It is hemoglobin.
What is hemoglobin and its composition?
In the red blood cells (erythrocytes) of vertebrates, hemoglobin, an iron-containing protein, transports oxygen from the lungs to the body's tissues and organs and returns carbon dioxide to the lungs. In a complete blood cell (CBC) test, the amount of hemoglobin in the blood is often measured.
A globin group is surrounded by four heme groups, which together form a tetrahedral shape in each hemoglobin molecule. Heme is an organic complex known as a porphyrin that has an iron atom linked to it. Heme makes up only 4% of the weight of the molecule. As the blood circulates between the lungs and the tissues, the iron atom is what holds oxygen in place. Each hemoglobin molecule contains four iron atoms, and as a result, it has the capacity to bind four oxygen molecules. Two linked pairs of polypeptide chains make up globin.
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