Researchers are studying the structure and function of hemoglobin. Arterial and venous blood samples are obtained from enrolled volunteers with no health problems. Red blood cells are isolated from whole blood by differential centrifugation, and the contents of the intact red blood cells are analyzed. It is found that the concentration of chloride is much lower in the red blood cells in the arterial sample compared to the venous sample. The activity of which of the following enzymes is most likely responsible for the observed finding?

Question

Researchers are studying the structure and function of hemoglobin.  Arterial and venous blood samples are obtained from enrolled volunteers with no health problems.  Red blood cells are isolated from whole blood by differential centrifugation, and the contents of the intact red blood cells are analyzed.  It is found that the concentration of chloride is much lower in the red blood cells in the arterial sample compared to the venous sample.  The activity of which of the following enzymes is most likely responsible for the observed finding?

Quizplus · Accepted Answer

11ec160b_3ce7_acba_9249_434a47c830bd_MD0001_00 Hemoglobin is only found within red blood cells (RBCs), and it is responsible not only for O₂ delivery to the tissues, but also for carrying CO₂ from the tissues to the lungs. Hemoglobin carries CO₂ in the form of carbaminohemoglobin, which is created as follows: CO₂ + Hb-NH₃ = 2H⁺ + Hb-NH-CO₂ However, only a small percentage of total blood CO₂ is carried by hemoglobin. The majority of total blood CO₂ is carried in the plasma as bicarbonate ion (HCO₃⁻) via the following process: The CO₂ produced by tissue metabolism enters RBCs and is hydrated by the enzyme carbonic anhydrase to form carbonic acid (H₂CO₃). H₂CO₃ then undergoes spontaneous conversion to HCO₃⁻ and H⁺. The excess HCO₃⁻ is then transferred out of RBCs into the plasma via band 3 protein in exchange for chloride ions (Cl⁻) to maintain electrical neutrality. This exchange is known as "chloride shift" and is the principal cause of high RBC chloride content in venous blood. (Choice A) Bisphosphoglycerate (BPG) mutase converts 1,3-BPG to 2,3-BPG. In RBCs, 2,3-BPG combines with hemoglobin to decrease its affinity for O₂ and facilitate O₂ unloading in the tissues. (Choices C and E) Glucose-6-phosphate dehydrogenase is the first enzyme in the pentose phosphate pathway (produces NADPH) and pyruvate kinase catalyzes the final step of glycolysis (produces ATP). Deficiency of either glucose-6-phosphate dehydrogenase or pyruvate kinase typically leads to shortened RBC lifespan and episodic hemolysis triggered by oxidative stressors. (Choice D) Na⁺/K⁺-ATPase is responsible for the maintenance of ionic concentration gradients across the plasma membrane in many cells in the body. This energy-requiring pump is electrogenic as it extrudes 3 sodium ions for every 2 potassium ions that enter the cell, which creates negative intracellular potential. Educational objective: The majority of CO₂ produced in the tissues is transported to the lungs as bicarbonate ion (HCO₃⁻). Within red blood cells (RBCs), the enzyme carbonic anhydrase forms HCO₃⁻ from CO₂ and water. The excess HCO₃⁻ is then transferred out of RBCs into the plasma via exchange with chloride ions (Cl⁻). This exchange is known as "chloride shift" and is the principal cause of high RBC chloride content in venous blood.

Researchers Are Studying the Structure and Function of Hemoglobin