Structural and functional analysis of the 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 haemog lobins of the Antarctic seabird south polar skua have been investigated. Th e two haemoglobins showed peculiar functional features, which were probably acquired to meet special needs in relation to the extreme environmental co nditions. Both haemoglobins showed a weak alkaline Bohr effect which, durin g prolonged flight, may protect against sudden and uncontrolled stripping o f oxygen in response to acidosis. We suggest that a weak Bohr effect in bir ds may reflect adaptation to extreme life conditions. The values of heat of oxygenation suggest different functional roles of the two haemoglobins. Th e experimental evidence suggests that both haemoglobins may bind phosphate at two distinct binding sites. In fact, analysis of the molecular models re vealed that an additional phosphate binding site, formed by residues NA1 al pha, G6 alpha and HC3 alpha, is located between the two cr chains. This add itional site may act as an entry/leaving site, thus increasing the probabil ity of capturing phosphate and transferring it to the main binding site loc ated between the two beta chains by means of a site-site migratory mechanis m, thereby favouring the release of oxygen. It is suggested that most haemo globins possess an additional phosphate binding site, having such a role in oxygen transport.