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Heme biosynthesis is the series of chemical interactions that result in the production of heme. A co-factor that is necessary for the biological activity of proteins, heme functions as a prosthetic group for hemeproteins. These hemeproteins are responsible for the transport and detection of diatomic gases, electron transfer and chemical catalysis in the body.
The process of heme biosynthesis begins in the mitochondria, which are the parts of a cell that produce energy. Glycine and succinyl coenzyme A are condensed by rate-limiting enzyme 5-aminolevulinic acid synthase (ALAs), with vitamin B6 as a co-enzyme. This interaction results in the production of D-aminolevulinic acid (dALA), which is then transported to the cytosol.
In the cytosol, two molecules of D-aminolevulinic acid combine and are dimerized by aminolevulinic acid dehydratase to produce the pyrrole ring compound porphobilinogen (PBG). Deamination by porphobilinogen deaminase combines four molecules of porphobilinogen into hydroxymethyl bilane (HMB), a linear tetrapyrrole. Heme biosynthesis then continues with the hydrolyzation of hydroxymethyl bilane into uroporphrinogen III with uroporphrinogen III synthase. This conversion results in the closure of the tetrapyrrole, forming a ring.
The last step of heme biosynthesis to occur in the cytosol is the removal of carboxylic groups from the uroporphrinogen III by uroporphrinogen III decarboxylase, producing carbon dioxide and coproporphyrinogen III. Synthesis is then transported back to the mitochondria, where the removal of the carboxyl group and two hydrogen atoms from coproporphyrinogen III by coproporphyrinogen oxidase produces protoporphyrinogen IX. Protoporphyrinogen IX oxidase then removes six hydrogen atoms from protoporphyrinogen IX, producing protoporphyrin IX. Finally, ferrochelatase inserts an iron atom into the protoporphyrin IX, resulting in heme.
After heme biosynthesis is complete, the heme binds to a protein to carry out a specific function. A common example of this is hemoglobin, a specialized hemeprotein that is composed of heme and four globin chains. Hemoglobin is a major component of red blood cells, and it gives the cells their red coloration. In the red blood cell, hemoglobin carries oxygen to the tissues in the body, where it is released for use. Hemoglobin then carries carbon dioxide from the tissues to be released into the lungs.
Heme biosynthesis is impaired by iron deficiency, because of the lack of available iron atoms. Subsequently, heme cannot be synthesized, and hemoglobin cannot be formed. If severe enough, anemia will develop as a consequence of a lack of functional red blood cells with enough hemoglobin to transport the required amounts of oxygen to the tissues. Iron deficiency preventing heme biosynthesis has many causes, including chronic blood loss, increased demands on the body such as in pregnancy, iron malabsorption and a poor diet that is deficient in iron-rich foods.
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