Characterization of the embryonic globin chains of the marsupial Tammar wallaby, Macropus eugenii

Abstract

The embryonic hemoglobins of the marsupial Tammar wallaby (Macropus eugenii) are known to aggregate, which was shown by the finding that the Hill coefficient, h, was greater than 4.0 in the upper part of the oxygen equilibrium curve. Here, we have undertaken a detailed primary structure analysis of the Tammar wallaby pouch young hemoglobin complement, which we hoped might provide clues into the residues that cause aggrregation and a high embryonic h. The Tammar wallaby embryonic hemoglobin complement is principally four major hemoglobins each with a different isoelectric point. Two early expressed hemoglobins contain the same embryonic β-like chain, epsilon (ε), but two separate α-like chains, termed zeta and zeta prime (ζ and ζ') both of which are N-terminally blocked. The later two expressed hemoglobins contain the same adult α-chain, but different β-like chains. The latest expressed hemoglobin contains the same β-like chain, ε, as the two early expressed forms, but the third expressed hemoglobin contains a unique β-like chain which we have termed omega (ω). A protein database similarity search using the first 54 N-terminal amino acids of the ω-chain showed a range of sequence identities of 57-72% to all known mammalian β-like chains, including the other marsupial ε-chains. The closest identity, reflected by both the highest percentage identity and Smith Waterman score, was with the embryonic β-chains of the aves. While the primary structures of the hemoglobins reported here do not explain the low hemoglobin-oxygen affinity in embryonic marsupial blood, the finding of the similarity with the bird globin-like sequence with one of the marsupial chains has implications on mammalian globin evolution. How many other marsupials and placental mammals are harboring a bird-like globin in their embryos?.