Heme and Porphyrin Metabolism
Iron serves numerous important functions in the body relating to the metabolism of oxygen, not the least of which is its role in hemoglobin transport of oxygen. Within the body iron exist in two oxidation states: ferrous (Fe2+) or ferric (Fe3+). Because iron has an affinity electronegative atoms such as oxygen, nitrogen and sulfur, these atoms are found at the heart of the iron-binding centers of macromolecules.
Under conditions of neutral or alkaline pH, iron is found in the Fe3+ state and at acidic pH the Fe2+ state is favored. When in the Fe3+ state, iron will form large complexes with anions, water and peroxides. These large complexes have poor solubility and upon their aggregation lead to pathological consequences. In addition, iron can bind to and interfere with the structure and function of various macromolecules. For this reason the body must protect itself against the deleterious effects of iron. This is the role served by numerous iron-binding proteins (see below).
Aside from its importance as the prosthetic group of hemoglobin and a small number of enzymes (e.g., redox cytochromes and the P450 class of detoxifying cytochromes), heme is importa nt because a number of genetic disease states are associated with deficiencies of the enzymes used in its biosynthesis. Some of these disorders are readily diagnosed because they cause d-aminolevulinic acid, (ALA) and other abnormally colored heme intermediates to appear in the circulation, the urine, and in other tissues such as teeth and bones. Some disorders of heme biosynthesis are more insidious such as the various porphyrias, a list of which can be found below and in the Inborn Errors Page.
Iron is associated with proteins either by incorporation into protoporphyrin IX of by binding to other ligands. When the ferrous form of iron and protoporphyrin IX are complexed the structure is referred to as heme. There are a number of heme containing proteins involved in the transport of oxygen (hemoglobin), oxygen storage (myoglobin) and enzyme catalysis such as nitric oxide synthase (NOS) and prostaglandin synthase (cyclooxygenase). A number of non-heme iron containing proteins are also known such as the iron-sulfur proteins of oxidative phosphorylation and the iron transport and storage proteins, transferrin and ferritin, respectively.
Iron consumed in the diet is either free iron or heme iron. Free iron in the intestines is reduced from the ferric (Fe3+) to the ferrous (Fe2+) state on the luminal surface of intestinal enterocytes and transported into the cells through the action of the divalent metal transporter, DMT1. When heme iron is absorbed the iron is released within the enterocytes. The iron can be stored within intestinal enterocytes bound to ferritin. Iron is transported across the basolateral membrane of intestinal enterocytes, through the action of the transport protein Ireg1 (also called ferroportin), following oxidation of the ferrous form back to the ferric form catalyzed by hephaestin (also called ferroxidase). Once in the circulation, iron is bound to transferrin and passes through the portal circulation of the liver. The liver is the major storage site for iron. The major site of iron utilization is the bone marrow where it is used in heme synthesis.
Transferrin, made in the liver, is the serum protein responsible for the transport of iron. Although several metals can bind to transferrin, the highest affinity of for the ferric (Fe3+) form of iron. The ferrous form of iron does not bind to transferrin. Transferrin can bind two moles of iron. Cells take up the transported iron through interaction of transferrin with cell-surface receptors. Internalization of the iron-transferrin-receptor complexes is initiated following receptor phosphorylation by PKC. Following internalization, the iron is released due to the acidic nature of the lysosomes. The transferrin receptor is then recycled back to the cell surface.
Ferritin is the major protein used for intracellular storage of iron. The majority of intracellularly stored iron is found in the liver, skeletal muscle and reticuloendothelial cells. If the storage capacity of the ferritin is exceded, iron will deposit adjacent to the ferritin-iron complexes in the cell. Histologically these amophous iron deposits are referred to as hemosiderin.
In humans approximately 70% of total body iron is found in hemoglobin. Because of storage and recycling very little (1-2mg) iron will need to be replaced from the diet on a daily basis. Any excess dietary iron is not absorbed or is stored in intestinal enterocytes. Refinement in our understanding of the regulation of iron absorption, recycling and release from intracellular stores has expanded recently with the discovery of the actions of the hepatic iron regulatory protein hepcidin. Hepcidin was initially described as a 25 amino acid peptide resembling cysteine-rich antimicrobial peptides. Recent evidence has demonstrated that hepcidin functions by inhibiting the presentation of one or more of the iron transporters (e.g. DMT1 and Ireg1) in intestinal membranes. With a high iron diet the level of hepcidin mRNA increases and conversely its levels decrease when dietary iron is low. This is occurring simultaneous to reciprocal changes in the levels of the transporters. Whether hepcidin regulates gene expression or the localization of the intestinal iron transporters is not yet fully understood.
