Structural and functional analysis of two haemoglobins of the Antarctic seabird Catharacta maccormicki. Characterization of an additional phosphate binding site by molecular modelling.

The amino-acid sequence and the oxygen-binding properties of the two haemoglobins of the Antarctic seabird south polar skua have been investigated. The two haemoglobins showed peculiar functional features, which were probably acquired to meet special needs in relation to the extreme environmental conditions. Both haemoglobins showed a weak alkaline Bohr effect which, during prolonged flight, may protect against sudden and uncontrolled stripping of oxygen in response to acidosis. We suggest that a weak Bohr effect in birds may reflect adaptation to extreme life conditions. The values of heat of oxygenation suggest different functional roles of the two haemoglobins. The experimental evidence suggests that both haemoglobins may bind phosphate at two distinct binding sites. In fact, analysis of the molecular models revealed that an additional phosphate binding site, formed by residues NA1a, G6a and HC3a, is located between the two a chains. This additional site may act as an entry/leaving site, thus increasing the probability of capturing phosphate and transferring it to the main binding site located between the two b chains by means of a site±site migratory mechanism, thereby favouring the release of oxygen. It is suggested that most haemoglobins possess an additional phosphate binding site, having such a role in oxygen transport.