The Bohr Effect

The movement of O2 from the blood to the mitochondria is by diffusion. Net movement occurs from regions where the partial pressure is high to regions 

OXYGEN CARRIAGE, RESPIRATION AND ACID-BASE PHYSIOLOGY 

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The transition between the T and R states of hemoglobin is also deeply involved in the Bohr effect and cooperativity. Therefore stabilization of either of the two stmctures should diminish these effects, which have important physiologic consequences. The clinical consequences of stabilization are not known. 

In 1982 a study of the usefulness of DBBF in the production of a blood substitute was reported (99). A single modification achieved the dual goals of reduced oxygen affinity and restricted tetramer—dimer dissociation. This work was confirmed in 1987 (98). The product, called aa-hemoglobin, was formulated in Ringer s lactate. P q under physiologic conditions is 3.7 kPa (28.0 torr). Hill s parameter is 2.2, and the Bohr effect was reduced (100). Plasma retention was increased, and the product appeared to be less heterogeneous than some of the other derivatives under study. Its production was scaled up by Baxter Healthcare Corp., under contract to the U.S. Army. 

The influence of pH on the affinity of Hb for oxygen known as the Bohr-effect indicates that protons retain the allosteric regulation of oxygen transport. It is also an indirect confirmation of the ability of Hb and Im Hb for transporting carbon dioxide. The values of the Bohr-effect d log P50/d pH for Hb and Im Hb are close to each other in the pH range 7.1-7.4. It is possible that the effect of the micro-environment of carboxylic CP on immobilized Hb and its polyfunctional interaction represents the interaction between Hb and the structural elements inside the red cell [99]. 

Protons responsible for the Bohr effect arise from rup-mre of salt bridges during the binding of O2 to T state... 

Figure 6-9. The Bohr effect. Carbon dioxide generated in peripheral tissues combines with water to form carbonic acid, which dissociates into protons and bicarbonate ions. Deoxyhemoglobin acts as a buffer by binding protons and delivering them to the lungs. In the lungs, the uptake of oxygen by hemoglobin releases protons that combine with bicarbonate ion, forming carbonic acid, which when dehydrated by carbonic anhydrase becomes carbon dioxide, which then is exhaled.

In hemoglobin M, histidine F8 (His F8) has been replaced by tyrosine. The iron of HbM forms a tight ionic complex with the phenolate anion of tyrosine that stabilizes the Fc3 form. In a-chain hemoglobin M variants, the R-T equilibrium favors the T state. Oxygen affinity is reduced, and the Bohr effect is absent. P Ghain hemoglobin M variants exhibit R-T switching, and the Bohr effect is therefore present. 

Hemoglobin s dioxygen binding is regulated by local concentrations of H+ (known as the Bohr effect), CO2 concentration, and organic phosphates such as diphosphoglycerate (DPG), whose structure is shown in Figure 4.2.17... 

Another, and more acute, trigger to the release of oxygen from haemoglobin is the pH of the blood surrounding the red cell. Protons arising from metabolic activity of tissues cause a right-shift in the position of the ODC and oxygen is released when the pH falls. This phenomenon is called the Bohr effect. 

Partial digestion of human haemoglobin by carbox rpeptidases A and B has shown that chemical modifications to the -chains are more effective in destroying haem-haem interaction than those to the a-chains while modifications to either chain decrease the magnitude of the Bohr effect. The C-terminal sequences of the two types of chain of human haemoglobin are as follows ... 

BPG binds selectively to deoxy-Hb, thereby increasing its amount of equilibrium. The result is increased O2 release at constant p02. In the diagram, this corresponds to a right shift of the saturation curve (2, curve 3). CO2 and act in the same direction as BPG. Their influence on the position of the curve has long been known as the Bohr effect. 

CO2. The CO2 lowers the pH, thus causing the Hb to release even more O2 to the Mb. The pH sensitivity (called the Bohr effect), as well as the progressive increase of the O2 binding constants in Hb, is due to interactions between the subunits ... 

Binding of allosteric modifiers, especially protons (the Bohr effect), organic phosphate (particularly diphosphoglycerate), and carbon dioxide. 

This reaction produces H+, contributing to the Bohr effect. The bound carbamates also form additional salt bridges (not shown in Fig. 5-9) that help to stabilize the T state and promote the release of oxygen. 

Hemoglobin also binds H+ and C02, resulting in the formation of ion pairs that stabilize the T state and lessen the protein s affinity for 02 (the Bohr effect). Oxygen binding to hemoglobin is also modulated by 2,3-bisphosphoglycerate, which binds to and stabilizes the T state. 

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