A "living fossil" sequence: primary structure of the coelacanth (Latimeria chalumnae) hemoglobin--evolutionary and functional aspects

The coelacanth (Latimeria chalumnae, Actinistia) has a single hemoglobin component. The primary structures of the alpha- and beta-chains are presented. They could be separated by reversed-phase HPLC. Peptides obtained by tryptic digestion of the native and oxidized chains were isolated by reversed-phase HPLC and sequenced in liquid and gas-phase sequenators. The alignment was achieved by employing the N-terminal sequences of the native chains and those of a beta-chain cyanogen bromide peptide as well as fragments obtained by acid hydrolysis. The Latimeria alpha-chains consist of 142 amino-acid residues, due to a fish-specific insertion between positions 46 and 47, whereas the beta-chains are of normal length (146 residues). Latimeria alpha- and beta-chains share 72 (51.1%) and 70 (47.9%) identical residues with human hemoglobin, respectively. Numerous heme contacts and positions involved in subunit interface contacts are replaced. The most interesting of them were studied by molecular modeling. The loss of an alpha 1/beta 2-contact by the exchanges alpha 92(FG4)Arg----Leu and beta 43(CD2)Glu----Lys might be responsible for the easy dissociation of the tetrameric hemoglobin molecule. A comparison of the residues replaced in contact positions with fishes and amphibians revealed the highest number of matches between Latimeria and tadpoles. The same result was obtained by the evaluation of other regions relevant for structure and function of the molecule, like exon-intron boundary regions, phosphate binding sites and salt bridges responsible for the Bohr effect.