Most adults have four to five grams of iron stored in the body. Of this, around 2.5 grams are localized within the red blood cells, namely haemoglobin to carry oxygen and to support the acid-base balance. Most of the rest lies bounded to ferritin in the rest o the human cells, mainly in the liver, the spleen and the bone marrow. Some minor percentage of human iron is distributed among enzymes and myoglobin. A small amount of iron circulates in the blood bounded to transferrin.
Organs involved in haem synthesis are par excellence the bone marrow and the liver, and the hepar also serves a physiologic reserve of iron in the body. Due to its high toxicity, free ions of iron are in extremely low concentrations in the body.
Haemoglobin plays the major role in supplying the human body with oxygen. Haemoglobin is a molecule that consists of a protein compound – globin, and non-protein part that carries atom of iron – haem. Erythrocytes bearing haemoglobin are the main cells to store the iron in the body, followed by macrophages and hepatocytes. As for the latter, ferritin is the protein to carry iron.
Porphyrins are derivatives of porphine, a cycle that contains four molecules of pyrrole shared by =CH – groups (metene bridges). A system of alternating double C=C bounds results:
These basic chemical structures are shared by all creatures, but in humans this is protoporphyrin type IX used to carry iron cation (Fe2+) in the haem.
In this molecule, side chains are formed by methyl (-CH3) and vinyl (-CH=CH2), as well as propionic acid (-CH2-CH2-COOH) derivates. Thus, haem is a plain molecule of a quadrate form.
Haem is produced due to a complex enzymatic process, which is conservative among biological species. In humans, porphyrin synthesis serves almost exclusively to produce haem, but in other species it results in vitamin B12. The pathway initiates from amino acid glycine and succinyl-CoA, a combination of succinic acid and coenzyme A, from the Krebs cycle.
These two form the ?-Aminolevulinic acid with the help of aminolevulinic acid synthase.
Aminolevulinic acid synthase is under negative influence of glucose and haem. As Kolluri (2004) demonstrated, this mechanism has clinical implications: infusion of heme or glucose can stop exacerbations of intermittent porphyria in those individuals with an inborn error of iron metabolism. This reaction takes place within the mitochondria. The rest of the chemical transformations up to protoporphyrinogen type III occur in the cytoplasm.
Haem synthesis in mitochondria and cytoplasm (from Daniell et al, 1997)
As Daniell at al (1997) note, haem is produced by a metabolic pathway that consists of enzyme-controlled steps:
– two ?-Aminolevulinic acid molecules are condensed to form porphobilinogen, a monopyrrole.
– four porphobilinogen molecules are polymerized to form the linear tetrapyrrole hydroxymethylbilane
– hydroxymethylbilane is converted to the cycle of tetrapyrrole uroporphyrinogen III
– decarboxylations of uroporphyrinogen III produce a series of 7-, 6-, and 5-carboxyl prophyrinogens and then coproporphyrinogen III
– decarboxylations of coproporphyrinogen III produce protoporphyrinogen IX.
– protoporphyrinogen IX undergoes oxidation to protoporphyrin IX.
– protoporphyrin IX is chelated with Fe and heme is produced.
Insufficient activity of a certain enzyme from this pathway results in accumulation of the chemical precursors that are proximal to the missing step, a condition that results in porphyria. Should iron deficiency occur, an increase in protoporphyrin bounded to zink is observed. Porphyria cutanea may develop at enormous exposure to iron.
It must be emphasized that ferric maximum coordination number is six (there may be six chemical bounds to one molecule of iron). As seen from the chemical structures above, atom of iron (Fe2+) chelates to the protoporphyrin molecule by means of four bounds, one bound is left for protein of globin and the resulting one free bound is available for oxygen.
Haem oxidation status determines haemoglobin capacities. Unbound to oxygen or deoxyhaemoglobin carries a ferro-ion (Fe2+). Totally oxygenated haemoglobin, called oxyhaemoglobin carries four molecules of oxygen (O2) per molecule of haemoglobin. Haemoglobin may be also bound to carbon monoxide (CO), and the derivate is named carboxyhaemoglobin. Fe2+ ion may undergo oxidation to Fe3+ producing methaemoglobin.
