Iron, the component of heme

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, the component of heme

Absorption and Transport of Iron in Human Body

Absorption and Transport of Iron in Human Body
Entry of iron into the body us carefully regulated by its absorption, a very complex and poorly understood process. This regulation of absorption in the normal, healthy individual is closely tied to the level of the iron stores; absorption increases when iron stores are low and decreases as stores become greater. The rate of erythropoiesis also appears to influence iron absorption.

Food iron is presented to the body either as heme iron, found only in animal products, or as non-heme iron, which comprises all the iron occurring in plant foods and about 60% of that in animal foods.

Whether the iron is heme or non-heme has a major influence in the amount of the mineral absorbed. Since heme iron is so limited in most diets absorption of non-heme iron deserves the greater emphasis.

Overall absorption is estimated to range from 10 – 15%. Iron absorption can occur throughout the small intestine but is most efficient in the proximal portion, particularly the duodenum. Heme iron is absorbed as an intact metalloporphyrin, perhaps having first been split from global chain while in lumen of the gastrointestinal (GI) tract.

Heme is well absorbed but rates of absorption are inversely related to iron stores and may range form 15% to 35%. Within the mucosal cells the absorbed heme is broken down by a heme oxygenase, and the iron released moves through the mucosal cells in the form of small molecules.

The non-heme iron is less well absorbed with rate of absorption ranging from approximately 2% to 20%. Its specific rate of absorptions highly dependent on the amount of iron stores and the influence of concomitantly ingested dietary components.

Also affecting rate of absorption are existing conditions within the lumen of the proximal GI tract. The chemical form of the non-heme iron that enters the mucosal cells, the nature of receptor sties, and the trans-mucosal transport are unknown.
Absorption and Transport of Iron in Human Body

Iron in Human Body

Iron in Human Body
The essentiality of the microminerals iron is due to its presence in heme, a molecule crucial to energy transformation and therefore crucial to life itself. The atom of iron in the center of the heme molecule allows the transport of oxygen to tissues (hemoglobin); the transitional storage of oxygen in tissues, particularly cardiac muscle (myoglobin); and the transport of electrons through the respiratory chain (cytochromes).

Iron has several oxidation states, depending on its chemical environment. The only states that are stable in the aqueous environment of the human body (and food) are the ferric form and the ferrous. Nevertheless, the iron atom in the center of heme is highly reactive and allows the formation of coordinate bonds with six other atoms.

Although most of the body’s iron is found as a component of heme, some of the mineral is found in storage form, and some is associated with certain nonheme enzymes.

In adult, functional components, hemoglobin comprises about 85%; myoglobin approximately 10%; the enzymes approximately 1%. The total amount of iron found in person is related not only to body weight but also is influenced by a variety of physiologic conditions, including age, sex, pregnancy, and state of growth. For example, the amount of iron stored in the premenopausal woman is likely to be much less than in the postmenopausal female or in the male.
Iron in Human Body