Molecular biologists studying the properties of hemoglobin are investigating the structural changes associated with oxygen loading and unloading. During the transition from point 1 to point 2 on the graph shown below, hemoglobin molecules are most likely to release which of the following?

Question

Molecular biologists studying the properties of hemoglobin are investigating the structural changes associated with oxygen loading and unloading.  During the transition from point 1 to point 2 on the graph shown below, hemoglobin molecules are most likely to release which of the following?

Quizplus · Accepted Answer

11ec160b_36f6_114f_9249_0b7e212caaa9_MD0001_00 The transition from point 1 to 2 on the graph represents the loading of O₂ onto partially deoxygenated hemoglobin as occurs in the lungs. At very low partial pressure of O₂ (pO₂), hemoglobin is mostly deoxygenated, and binding of O₂ molecules is relatively difficult (as indicated by the early flatness of the curve). As the partial pressure of O₂ increases, O₂ binds to 1 of the 4 binding sites on hemoglobin and increases the O₂-binding affinity of the other available binding sites (steepening of the curve), a phenomenon known as cooperative binding. Subsequently, additional O₂ molecules bind more easily as the pO₂ increases until hemoglobin becomes nearly fully saturated. The binding of O₂ molecules to hemoglobin in the lungs has two consequences, known as the Haldane effect: The affinity of hemoglobin for CO₂ is decreased, resulting in unloading of CO₂ from hemoglobin (this accounts for a small percentage of overall CO₂ in the blood and is not pictured above). The acidity of the hemoglobin molecule is increased; in response, protons (H⁺ ions) are released from the hemoglobin binding sites. The H⁺ ions combine with bicarbonate ions (the primary form of CO₂ in the blood) in the lungs to facilitate the production of water (H₂O) and CO₂. The CO₂ is then transferred to the alveoli and expired while oxygen is taken up by hemoglobin. In the peripheral tissues, high levels of CO₂ create an increase in ambient acidity that shifts the hemoglobin dissociation curve to the right and facilitates the unloading of O₂ (Bohr effect). The CO₂ (and water) are converted into H⁺ and HCO₃⁻. The H⁺ ions are carried by hemoglobin while the HCO₃⁻ is transferred to the plasma for transport back to the lungs. (Choice A) In the lungs, chloride ions shift out of red blood cells and into the plasma in exchange for HCO₃⁻. These chloride ions are not bound to or released by hemoglobin. (Choice B) Heme is not released from hemoglobin during O₂ loading or unloading. However, it is released during the normal destruction of aged red blood cells in the spleen. (Choice C) O₂ molecules bind to hemoglobin to increase hemoglobin saturation. (Choice D) 2,3-diphosphoglycerate contains 2 phosphate groups and binds to hemoglobin to facilitate unloading of O₂ in the tissues. However, phosphate molecules are not directly bound or released by hemoglobin. Educational objective: The binding of O₂ to hemoglobin increases the affinity for binding of subsequent O₂ molecules (cooperative binding). In the lungs, the binding of O₂ to hemoglobin drives the release of H⁺ and CO₂ from hemoglobin (Haldane effect). In the peripheral tissues, high concentrations of CO₂ and H⁺ facilitate O₂ unloading from hemoglobin (Bohr effect).

Molecular Biologists Studying the Properties of Hemoglobin Are Investigating the Structural