An extended Monod-Wyman-Changeaux-model expressed in terms of the Herzfeld-Stanley formalism applied to oxygen and carbonmonoxide binding curves of hemoglobin trout IV.

An extended Monod, Wyman, Changeaux (MWC)-model, the mathematical basis of which had been formulated by Herzfeld and Stanley (J. Mol. Blol. 82:231. 1974.) was used to fit oxygen and CO-binding curves of hemoglobin trout IV measured at different pH-values between pH = 8.0 and 6.0. From this calculation one obtains that even the fully liganded molecule exhibits a R----T quaternary transition upon approaching the acid pH-region. In the case of O2-binding, the cooperativity becomes negative below pH = 6.5. This can be related to the difference between the equilibrium constants of proton binding to the alpha- and beta-subunits. Furthermore, it can be shown that the interaction between the quaternary T----R- and the tertiary t----r-transitions is different for the alpha- and beta-subunits.