The effect of alkaline earth ions on the co-operativity of the oxygenation of Helix pomatia haemocyanin

Abstract

1. 1. The most pronounced sigmoidal oxygenation curve is obtained for Helix pomatia haemocyanin, in the absence of buffer, at pH 7·9 in the presence of 6 mM Ca 2+ or 10 mM Sr 2+. Mg 2+ and Ba 2+, on the contrary, have no effect on the co-operativity of the oxygenation. 2. 2. The influence of alkaline earth ions on the stabilization of H. pomatia haemocyanin at pH 7·9 follows a different order: Ca 2+>Mg 2+>Sr 2+ = Ba 2+, suggesting two types of binding sites. The type with the higher affinity could be responsible for the stabilization, the type with the lower affinity for the co-operativity of the oxygen binding. 3. 3. An interpretation is suggested for the co-operative oxygen binding of H. pomatia haemocyanin by postulating a deoxy and an oxy structure in a relaxed and in a tense state. In the relaxed state the transition between the two structures is easy and the oxygen affinity high, in the tense state, stabilized at pH 7·9 by Ca 2+ or Sr 2+, the transition is difficult and the oxygen affinity low.