BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL UNIVERSITIES - E. S. Severin - 2004

SECTION 13. HEME METABOLISM AND IRON HOMEOSTASIS

II. Iron Metabolism

In heme-containing Proteins, iron is a structural component of the heme group. In Non-heme iron proteins, iron is directly bound to the polypeptide chain. Such proteins include transferrin, ferritin, the oxidative Enzymes Ribonucleotide reductase and xanthine oxidase, and the iron-sulfur proteins NADH dehydrogenase and succinate dehydrogenase.

The body of an adult human contains 3–4 g of iron, of which only about 3.5 mg is present in Blood Plasma. Hemoglobin accounts for approximately 68% of the total body iron, ferritin for 27%, Myoglobin for 4%, and transferrin for 0.1%. All iron-containing enzymes combined account for merely 0.6% of the body's iron pool. The sources of iron for The Biosynthesis of iron-containing proteins are dietary iron and iron released from the continuous breakdown of erythrocytes in Liver and Spleen Cells.

In a neutral or alkaline environment, iron exists in its oxidized state as Fе³⁺, forming large, readily aggregating complexes with ОН^-, other anions, and Water. At low pH values, iron is reduced and readily dissociates. The redox processes of iron ensure its redistribution among macromolecules within the Organism. Iron ions exhibit a high affinity for numerous compounds, forming chelate complexes with them that alter their Properties and Functions. Consequently, specialized proteins mediate the TRANSPORT AND STORAGE of iron in the body. Within cells, iron is stored by the protein ferritin, whereas in the bloodstream it is transported by transferrin.

A. Intestinal Iron Absorption

Dietary iron is primarily present in the oxidized state (Fе³⁺) bound to proteins or organic acid salts. The acidic environment of gastric juice facilitates the release of iron from organic acid salts. The duodenum is the primary site of iron absorption. Dietary ascorbic acid reduces iron and enhances its absorption, since only Fе²⁺ is taken up by the intestinal mucosal cells. A normal daily diet typically contains 15–20 mg of iron, of which only about 10% is absorbed. An adult human loses approximately 1 mg of iron per day.

The amount of iron absorbed by the intestinal mucosal cells generally exceeds the body's immediate requirements. The transfer of iron from enterocytes into the bloodstream depends on The rate of Synthesis of the protein apoferritin within these cells. Apoferritin "captures" iron inside enterocytes and is converted into ferritin, which remains sequestered in the enterocyte. This mechanism limits The entry of iron from intestinal cells into blood capillaries. When the demand for iron is low, the rate of apoferritin synthesis increases (see "Regulation of cellular Iron Uptake" below). The constant shedding of mucosal cells into the intestinal lumen rids the body of excess iron. Conversely, in iron deficiency, virtually no apoferritin is synthesized in enterocytes. Iron released from enterocytes into the bloodstream is bound and transported by the plasma protein transferrin (Fig. 13-7).

Class="center">Fig. 13-7. Uptake of exogenous iron into Tissues. In the intestinal lumen, iron is liberated from dietary proteins and organic acid salts. Ascorbic acid facilitates iron assimilation by reducing it. In the intestinal mucosal cells, excess absorbed iron binds to the protein apoferritin to form ferritin, with ferritin oxidizing Fе²⁺ to Fе³⁺. The transfer of iron from intestinal mucosal cells into the blood is accompanied by its oxidation by the serum enzyme ferroxidase. In the bloodstream, Fе³⁺ is transported by the serum protein transferrin. In tissues, Fе²⁺ is utilized for the synthesis of iron-containing proteins or stored in ferritin.

B. Iron Transport in Blood Plasma and Cellular Uptake

Iron is transported in blood plasma by the protein transferrin. Transferrin is a glycoprotein synthesized in the liver that specifically binds oxidized iron (Fе³⁺). Iron entering the blood is oxidized by the enzyme ferroxidase, also known as the copper-containing plasma protein ceruloplasmin. A single transferrin molecule can bind one or two Fе³⁺ ions, obligatorily coupled with the СO₃^2- anion to form a transferrin-2 complex (Fе³⁺-СO₃^2-). Under normal conditions, blood transferrin is approximately 33% saturated with iron.

Transferrin interacts with specific Cell Membrane Receptors. This interaction triggers The formation of a Ca²⁺-calmodulin-PKC complex in The Cell Cytosol, which phosphorylates the transferrin receptor and induces endosome formation. An ATP-dependent proton pump residing in the endosomal membrane establishes an acidic interior within the endosome. In this acidic environment, iron dissociates from transferrin. Subsequently, the receptor-apotransferrin complex recycles back to The Plasma Membrane. At the neutral pH of the extracellular fluid, apotransferrin undergoes a conformational change, detaches from the receptor, enters the blood plasma, and regains The ability to bind iron ions and embark on a new cycle of cellular transport. Inside the cell, iron is utilized for the synthesis of iron-containing proteins or stored within the protein ferritin.

Ferritin is an oligomeric protein with a Molecular Weight of 500 kDa. It consists of heavy (21 kDa) and light (19 kDa) polypeptide chains arranged into 24 protomers. Variations in the subunit COMPOSITION OF THE ferritin oligomer give rise to multiple isoforms of this protein across different tissues. Ferritin is structured as a hollow protein shell capable of housing up to 4,500 ferric iron ions, though it typically contains fewer than 3,000. The heavy chains of ferritin catalyze The oxidation of Fе²⁺ to Fе³⁺. Iron is sequestered in the center of the sphere as a hydrous ferric oxide-phosphate core, enclosed by the protein shell. It enters and exits the interior via hydrophilic channels traversing the apoferritin protein shell, although iron can also be deposited within the protein moiety itself. Ferritin is found in virtually all tissues, with the highest concentrations in the liver, spleen, and Bone Marrow. A minor fraction of ferritin is secreted from tissues into blood plasma. Because the release of ferritin into the blood is proportional to its tissue content, blood ferritin concentration serves as an important diagnostic marker of body iron stores in iron-deficiency anemia. Whole-body iron METABOLISM is illustrated in Fig. 13-8.

