BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 4. HEMOGLOBIN: AN ALLOSTERIC PROTEIN

4.3. Cooperative Oxygen Binding by Hemoglobin Enhances Oxygen Transport

Cooperative oxygen binding increases the efficiency of Hemoglobin as an oxygen carrier. As the partial pressure of oxygen changes, the oxygen saturation of hemoglobin changes more rapidly than it would if all binding sites acted independently of one another. Let us consider the following example. Suppose that the pO₂ in the lung alveoli is 100 torr, and in the capillaries of an active Muscle, 20 torr. Let P₅₀ = 30 torr and n = 2.8. Then Y in the alveolar capillaries will be 0.97, and in the muscle capillaries, 0.25. The amount of oxygen released in the Tissues is proportional to the difference in Y values, i.e., 0.72. Now let us make the same calculation for a hypothetical oxygen carrier characterized by the same P₅₀ = 30 torr, but binding oxygen noncooperatively (n = 1). In this case, Y_alveoli = 0.77, Y_muscle = 0.41, and ∆Y = 0.36. It is clearly seen from this that cooperative oxygen binding by hemoglobin doubles the release of oxygen in tissues compared to noncooperative binding.

4.4. H⁺ and CO₂ Promote O₂ Release (The Bohr Effect)

Changes in pH over a wide range, as well as the concentration of CO₂, have no noticeable effect on the oxygen-binding properties of Myoglobin. In the case of hemoglobin, by contrast, acidification of the medium promotes oxygen dissociation. A decrease in pH within physiological limits shifts the oxygen dissociation curve to the right, meaning that oxygen affinity decreases (Fig. 4.6). An increase in CO₂ concentration (at constant pH) also decreases oxygen affinity. In tissues with a High Metabolic Rate, such as exercising muscle, large amounts of CO₂ and acids are produced. The elevated levels of CO₂ and H⁺ in the capillaries of actively metabolizing tissues facilitate the release of O₂ from oxyhemoglobin. This vital mechanism addressing the increased oxygen demand characteristic of metabolically active tissues was discovered in 1904 by Christian Bohr.

Class="center">Fig. 4.6. Effect of pH on the oxygen affinity of hemoglobin. A decrease in pH from 7.6 to 7.2 leads to the release of oxygen from oxyhemoglobin

The opposite effect, discovered 10 years later by John Haldane, takes place in the lung capillaries. Here, a high concentration of O₂ promotes the release of H⁺ and CO₂ from hemoglobin, in a manner entirely analogous to how a high concentration of H⁺ and CO₂ in actively metabolizing tissues promotes

the release of O₂. These interrelationships among the binding of O₂, H⁺, and CO₂ are known as the Bohr effect (Fig. 4.7).

Fig. 4.7. The Essence of the Bohr effect. The mechanism and Stoichiometry of the processes are presented in a simplified form

4.5. Bisphosphoglycerate Lowers Oxygen Affinity

The oxygen affinity of hemoglobin located within erythrocytes is lower than that of hemoglobin in solution. As early as 1921, Joseph Barcroft raised the question: "Is there not some third substance [present in erythrocytes]... forming an integral part of the oxygen-hemoglobin complex?" Indeed, such a substance exists. In 1967, Reinhold Benesch and Ruth Benesch demonstrated that 2,3-bisphosphoglycerate (BPG) binds to hemoglobin and thereby exerts a profound effect on its oxygen affinity. BPG is present in erythrocytes at roughly the same molar concentration as hemoglobin. In the absence of BPG, the P₅₀ for hemoglobin is 1 torr, just as for myoglobin; in the presence of BPG, the P₅₀ for hemoglobin rises to 26 torr (Fig. 4.8). Thus, BPG reduces the oxygen affinity of hemoglobin 26-fold. BPG plays a crucial physiological role: in its absence, hemoglobin passing through tissue capillaries where the pO₂ is ~26 torr would release very little O₂. BPG influences the oxygen affinity of hemoglobin by binding to deoxyhemoglobin rather than oxyhemoglobin. Oxygen binding and BPG binding are mutually exclusive processes. To a first approximation, the Oxygenation of hemoglobin in the presence of BPG is expressed by the following equation:

Hb—BPG + 4O₂ ⇄ Hb(O₂)₄ + BPG.

Fig. 4.8. Bisphosphoglycerate lowers the oxygen affinity of hemoglobin