The regulation of iron utilization in the body is primarily controlled via iron-mediated regulation of mRNA translation. The description of this process can be found in the Protein Synthesis page. Both the transferrin receptor and the ferritin mRNAs contain stem-loop structures termed iron responsive elements, IREs. These IREs are recognized by an iron-binding protein containing an iron-sulfur center similar to that of the TCA cycle enzyme aconitase. Other IRE containing mRNAs include those encoding the erythrocyte protoporphyrin synthesis enzyme, ALA synthase (see below), mitochondrial aconitase and ferroportin 1.
Clinical Aspects of Iron Metabolism
Iron can bind to and form complexes with numerous macromolecules, the consequences of which can be a disruption in normal activities of the affected complexes. Excess intracellular iron results in formation and deposition of hemosiderin which can lead to cellular dysfunction and damage. Thus, the consequences of excess iron intake and storage can have profound consequences. However, one must also consider that a reduction in iron intake can also lead to untoward consequences. Most notably, a reduced iron level negatively affects the function of oxygen transport in red blood cells. Defects in iron metabolism can result from impaired intestinal absorption, excess loss of heme iron due to bleeding as well as to mutations in the iron response elements of iron regulated mRNAs.
Hemochromatosis is defined as a disorder in iron metabolism that is characterized by excess iron absorption, saturation of iron-binding proteins and deposition of hemosiderin in the tissues. The primary affected tissues are the liver pancreas and skin. Iron deposition in the liver leads to cirrhosis and in the pancreas causes diabetes. The excess iron deposition leads to bronze pigmentation of the organs and skin. In fact, the bronze skin pigmentation seen in hemochromatosis, coupled with the resulatant diabetes lead to the designation of this condition as bronze diabetes.
The primary cause of hemochromatosis is the inheritance of an autosomal recessive allele. The locus causing hemochromatosis has been designated the HFE and is a major histocompatibility complex (MHC) class-1 gene. The gene encodes an a chain protein with three immunoglobulin-like domains. This a chain protein associates with b2-microglobulin. Normal HFE has been shown to form a complex with the transferrin receptor and in so doing is thought to regulate the rate of iron transfer into cells. A mutation in HFE will therefore, lead to increased iron uptake and storage. The majority of hereditary hemochromatosis patients have inherited a mutation in HFE that results in the substitution of Cys 282 for a Tyr. This mutation causes loss of conformation of one of the immunoglobulin domains in HFE. Another mutation found in HFE causes a change of His 68 to Asp. Secondary hemochromatosis (not caused by mutation in HFE and thus not inherited) can result from excess oral intake of iron or in patients receiving blood transfusions. Iron deficiency anemia is characterized by microcytic (small) and hypochromic (low pigment) red blood cells. Reduced iron intake and/or excess iron excretion results in a decreased globin protein content in red blood cells as a consequence of the heme control of globin synthesis (see the Protein Synthesis page for details).The most common causes of iron deficient anemia are excess menstrual flow or gastrointestinal (GI) bleeding. Causes of GI bleeding can include the use of medications that lead to ulceration or erosion of the gastric mucosa, peptic ulcer disease, gastric tumors, hiatal hernia or the gastritis associated with chronic alcohol consumption.Treatment of iron deficiency anemia is to first determine the cause and source of the excess bleeding. Oral administration of ferrous sulfate is commonly used to supplement the iron loss, however, intravenous iron therapy may be called for in some cases. Severe iron deficiency anemia may necessitate transfusion with packed red blood cells.
Synthesis of Porphobilinogen and Heme
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| Heme pathway |
The first reaction in heme biosynthesis (1) takes place in the mitochondrion and involves the condensation of 1 glycine and 1 succinylCoA by the pyridoxal phosphate-containing enzyme, d-aminolevulinic acid synthase (ALA synthase).
This reaction is both the rate-limiting reaction of heme biosynthesis, and the most highly regulated reaction (see Regulation below). Mitochondrial d-aminolevulinic acid (ALA) is transported to the cytosol, where ALA dehydratase (2)(also called porphobilinogen synthase or hydroxymethylbilane synthase) dimerizes 2 molecules of ALA to produce the pyrrole ring compound porphobilinogen.
The next step (3) in the pathway involves the head-to-tail condensation of 4 molecules of porphobilinogen to produce the linear tetrapyrrole intermediate, hydroxymethylbilane. The enzyme for this condensation is porphobilinogen deaminase (PBG deaminase). This enzyme is also called uroporphyrinogen I synthase.
