Amino Acid Sequence of the ß-Chain of the Tetrameric Haemoglobin of the Bivalve Mollusc, Anadara trapezia

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Abstract
The amino acid sequence of the .beta.-chain of the principal haemoglobin from A. trapezia has been determined. The sequence was deduced from the sequences of tryptic peptides, which were fractionated using high-performance liquid chromatography and peptide mapping. Additional sequence data, particularly for the large tryptic peptides, was obtained from enzyme digests of both cyanogen bromide fragments and large citraconyltryptic peptides. The .beta.-chain has 151 residues which is longer than all the other sequenced haemoglobin chains except the .alpha.-chain of A. trapezia, which is 153 residues in length. The residues corresponding to those normally in the D helix are absent in this .beta.-chain. The additional residues are contributed by an extension of the N-terminal region, which was also found to be acetylated. Comparison of the .beta.-chain amino acid sequence with that of the .alpha.-chain of A. trapezia, the dimeric chain of A. trapezia, and the dimeric chain of A. broughtonii showed 53% identity in each case. In the E and F helices, the homology is particularly noticeable. There is 100% homology in the F helix of all four chains. The dimeric globin of A. trapezia also shows 100% homology with the .beta.-chain in the E helix, while the .alpha.-chain shows 75%. If the tertiary structure of the .alpha.- and .beta.-chains of A. trapezia haemoglobin is the same as that of horse haemoglobin, then there are many changes in the .alpha.1.beta.2 contact site residues.