Oxygen Binding of Monotrene Haemoglobins II. Fixation of Functionally Non-equivalent Haemoglobins in the Echidna Population

The echidna, Tachyglossus aculeatus, has two haemoglobins and polymorphic variants of both have been described. Haemoglobins Hb-IA, Hb-IB and Hb-IIA have been separated and their oxygen binding parameters studied. Hb-IIA has a markedly higher oxygen affinity and lower cooperativity than the polymorphs ofhaemoglbin I. In phosphate buffer, pH 7·1 and 25°C, Hb-IIA has an oxygen half-saturation pressure (Pso) of 1·3 kPa and a Hill coefficient of 2·4, whereas the polymorphic forms of Hb-I have P so values of about 1·6 kPa and Hill coefficients of about 2·9. Differences in diphosphoglyceric acid interactions account for some of these differences. When the polymorphic forms of haemoglobin I were compared at pH 7 ·1, Hb-IA had a AHo value of -27·9 kJjmol and a Hill coefficient of 2·9, whereas Hb-IB had a AHo value of -31·7 kJjmol and a Hill coefficient of 3·0