Structure of binding sites for heterotropic effectors in fish haemoglobins

Abstract

Haemoglobin binds oxygen cooperatively. In mammalian and bird haemoglobins the oxygen affinity is modulated by hydrogen and chloride ions, organic phosphates and CO₂ (ref. 1), and the amino acid residues responsible for these so-called heterotropic effects have been identified^2,3. In some fish haemoglobins these heterotropic effects are modified to adapt the fish to their specific habitats⁴. We have determined the functional properties and amino acid sequences of the haemoglobins in trout blood and in the Amazonian fish Pterygoplichthys pardalis (Pt HbI), which is a facultative air breather^5–9. Trout HbI binds oxygen cooperatively, but shows no Bohr effect over the pH range 5–9 and no organic phosphate effect¹⁰. Pt HbI also binds oxygen cooperatively; its Bohr effect is small and reversed and its oxygen affinity is lowered by organic phosphates below pH 7.0–7.5 (ref. 9). Both haemoglobins represent substantial fractions of the total pigments in the red cells. We have now analysed the amino acid sequences of these two proteins to discover whether the particular residues thought to be responsible for the Bohr effect and organic phosphate binding in mammalian haemoglobins have been replaced by others in fish haemoglobins^6,7,11.