Evolution of allosteric models for hemoglobin

yWe compare various allosteric models that have been proposed to explain cooperative oxygen binding to hemoglobin, including the two- state allosteric model of Monod, Wyman, and Changeux ( MWC), the Cooperon model of Brunori, the model of Szabo and Karplus ( SK) based on the stereochemical mechanism of Perutz, the generalization of the SK model by Lee and Karplus ( SKL), and the Tertiary Two- State ( TTS) model of Henry, Bettati, Hofrichter and Eaton. The preponderance of experimental evidence favors the TTS model which postulates an equilibrium between high ( r)- and low ( t)a. nity tertiary conformations that are present in both the T and R quaternary structures. Cooperative oxygenation in this model arises from the shift of T to R, as in MWC, but with a signifi. cant population of both r and t conformations in the liganded T and in the unliganded R quaternary structures. The TTS model may be considered a combination of the SK and SKL models, and these models provide a framework for a structural interpretation of the TTS parameters. The most compelling evidence in favor of the TTS model is the nanosecond - millisecond carbon monoxide ( CO) rebinding kinetics in photodissociation experiments on hemoglobin encapsulated in silica gels. The polymeric network of the gel prevents any tertiary or quaternary conformational changes on the sub- second time scale, thereby permitting the subunit conformations prior to CO photodissociation to be determined from their ligand rebinding kinetics. These experiments show that a large fraction of liganded subunits in the T quaternary structure have the same functional conformation as liganded subunits in the R quaternary structure, an experimental. finding inconsistent with the MWC, Cooperon, SK, and SKL models, but readily explained by the TTS model as rebinding to r subunits in T. We propose an additional experiment to test another key prediction of the TTS model, namely that a fraction of subunits in the unliganded R quaternary structure has the same functional conformation ( t) as unliganded subunits in the T quaternary structure.