Free energies of quaternary assembly (dimers to tetramers) were determined for the 10 ligation species of CN-methemoglobin in the region of the alkaline Bohr effect (pH 7.0-9.5). Analysis of this database yielded the following principal findings: (1) At each pH, the nine CN-met species exhibit two distinct values of Bohr proton release and Bohr free energy. The two Bohr effects are found to distribute in a fashion that coincides with predictions of a symmetry rule (Ackers et al., 1992), ie, the first value reflects a ''tertiary Bohr effect'' arising from ligation within the quaternary T tetramer and a second Bohr effect arises from the quaternary transition (T --> R) which occurs when both dimeric half-molecules acquire at least one ligated subunit. (2) The Bohr effects for CN-met ligation are in good agreement with previously-established Bohr effects for stepwise Oz binding under identical conditions (Chu et al., 1984). (3) In combination with recent studies which show that CN-met species [21] has a quaternary T structure (Daugherty et al., 1991; Doyle and Ackers, 1992; LiCata et al., 1993), the present results show that the ''tertiary Bohr effect'' within quaternary T exceeds the Bohr effect of dissociated dimers, as suggested by Lee and Karplus (1983). (4) The tertiary Bohr effect is found tb account for the pH dependence of tertiary constraint energy, Delta G(tc), which ''pays'' for ligand-binding cooperativity prior to the quaternary (T --> R) switchover. Possible origins of the tertiary Bohr effect and its relationship to the quaternary Bohr effect are considered.
