What are the components of hemoglobin?

Hemoglobin is the protein that transports oxygen (O2) in human blood from the lungs to the tissues of the body. A hemoglobin molecule is composed of four polypeptide globin chains. Each contains a heme moiety which has an organic part (a protoporphyrin ring made up of four pyrrole rings) and a central iron ion in the ferrous state (Fe2+).

Heme synthesis occurs both in cytosol and in mitochondria of erythrocytes. Protoporphyrin is synthesized from the condensation of glycine and succinyl coenzyme A, eight molecules of each being required to form a linear tetrapyr-role molecule, which finally cyclizes into the protoporphyrin ring. The protoporphyrin then binds to a Fe2+ ion to form heme.

Each heme group contains an iron atom that is able to bind to one oxygen (O2) molecule. Because hemoglobin contains four heme groups, each hemoglobin protein can bind four oxygen molecules.

Fe2+ is a major component of hemoglobin that carries oxygen to all parts of the body. Fe2+ also has a critical role within cells assisting in oxygen utilization, enzymatic systems, especially for neural development, and overall cell function everywhere in the body.
What are the components of hemoglobin?