Subunit assembly of giant haemoglobin from the polychaete Tylorrhynchus heterochaetus.

The subunit assembly of the giant haemoglobin of the polychaete Tylorrhynchus heterochaetus is presented. Tylorrhynchus haemoglobin consists of two types of subunits: a "monomeric" chain I and a disulphide-bonded "trimer" of chains IIA, IIB and IIC. The molar ratio of the four constituent chains was determined by statistical comparison of the accurate amino acid composition calculated from the sequence of each chain and the observed composition measured by amino acid analysis of the whole molecule. On the basis of the molar ratio and the molecular weight of each chain, deduced from the amino acid sequence, a symmetrical model for the molecular assembly of the haemoglobin was constructed. The proposed model consists of four species of chains of 192 polypeptides and has a molecular weight of 3,275,808. The minimum structural entity is a "tetramer" consisting of the "monomeric" chain and the disulphide-bonded "trimer". Each chain contains one haem.