title: Experimental basis for a new allosteric model for multisubunit proteins.
abstract: Monod, Wyman, and Changeux (MWC) explained allostery in multisubunit proteins with a widely applied theoretical model in which binding of small molecules, so-called allosteric effectors, affects reactivity by altering the equilibrium between more reactive (R) and less reactive (T) quaternary structures. In their model, each quaternary structure has a single reactivity. Here, we use silica gels to trap protein conformations and a new kind of laser photolysis experiment to show that hemoglobin, the paradigm of allostery, exhibits two ligand binding phases with the same fast and slow rates in both R and T quaternary structures. Allosteric effectors change the fraction of each phase but not the rates. These surprising results are readily explained by the simplest possible extension of the MWC model to include a preequilibrium between two tertiary conformations that have the same functional properties within each quaternary structure. They also have important implications for the long-standing question of a structural explanation for the difference in hemoglobin oxygen affinity of the two quaternary structures.
authors.affiliation: Departments of Physics and Earth Sciences, NEST, Istituto Nanoscienze, Consiglio Nazionale delle Ricerche, 56127 Pisa, Italy; and cristiano.viappiani@unipr.it eaton@helix.nih.gov.
authors.forename: Cristiano
authors.initials: C
authors.lastname: Viappiani
authors.affiliation: Departments of Physics and Earth Sciences, NEST, Istituto Nanoscienze, Consiglio Nazionale delle Ricerche, 56127 Pisa, Italy; and.
authors.forename: Stefania
authors.initials: S
authors.lastname: Abbruzzetti
authors.affiliation: Neurosciences, and.
authors.forename: Luca
authors.initials: L
authors.lastname: Ronda
authors.affiliation: Neurosciences, and.
authors.forename: Stefano
authors.initials: S
authors.lastname: Bettati
authors.affiliation: Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520.
authors.forename: Eric R
authors.initials: ER
authors.lastname: Henry
authors.affiliation: Pharmacy, University of Parma, 43124 Parma, Italy;
authors.forename: Andrea
authors.initials: A
authors.lastname: Mozzarelli
authors.affiliation: Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520 cristiano.viappiani@unipr.it eaton@helix.nih.gov.
authors.forename: William A
authors.initials: WA
authors.lastname: Eaton
mesh_terms: D000494:Allosteric Regulation; D000495:Allosteric Site; D006441:Hemoglobin A / Q000737:chemistry* / Q000378:metabolism*; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D007834:Lasers; D008024:Ligands; D008956:Models, Chemical; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism; D010782:Photolysis; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D058428:Silica Gel / Q000737:chemistry / Q000378:metabolism
chemical_list: D006454:Hemoglobins; D008024:Ligands; D021122:Protein Subunits; D058428:Silica Gel; D006441:Hemoglobin A; C020622:hemoglobin B; D010100:Oxygen