Two sites for GTP binding in cathodic haemoglobins from Anguilliformes

Cathodic haemoglobins of four species of anguilliform fish were characterized from a functional point of view, with special regard to the interaction with their physiological effectors. A series of oxygen-binding experiments at increasing GTP concentrations was carried out in order to compare GTP-binding activities in the absence and presence of saturating amounts of chloride. The results indicated that the cathodic haemoglobin of three species (Anguilla anguilla, Conger conger and Muraena helena) do have two sites for GTP-binding. In the absence of chloride, the two sites cannot be discriminated, whereas in the presence of chloride, a competition between the two anions occurred for the second GTP-binding site. The cathodic haemoglobin of Gymnothorax unicolor, which showed lower GTP sensitivity than the other haemoglobins examined, displayed only one GTP-binding site. The presence of an additional phosphate-binding site is not exceptional, although the way haemoglobin interacts with the two organic phosphate molecules may differ among species. This property may provide an auxiliary means of haemoglobin modulation for species that inhabit environments where oxygen availability is highly variable and haemoglobin-oxygen affinity needs to be modulated to different extents in order to satisfy physiological oxygen requirements.