A molecular model for the d chain of the giant haemoglobin from Lumbricus terrestris and its implications for subunit...

A structural model for the monomeric d chain of the giant haemoglobin from Lumbricus terrestris is described. Based on the crystal structures of other globins, the model provides evidence for the existence of a novel tryptophan-haem interaction. The observation that all three tryptophans are buried within the hydrophobic core is consistent with fluorescence data on the isolated monomer and the intact molecule. The model has also been used to predict the probable arrangement of the abcd tetramer as being similar to that observed in the clam Hb II structure. Such predictions allow the identification of four residues of particular importance in stabilising one of the subunit-subunit interfaces: Arg48, Arg97, His89 and Gln93. The latter two may be of special importance in the mediation of cooperative effects within the tetramer and indeed the intact molecule.