BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 4. HEMOGLOBIN: AN ALLOSTERIC PROTEIN

Summary

Tetrameric Hemoglobin exhibits novel properties that are absent in monomeric Myoglobin. In addition to its ability to transport O₂, hemoglobin can also carry H⁺ and CO₂. Furthermore, the binding of the latter is regulated by allosteric shifts, which represent interactions between spatially distant sites mediated by Conformational Changes in the protein. Hemoglobin is the most thoroughly studied allosteric protein. It displays three major allosteric effects. First, the oxygen-binding curve of hemoglobin is sigmoidal, indicating Cooperative oxygen binding. The binding of oxygen to one heme facilitates The addition of O₂ to the remaining Hemes of the same protein molecule. This cooperativity increases The amount of oxygen transported. Second, H⁺ and CO₂ promote the release of oxygen from hemoglobin—an effect of great physiological significance, as it enhances oxygen delivery in metabolically active Tissues, such as working Muscles. The reverse effect also occurs: O₂ promotes the release of H⁺ and CO₂ in the pulmonary capillaries of the Lungs. The allosteric linkage between the binding of H⁺, CO₂, and O₂ is known as the Bohr effect. Third, the oxygen affinity of hemoglobin is also regulated by 2,3-bisphosphoglycerate (BPG), a low-molecular-weight compound with a high density of negative charges. BPG binds to deoxyhemoglobin but not to oxyhemoglobin. It follows that BPG decreases the oxygen affinity of hemoglobin. BPG plays a vital role in adaptation to high altitude and Hypoxia. Fetal hemoglobin has a higher oxygen affinity than adult hemoglobin because it binds less BPG.

The allosteric properties of hemoglobin stem from the interactions between its α and β subunits. The rigidity of the T (Tense) quaternary Structure is maintained by salt bridges formed between the subunits, which result in low oxygen affinity. In the R (Relaxed) form, these intersubunit bonds are absent, and the oxygen affinity is high. Upon oxygenation, the iron atom moves into the plane of the heme, pulling the proximal Histidine along with it. This displacement triggers the disruption of salt bridges and shifts the equilibrium from the T state to the R state. The binding of the fourth O₂ molecule to hemoglobin occurs much more readily than the binding of the first, as it requires the rupture of fewer salt bridges. BPG binds to positively charged groups on two β chains that line the central cavity of hemoglobin. The binding of BPG stabilizes the T state and thus reduces the oxygen affinity of hemoglobin. Another allosteric effector, carbon dioxide, binds to the terminal amino groups of all four chains, forming readily dissociable carbamate linkages. For hydrogen ions, which also contribute to the Bohr effect, there are three pairs of binding sites. The immediate microenvironment of two terminal amino groups and two pairs of histidine side chains in deoxyhemoglobin has a higher negative charge than in oxyhemoglobin, which causes H⁺ to bind to these sites following the release of O₂. CO₂ and H⁺, much like BPG, decrease oxygen affinity by stabilizing the T conformation of hemoglobin.