Globin Superfamily Research Articles

Isolation and amino acid sequence of a monomeric hemoglobin in heart muscle of the bullfrog, Rana catesbeiana.

A monomeric heme-containing protein was isolated from heart muscles of the bullfrog, Rana catesbeiana, by gel filtration on Sephadex G-75 followed by DE52 ion-exchange column chromatography. The protein is composed of 132 amino acid residues and has Mr = 14,000 estimated by gel filtration on Sephadex G-50. This is the shortest heme globin so far known. The complete amino acid sequence of the apoprotein was deduced from the amino acid sequences of cyanogen bromide fragments and tryptic peptides. In order to align its amino acid sequence with other proteins in the globin superfamily (Hunt, L. T., Hurst-Calderone, S., and Dayhoff, M. O. (1978) Atlas of Protein Sequence and Structure, Vol. 5, Suppl. 3, 229-249, National Biomedical Research Foundation, Washington, D.C.), three gaps common to alpha-hemoglobin are required plus three more gaps unique in this protein. Of the latter, one is at the end of the EF region, the second near the beginning of the H-region, and the third is at the COOH terminus. With 62 amino acid residues in common, the amino acid sequence of this monomer is more homologous to the alpha-hemoglobin of the tadpole of R. catesbeiana (Maruyama, T., Watt, K. W. K., and Riggs, A. (1980) J. Biol. Chem. 255, 3285-3293), than to any other globin. A phylogenetic study of it and other globins clearly reveals that it arose via a gene duplication of hemoglobin near the time of the duplication that gave rise to the alpha and beta genes. But residues in contact with the heme group are rather conserved while the residues in the alpha 1 beta 1, alpha 1 beta 2 subunit contact regions are significantly substituted, frequently reverting to a myoglobin-like residue. The absence of this monomeric protein from the blood and the absence of myoglobin in heart muscle may indicate the protein functions as a myoglobin.