Haemoglobins of the Shark, Heterodontus portusjacksoni III. Amino Acid Sequence of the ß -Chain

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Abstract
The amino acid sequence of the .beta.-chain has 141 residues, the same as that of mammalian .alpha.-chains and less than the 146 residues of mammalian .beta.-chains or the 148 residues of the .alpha.-chain from the tetrameric shark Hb. The sequence was deduced from the sequences of peptides obtained by digestion of the globin or its cyanogen bromide fragments with trypsin, chymotrypsin, pepsin and papain. The difference in length of the .beta.-chain is most readily accounted for by the absence of the D helix. This small helical section is normally present in myoglobins and .beta.-globins but absent in .alpha.-chains. The deduction that it is absent from shark .beta.-chain is based on considerations of homology. The .beta.-chain shows the insertion of histidine .beta.2 and the deletions corresponding to residues A17 and AB1 relative to .alpha.- and myoglobin chains. The reactive thiol group in shark Hb was shown, by radioactive labeling, to be residue 51 in the .beta.-chain, immediately preceding the E helix. The amino acid sequence of shark .beta.-chain showed 92 differences from human .beta.-chain, significantly more differences than shown by chicken or frog .beta.-chains, in line with its earlier time of divergence. If the tertiary structure of the shark .beta.-chain is the same as that of the horse, then there are 2 changes in the .alpha.1.beta.2 contact site in HbO2 and an additional one in deoxyhemoglobin. When both .alpha.- and .beta.-chain contacts are considered, there were a total of 9 changes in residues involved in the .alpha.1.beta.2 contacts. There was no Bohr effect in shark Hb, and of the residues normally involved in this effect, the C-terminal histidine residue of the .beta.-chain was present, but the aspartyl (FG1) residue to which it is salt-linked is not, being replaced by a glutamyl residue.

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