Diving behaviour and haemoglobin function: the primary structure of the α- and β-chains of the sea turtle (Caretta caretta) and its functional implications

The amino acid sequence of the α- and β-chains of haemoglobin (Hb) from the loggerhead sea turtle (Caretta caretta) has been determined. Comparison with that of human Hb shows differences in several residues involved in both α1β1 and α1β2 packing contacts. On the whole, in spite of the mutations, the essential characteristics of both interfaces seem to be maintained. The functional properties of the sea turtle Hb have been investigated at different temperatures and as a function of proton, chloride and organic phosphate concentrations. In addition to overall similarities shared with most of the vertebrate Hbs previously described, this molecule shows significant differences which could be related to the life behaviour of the turtle. In fact, while the shape of the Bohr-effect curve is well adapted for gas exchange during prolonged dives, the very small enthalpy change for O2 binding ensures that O2 delivery becomes essentially insensitive to the temperature changes of the environment. Moreover, and similarly to the case of emperor penguin Hb, the small alkaline Bohr effect appears to be only choride-linked, since the pH dependence of the O2 affinity is abolished in the absence of this ion. These functional characteristics are discussed on the basis of the primary structure of α- and β-chains.