STUDIES ON ERYTHROCRUORIN .3. OXYGEN EQUILIBRIUM OF EARTHWORM ERYTHROCRUORIN

The oxygen equilibrium of Lumbricus erythrocruorin in several experimental conditions has been correlated with the structural properties of the protein. The whole blood and the isolated protein show a high co-operativity (n = 4) and a sigmoidal Hill plot at pH 7.8. At higher and lower pH values the shape of the oxygen equilibrium curve is modified irreversibly; the value of n drops to 2 and the Hill plot becomes linear. This irreversible change is not correlated with the state of aggregation of the protein. From the experimental data it appears that the “native” protein is in a metastable state in conformation; the “native” protein may be converted into a stable form with different functional properties both by changes in pH and in temperature. The functional properties of the stable form are completely reversible. From a general point of view it would seem that very large proteins, in contrast to small proteins, may be frozen in vivo in a conformation that does not correspond to a minimum in free energy