sequence of the beta chain of a major component.

The amino acid sequence of the beta chain of component III of the hemoglobin of the tadpole of the bullfrog. Rana catesbeiana, has been determined. Comparison of this sequence with the 117 identified residues of the beta chain in the adult bullfrog shows that 50% (59 of 117) of the residues are identical; 55% (81 of 146) are identical in the comparison with the human beta chain. Tadpole hemoglobin lacks most of the residues believed responsible for the alkaline Bohr effect in human hemoglobin: the NH2 terminus is acetylated and histidine H5 (alpha 122) in the human alpha chain is replaced by glutamine in the tadpole. Although the tadpole beta chain has a COOH-terminal histidine, the hydrogen bond responsible in human hemoglobin for about one-half the normal alkaline Bohr effect cannot form, because asparagine rather than aspartate is present at position 94. However, histidyl residue H21 (beta 143), invoked to explain the reversed "acid" Bohr effect in human hemoglobin, is present in the tadpole chain and is adjacent to a seryl residue (beta 144) rather than lysyl residue, so that the pK of the beta 143 histidine should be higher than in human hemoglobin. This could explain the substantial acid Bohr effect in tadpole hemoglobin.