Iron heme binding in human body

In humans eating an average normal diet, more than one third of the body’s total daily iron requirement of 3 mg comes from dietary heme-iron.

The duodenum and proximal jejunum are considered the main sites of absorption, not the stomach.

The most important group of iron binding proteins contains the heme molecules. Heme proteins contain one iron atom at the center of a porphyrin ring, which provides four equatorial nitrogen ligands to the metal. Heme functions as a prosthetic group for some proteins.

Heme proteins are generally responsible for oxygen binding (hemoglobin) and activation (cytochrome P450). Heme iron accounts for the majority of the iron in human body, and hemoglobin contains as much as 70% of the total iron content of a normal adult.

The atom of iron in the center of the heme molecules enables oxygen transport to tissues (hemoglobin); transitional storage of oxygen in tissues particularly muscle (myoglobin) and transport of electron through the respiratory chain (cytochrome).

Hemoglobin, a conjugated tetrametric protein, consists of four heme groups and globin, which is made up of four polypeptide chains. Each polypeptide chain is associated with one of the heme molecules.

These heme carrier proteins also carry out the redox reactions and electron transport required for oxidative phosphorylation, the process that is the principle source of energy for cells.

In addition to serving as a prosthetic group of hemoglobin, heme also regulates translation erythroid precursors. Iron in incorporated into protoporphyrin IX in the last step of heme biosynthesis by ferrochelatase to form heme.
Iron heme binding in human body