It should be noted haem is an unstable compound and may spontaneously oxidize to hemin, a Fe3+ derivative. Free haem is a highly toxic substance: when free haem is released from haemoglobin iron atom induces Fenton reaction to destroy many organic compounds. This kind of chain link is a self-supporting process that involves free radicals release in an ongoing manner.
Fenton reaction involving haem might be the exact mechanism to sensitize cells to apoptosis, a programmed cell death.
A red blood cell life span takes around 120 days, so there is a turnover of 6 grams of haemoglobin a day. Thus, haem is to be degraded effectively and iron must be stored properly. Normally, cells of the reticuloendothelial system fulfill the task (located mainly in the spleen). Haem oxygenase will destruct haem into biliverdin.
As seen from this scheme, the ring is torn releasing the ion of iron Fe3+. Carbon monoxide is also reduced, and this is the only reaction in the human body known to release CO. Next, biliverdin is converted to bilirubin treating NADPH as a source for reduction potential.
Bilirubin attached to albumin from peripheral cells is transported to the liver via the portal vein and in the hepatocytes, bilirubin-UDP-glucuronyltransferase would add two equivalents of glucuronic acid to produce a water soluble derivate of bilirubin to be excreted by the bile:
Haem is incorporated into globin ?- and ?- chains in such way that each protein globule contains a haem group.
Haemoglobin structure displaying haem within the protein subunits
Molecular graphics of the haemoglobin is now available due to crystallography techniques.
Three of four haem chains are shown by arrows in this haemoglobin image
It needs to be noted, that in spite of the fact the enzymes responsible for haem synthesis have been identified, transport mechanisms for iron, haem, or haem synthesis intermediates are only emerging.
The second most significant storage of iron in the human body is the ferritin molecule. It must be noted again, that free iron cations are highly dangerous and lead to toxic free radicals exposure. Ferritin is a protective measure elaborated to bind iron within the cell. Ferreira (2000) has shown ferritin gene deletion results in lethality in the embryonic mice. In human, only minor ferritin gene deviations are compatible with survival (Theil, 2003). Thus, iron is safely preserved within the molecule of ferritin.
Ferritin is a protein that stores ferrum and releases it in a controlled fashion. Due to ferritin stores the body composed a buffer against iron deficiency. Should the blood contain low iron, ferritin is to release the debt. On the other hand, as soon as the blood and tissues of contain too much of iron, ferritin may store the excess of ferrum.
Ferritin is a protein that consists of 24 subunits that organize themselves to form a sphere with special channels within its walls. Inside the shell of ferritin, ferric ions bond to phosphate and hydroxide ions. Each ferritin particle is capable to store about 4500 iron ions (Theil, 1987). In general, L-subunit of ferritin store iron for a prolonged period in the liver or the spleen. Usually, these molecules of ferritin contain 1,500 Fe atoms per ferritin molecule. H-subunits of ferritin are located in the brain and in the heart and show lower concentrations of iron (less than 1,000 Fe atoms per ferritin molecule).
Inside the sphere of ferritin iron is stored in a form of ferrihydrite FeO(OH)]8[FeO(H2PO4), which is attached to the inner surface of the molecule. Thus, to be released into the body the iron atom must be changed from Fe3+ to Fe2+ oxidation state. As soon as lysosomes degrade ferritin sphere iron becomes available for utilisation by other proteins. Albeit, ferritin may release iron in a controlled fashion through its channels.
Ferritin shows ferroxidase activity, thus can convert Fe2+ to Fe3+ that limits the deleterious Fenton reaction. Mitochondrial ferritin has been recognized recently. It is assumed, that the aim of this kind of ferritin is to protect cells that generate a high rate of mitochondrial reactive oxygen.
Protein aggregates of ferritin are named hemosiderin. However, the latter is much less accessible for iron to be extracted and recycled. Hemosiderin is considered to be a degradation product of ferritin. Small amounts of ferritin circulate in the serum and normally it is derived from the liver and lymphoid tissues. Nevertheless, tumours may release ferritin into the blood circulation: hepatoma cells excrete ferritin.