In the hemoglobin molecule, what does oxygen bind to?

• A. The porphyrin ring without iron
• B. The globin protein chains
• C. The carbohydrate side chains
• D. The iron (Fe^{2+}) atom in the heme group

Answer: (D)

Verified step by step guidance

1. Understand the structure of hemoglobin: Hemoglobin is a protein composed of four subunits, each containing a heme group. The heme group is a complex structure that includes a porphyrin ring and an iron (Fe^{2+}) atom at its center.
2. Learn the role of the iron atom: The iron atom (Fe^{2+}) in the heme group is the key site for oxygen binding. It is capable of forming a reversible bond with oxygen molecules, which is essential for oxygen transport in the blood.
3. Clarify why oxygen does not bind to other components: Oxygen does not bind to the porphyrin ring without iron, the globin protein chains, or the carbohydrate side chains because these structures lack the chemical properties required for reversible oxygen binding.
4. Explore the chemical interaction: When oxygen binds to the Fe^{2+} atom, it forms a coordination complex. This interaction is facilitated by the electronic configuration of Fe^{2+}, which allows it to temporarily hold the oxygen molecule.
5. Relate this to hemoglobin's function: The reversible binding of oxygen to the Fe^{2+} atom in the heme group enables hemoglobin to pick up oxygen in the lungs and release it in tissues, supporting cellular respiration.