Tremolitweg organizes what is documented about iron metabolism: the binding at the heme, the path through the circulatory system, and the wording the European Union has authorized for this element.
Open the guideOxygen dissolves in aqueous fluids only to a small degree. In 100 milliliters of blood plasma at normal air pressure, roughly 0.3 milliliters are physically dissolved. At rest, however, the organism turns over about 250 milliliters of oxygen per minute. The red blood pigment closes this gap, and at its center sits an iron ion.
Hemoglobin consists of four protein subunits. Each carries a heme group: a flat porphyrin ring at whose center a divalent iron ion is held by four nitrogen atoms. An oxygen molecule attaches to this ion without the iron changing its charge in the process. The binding is reversible — that is the difference between transport and combustion. A single hemoglobin molecule takes up as many as four oxygen molecules in this way.
The four subunits do not work independently of one another. As soon as the first binding site is occupied, the spatial arrangement of the entire molecule changes, and the remaining sites take up oxygen more readily. Plotting saturation against the partial pressure of oxygen therefore yields not a straight line but a curved one.
In the pulmonary capillaries, at a partial pressure of around 100 millimeters of mercury, the blood pigment is about 97 percent loaded. In resting tissue, at around 40 millimeters, it is still about 75 percent. The difference between the two values is the amount actually released. In adults the half-saturation pressure lies at approximately 26 millimeters.
Where muscle performs work, carbon dioxide is produced, the pH falls slightly, and the temperature rises. All three factors lower hemoglobin's binding affinity — the blood pigment releases its cargo where it is needed. This relationship bears the name of the Danish physiologist Christian Bohr. A further participant is 2,3-bisphosphoglycerate, an intermediate product of sugar breakdown in the red blood cell.
Alongside the hemoglobin of the blood, there is a second iron-containing pigment protein in skeletal and cardiac muscle. Myoglobin has only one subunit and a single heme group. It holds oxygen more tightly than hemoglobin and releases it only at very low partial pressure, that is, precisely when the cell runs into a bottleneck. About five percent of total body iron is bound in this protein; meat owes its red color to it.
An adult body carries three to five grams of iron. That this amount remains largely constant over decades is due less to intake than to a very precise internal reuse.
The stock is distributed across a few large items:
Red blood cells live about 120 days. Afterwards they are broken down by phagocytes in the spleen and liver, the heme is split, and the iron is returned to circulation. By this route roughly 20 to 25 milligrams are moved day after day — a multiple of what is added from food. From a mixed diet the intestine absorbs only about one to two milligrams, and precisely this order of magnitude is lost again via shed skin and intestinal cells.
The organism has no dedicated excretion pathway for excess iron. Regulation therefore takes place at the entrance: the liver hormone hepcidin determines how much iron the intestinal cell passes on to the blood. When stores are well filled, transfer is throttled; when they are scarce, the absorption rate rises.
The European Commission maintains a list of authorized health claims. Four formulations are entered in it for iron. Each belongs to a separate opinion of the European Food Safety Authority and is reproduced here in its original wording.
The journey of an oxygen molecule from the pulmonary alveolus to the mitochondrial membrane takes less than a minute. It is carried by an iron ion that must take it up in the lungs and let it go again in the tissue. Both — the holding on and the letting go — depend on the same chemical property of the metal: it binds strongly enough to cover the distance, and weakly enough not to block at the destination. Roughly 250 million hemoglobin molecules work on this task in every single red blood cell.
Pursuant to Regulation (EU) No 432/2012
Nervous tissue does not contain iron evenly distributed: the highest concentrations are found in the basal ganglia and in the oligodendrocytes, those cells that form the myelin sheaths of the nerve fibers. Tyrosine hydroxylase, too, the slowest enzyme on the path to dopamine, works with an iron ion in its active center.
Pursuant to Regulation (EU) No 432/2012
In the inner mitochondrial membrane, electrons travel across a series of protein complexes. The first complex alone carries eight iron-sulfur centers, the second three, and the Rieske center sits in the third. Added to these are the heme groups of the cytochromes. Without these metal centers the electron flow from which the cell obtains its ATP would not get going.
Pursuant to Regulation (EU) No 432/2012
Before a cell divides, it must duplicate its genome. The deoxyribonucleotides needed for this arise from their precursors through an enzyme whose active center contains two iron atoms and a radical coupled to them. Tissues with a high division rate — bone marrow, intestinal lining, hair root — call on this metal correspondingly often.
Pursuant to Regulation (EU) No 432/2012
Calculations for the daily balance, content tables by portion size, and a dedicated chapter on absorption in the small intestine — compiled by the Tremolitweg editorial team and available as a PDF file.
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