Fig. 13-8. Iron metabolism in The Human Body.

C. Regulation of Cellular Iron Uptake

Intracellular iron levels are determined by the balance between rates of uptake, utilization, and storage, and are governed by two molecular mechanisms. The rate of iron uptake in non-erythroid cells depends on the Abundance of transferrin receptor proteins on their surface membranes. Excess intracellular iron is sequestered by ferritin. The synthesis of both apoferritin and transferrin receptors is regulated at the translational level in response to intracellular iron availability.

Stem-loop structures known as iron-responsive elements (IREs) are located in the untranslated 3'-termini of both transferrin receptor mRNA and apoferritin mRNA (Figs. 13-9 and 13-10). Notably, transferrin receptor mRNA contains 5 IREs, whereas apoferritin mRNA contains only 1.

These mRNA regions interact with regulatory IRE-binding proteins. At low intracellular iron concentrations, the IRE-binding protein binds to the IRE of apoferritin mRNA, sterically blocking the attachment of Translation initiation factors (Fig. 13-9, A). Consequently, the rate of apoferritin translation and its cellular content decrease. Conversely, under low iron conditions, the IRE-binding protein binds to the iron-responsive elements of transferrin receptor mRNA, protecting it from degradation by ribonucleases (Fig. 13-10, A). This leads to an upregulation of transferrin receptors and accelerates iron uptake into cells.

When intracellular iron levels rise, its interaction with the IRE-binding protein induces the oxidation of sulfhydryl (SH) groups in the Active Site of the protein, diminishing its affinity for the IREs of mRNAs. This triggers two major consequences:

✵ first, the translation of apoferritin is accelerated (Fig. 13-9, B);

Fig. 13-9. Regulation of apoferritin synthesis. A — when intracellular iron levels are low, the iron-binding protein exhibits high affinity for the IRE and binds to it. This prevents the recruitment of translation initiation factors to the apoferritin mRNA, halting apoferritin synthesis; B — when intracellular iron levels increase, iron binds to the iron-binding protein, reducing its affinity for the IRE. Translation initiation factors then bind to the apoferritin mRNA and initiate the translation of apoferritin.

✵ secondly, the IRE-binding protein releases the hairpin loops of transferrin receptor mRNA, causing it to be degraded by the enzyme Ribonuclease, which consequently slows down transferrin receptor synthesis (Fig. 13-10, B). The acceleration of apoferritin synthesis and the inhibition of transferrin receptor synthesis lead to a reduction in cellular iron levels.

Overall, these mechanisms regulate intracellular iron levels and its utilization for the synthesis of iron-containing proteins.

Fig. 13-10. Regulation of transferrin receptor synthesis. A — when cellular iron levels are low, the iron-responsive protein exhibits a high affinity for the IRE of the mRNA encoding the transferrin receptor protein. Binding of the iron-binding protein to the mRNA IRE prevents its degradation by ribonuclease, allowing transferrin receptor Protein Synthesis to continue; B — When cellular iron levels are high, the affinity of the iron-binding protein for the IRE decreases, making the mRNA accessible to ribonuclease, which hydrolyzes it. mRNA degradation leads to a decrease in transferrin receptor protein synthesis.

G. Disorders of iron metabolism

Iron-deficiency anemia can occur As a result of recurrent bleeding, Pregnancy, frequent childbirth, gastrointestinal ulcers and tumors, or following gastrointestinal surgery. In iron-deficiency anemia, red Blood Cells are reduced in size and pigmentation (microcytic hypochromic erythrocytes). Erythrocytes show a decreased hemoglobin content, transferrin iron saturation drops, and the concentration of ferritin in tissues and blood plasma declines. These changes are caused by iron deficiency in the body, which leads to impaired synthesis of heme and ferritin in non-erythroid tissues and hemoglobin in erythroid cells.

Hemochromatosis. When the amount of iron in cells exceeds the storage capacity of ferritin, iron is deposited within the protein moiety of the ferritin molecule. The formation of such amorphous deposits of excess iron converts ferritin into hemosiderin. Hemosiderin is poorly soluble in water and contains up to 37% iron. The accumulation of hemosiderin granules in the liver, Pancreas, spleen, and liver leads to tissue damage known as hemochromatosis. Hemochromatosis can be caused by a hereditary increase in intestinal iron absorption, with total body iron in affected individuals reaching up to 100 g. This condition is inherited in an autosomal recessive manner, with approximately 0.5% of Caucasians being homozygous for the hemochromatosis Gene. Hemosiderin accumulation in the pancreas leads to the destruction of the β-Cells of the islets of Langerhans and, consequently, to Diabetes Mellitus. Hemosiderin deposition in hepatocytes causes liver cirrhosis, while its accumulation in cardiomyocytes leads to Heart Failure. Patients with hereditary hemochromatosis are treated with regular phlebotomies, performed weekly or monthly depending on the severity of the condition. Frequent blood transfusions can also lead to hemochromatosis; in such cases, patients are treated with iron-chelating agents.