How many oxygen molecules can hemoglobin bind?

How many oxygen molecules can hemoglobin bind?

• A. Two
• B. One
• C. Eight
• D. Four

Answer: (D)

Hemoglobin can bind multiple oxygen molecules because it is a tetramer with four heme groups, and each heme group contains an iron ion that can reversibly bind one O2 molecule. This means a single hemoglobin molecule can carry up to four O2 molecules—one per heme group. The structure also makes the binding cooperative: once the first oxygen binds, the protein changes shape in a way that makes it easier for the remaining sites to bind oxygen. This cooperative binding helps Hb become highly saturated in the lungs and release oxygen in tissues where it’s needed. In contrast, myoglobin has only one heme group, so it binds a single O2 molecule.

Hemoglobin can bind multiple oxygen molecules because it is a tetramer with four heme groups, and each heme group contains an iron ion that can reversibly bind one O2 molecule. This means a single hemoglobin molecule can carry up to four O2 molecules—one per heme group. The structure also makes the binding cooperative: once the first oxygen binds, the protein changes shape in a way that makes it easier for the remaining sites to bind oxygen. This cooperative binding helps Hb become highly saturated in the lungs and release oxygen in tissues where it’s needed. In contrast, myoglobin has only one heme group, so it binds a single O2 molecule.