Mechanistic Basis for Allosteric O2 Binding to Hemoglobin coordination of 6th ligand (4 heme N’s, 1 histidine N and 1 O2) causes Fe3+ radius to shrink so it fits inside heme ring movement of the Fe atom is transmitted to other substituents by the proximal histidine upon O2 binding

Mechanistic Basis for Allosteric O2 Binding to Hemoglobin Salt Bridges Between Deoxyhemoglobin Subunits movement of the Fe atom requires breakage of some of the eight salt bridges that exist between the  and  subunits of deoxyhemoglobin binding of the first O2 atom is most difficult due to the energetically unfavorable requirement of breaking salt bridges to accommodate O2 ligation to Fe binding of successive O2 molecules becomes progressively easier as the number of salt bridges between the  and  subunits is reduced with each successive O2 binding event the fourth O2 molecule is bound 300 times more tightly than the first due to this effect this sequential increase in O2 affinity accounts for the sigmoidal binding curve of O2 to hemoglobin the net effect of this cooperative interaction of hemoglobin oxygenation is to increase the binding of O2 in the lungs and release it in areas of low oxygen pressure

2,3 Bisphosphoglycerate Reduces O2 Affinity by Crosslinking Deoxyhemoglobin BPG binds to deoxyhemoglobin with a 1:1 stoichiometry via stereochemical charge complementarity the cationic histidines salt bridge with the terminal phosphates of BPG neighboring lysine residues stabilize the anionic charge on the central BPG carboxylate BPG is extruded upon binding O2  the central cavity becomes too small when the subunits move  BPG stabilizes the deoxyhemoglobin by additional salt bridges & promotes O2 release at low [O2]

Mechanism of Bohr Effect Basic summary of Bohr effect CO2 stabilizes the deoxyhemoglobin form by forming carbamates at the amino terminii of the  dimers which increases salt bridge formation Basic summary of Bohr effect O2 affinity decreases with decreasing pH due to protonation of 1 His 146 decreasing pH facilitates O2 release from oxyhemoglobin

Cooperative Binding Analysis Hill Plot where: f = # binding sites occupied n = Hill coefficient K = a constant (i.e., not the binding constant for one A ligand) log = n log [A] + log K

Model of Allosteric Interaction in the Binding of Four Molecules of O2 to Hemoglobin Postage Stamp Analogy 1 requires 2 tears, 2 & 3rd require a single tear, last is “free”