The binding of carbon dioxide by horse haemoglobin.

1. Three modified horse haemoglobins have been prepared: (i) alpha(c) (2)beta(c) (2), in which both the alpha-amino groups of the alpha- and beta-chains have reacted with cyanate, (ii) alpha(c) (2)beta(2), in which the alpha-amino groups of the alpha-chains have reacted with cyanate, and (iii) alpha(2)beta(c) (2), in which the two alpha-amino groups of the beta-chain have reacted with cyanate. 2. The values of n (the Hill constant) for alpha(c) (2)beta(c) (2), alpha(2)beta(c) (2) and alpha(c) (2)beta(2) were (respectively) 2.5, 2.0 and 2.6, indicating the presence of co-operative interactions between the haem groups for all derivatives. 3. In the alkaline pH range (about pH8.0) all the derivatives show the same charge as normal haemoglobin whereas in the acid pH range (about pH6.0) alpha(c) (2)beta(c) (2) differs by four protonic charges and alpha(c) (2)beta(2), alpha(2)beta(c) (2) by two protonic charges from normal haemoglobin, indicating that the expected number of ionizing groups have been removed. 4. alpha(c) (2)beta(2) and alpha(c) (2)beta(c) (2) show a 25% decrease in the alkaline Bohr effect, in contrast with alpha(2)beta(c) (2), which has the same Bohr effect as normal haemoglobin. 5. The deoxy form of alpha(c) (2)beta(c) (2) does not bind more CO(2) than the oxy form of alpha(c) (2)beta(c) (2), whereas alpha(c) (2)beta(2) and alpha(2)beta(c) (2) show intermediate binding. 6. The results reported confirm the hypothesis that, under physiological conditions, haemoglobin binds CO(2) through the four terminal alpha-amino groups and that the two terminal alpha-amino groups of alpha-chains are involved in the Bohr effect.