Ligand-binding equilibria, kinetics and 13C n.m.r. spectra of bound 13CO, of the haemoglobins from two fishes that are very distant on the evolutionary scale, i.e. the fourth haemoglobin component from Salmo irideus and the single component from Osteoglossum bicirrhosum, were studied. The C-terminal sequence was also determined for the haemoglobin from Osteoglossum. The results show that (i) the C-terminal residues of both chains are not directly responsible for the characteristic heterotropic effect known as Root effect, since for both fish haemoglobins these residues are identical with those of human haemoglobins. (ii) In all haemoglobins characterized by the Root effect a dependence of the 13CO n.m.r. resonances on pH is observed. However, the extent of the shift(s) depends on the particular protein, and is probably the result of a combination of both tertiary and quaternary conformational changes. (iii) Both haemoglobins from trout and Osteoglossum manifest a functional heterogeneity between the two types of chains in the tetramer, which increases with proton activity. For CO, the effect is very small for trout haemoglobin IV, and very marked for Osteoglossum haemoglobin; for O2 strongly heterogeneous binding curves were obtained at approx. pH6.2 with both haemoglobins. (iv) Estimations of the relative values of the affinity constants for the α and β chains in the tetramer were obtained for both haemoglobins from 13CO n.m.r. spectra at low fractional saturation. On the basis of these findings the molecular mechanism underlying the Root effect is discussed.
Discriminating between negative cooperativity and ligand binding to independent sites using pre-equilibrium properties of binding curves
