Hemoglobins, XLVIIII. The primary structure of a monomeric hemoglobin from the hagfish, Myxine glutinosa L.: evolutionary aspects and comparative studies of the function with special reference to the heme linkage.
Hagfish hemoglobin has three main components, one of which is Hb III. It is monomeric and consists of 148 amino acid residues (M = 17 350). Its complete primary structure, previously published, is discussed here. The proximal amino acid (F8) of the heme linkage is histidine as always in the hemoglobins, but the regularly expected distal histidine E7 is substituted by glutamine. This substitution, leading to a new kind of heme linkage, has hitherto only been demonstrated in opossum hemoglobin. It is suggested that E7, Gln, is directed out of the heme pocket, and that the adjacent Ell, Ile, is directed toward the inside of the pocket, giving the distal heme contact instead of histidine. Myxine Hb III has an additional tail of 9 amino acid residues at its N-terminal end, as has the hemoglobin of Lampetra fluviatilis. The genetic codes of Myxine and Lampetra hemoglobins show 117 differences, in spite of many morphological resemblances between hagfish and lamprey. Their primary hemoglobin structures show differences substantial enough to be compatible with the divergence of the two families some 400-500 million years ago.