How many oxygen molecules can one hemoglobin molecule bind?

One hemoglobin molecule can bind four oxygen molecules. This capacity is due to the structure of hemoglobin, which is a tetramer composed of four polypeptide subunits. Each subunit has a heme group, which contains an iron atom capable of forming a reversible bond with an oxygen molecule. When oxygen binds to one of the heme groups, it induces a conformational change in the hemoglobin molecule that increases the affinity of the remaining heme groups for oxygen. As a result, hemoglobin can effectively transport oxygen from the lungs to the tissues, making its ability to bind four oxygen molecules integral to its role in the body.