Iron is physiologically active only in the ferrous form in which protein?

Iron is physiologically active only in the ferrous form in which protein?

• A. Hemoglobin
• B. Cytochromes
• C. Ferritin
• D. Transferrin

Answer: (A)

Iron must be in the ferrous state (Fe2+) to bind oxygen reversibly in the heme pocket. In hemoglobin, iron sits as Fe2+ and coordinates with the porphyrin nitrogens and a proximal histidine; oxygen binding converts the iron-oxygen complex without permanently changing the iron’s oxidation state. If iron becomes ferric (Fe3+), it cannot bind O2, forming methemoglobin and impairing oxygen transport. Other proteins in the list handle iron differently: transferrin ferries iron after oxidation to Fe3+, ferritin stores iron as Fe3+, and cytochromes shuttle electrons by cycling between Fe2+ and Fe3+. Thus, among these, the protein whose physiologic activity hinges on iron being ferrous for its primary function is hemoglobin.

Iron must be in the ferrous state (Fe2+) to bind oxygen reversibly in the heme pocket. In hemoglobin, iron sits as Fe2+ and coordinates with the porphyrin nitrogens and a proximal histidine; oxygen binding converts the iron-oxygen complex without permanently changing the iron’s oxidation state. If iron becomes ferric (Fe3+), it cannot bind O2, forming methemoglobin and impairing oxygen transport. Other proteins in the list handle iron differently: transferrin ferries iron after oxidation to Fe3+, ferritin stores iron as Fe3+, and cytochromes shuttle electrons by cycling between Fe2+ and Fe3+. Thus, among these, the protein whose physiologic activity hinges on iron being ferrous for its primary function is hemoglobin.