Cofactor (biochemistry)

From Wikipedia, the free encyclopedia

The succinate dehydrogenase complex showing several cofactors, including flavin, iron-sulfur centers, and heme.

 

A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's activity. Cofactors can be considered "helper molecules" that assist in biochemical transformations. The rates at which these happen are characterized by enzyme kinetics. 

Cofactors can be subclassified as either inorganic ions or complex organic molecules called coenzymes, the latter of which is mostly derived from vitamins and other organic essential nutrients in small amounts. A coenzyme that is tightly or even covalently bound is termed a prosthetic group. Cosubstrates are transiently bound to the protein and will be released at some point, then get back in. The prosthetic groups, on the other hand, are bound permanently to the protein. Both of them have the same function, which is to facilitate the reaction of enzymes and protein. Additionally, some sources also limit the use of the term "cofactor" to inorganic substances. An inactive enzyme without the cofactor is called an apoenzyme, while the complete enzyme with cofactor is called a holoenzyme.

Some enzymes or enzyme complexes require several cofactors. For example, the multienzyme complex pyruvate dehydrogenase at the junction of glycolysis and the citric acid cycle requires five organic cofactors and one metal ion: loosely bound thiamine pyrophosphate (TPP), covalently bound lipoamide and flavin adenine dinucleotide (FAD), and the cosubstrates nicotinamide adenine dinucleotide (NAD⁺) and coenzyme A (CoA), and a metal ion (Mg²⁺).

Organic cofactors are often vitamins or made from vitamins. Many contain the nucleotide adenosine monophosphate (AMP) as part of their structures, such as ATP, coenzyme A, FAD, and NAD⁺. This common structure may reflect a common evolutionary origin as part of ribozymes in an ancient RNA world. It has been suggested that the AMP part of the molecule can be considered to be a kind of "handle" by which the enzyme can "grasp" the coenzyme to switch it between different catalytic centers.

Classification

Cofactors can be divided into two major groups: organic Cofactors, such as flavin or heme, and inorganic cofactors, such as the metal ions Mg²⁺, Cu⁺, Mn²⁺, or iron-sulfur clusters. 

Organic cofactors are sometimes further divided into coenzymes and prosthetic groups. The term coenzyme refers specifically to enzymes and, as such, to the functional properties of a protein. On the other hand, "prosthetic group" emphasizes the nature of the binding of a cofactor to a protein (tight or covalent) and, thus, refers to a structural property. Different sources give slightly different definitions of coenzymes, cofactors, and prosthetic groups. Some consider tightly bound organic molecules as prosthetic groups and not as coenzymes, while others define all non-protein organic molecules needed for enzyme activity as coenzymes, and classify those that are tightly bound as coenzyme prosthetic groups. These terms are often used loosely. 

A 1980 letter in Trends in Biochemistry Sciences noted the confusion in the literature and the essentially arbitrary distinction made between prosthetic groups and coenzymes group and proposed the following scheme. Here, cofactors were defined as an additional substance apart from protein and substrate that is required for enzyme activity and a prosthetic group as a substance that undergoes its whole catalytic cycle attached to a single enzyme molecule. However, the author could not arrive at a single all-encompassing definition of a "coenzyme" and proposed that this term be dropped from use in the literature.

Inorganic

Metal ions

Metal ions are common cofactors. The study of these cofactors falls under the area of bioinorganic chemistry. In nutrition, the list of essential trace elements reflects their role as cofactors. In humans this list commonly includes iron, magnesium, manganese, cobalt, copper, zinc, and molybdenum. Although chromium deficiency causes impaired glucose tolerance, no human enzyme that uses this metal as a cofactor has been identified. Iodine is also an essential trace element, but this element is used as part of the structure of thyroid hormones rather than as an enzyme cofactor. Calcium is another special case, in that it is required as a component of the human diet, and it is needed for the full activity of many enzymes, such as nitric oxide synthase, protein phosphatases, and adenylate kinase, but calcium activates these enzymes in allosteric regulation, often binding to these enzymes in a complex with calmodulin. Calcium is, therefore, a cell signaling molecule, and not usually considered a cofactor of the enzymes it regulates.

Other organisms require additional metals as enzyme cofactors, such as vanadium in the nitrogenase of the nitrogen-fixing bacteria of the genus Azotobacter, tungsten in the aldehyde ferredoxin oxidoreductase of the thermophilic archaean Pyrococcus furiosus, and even cadmium in the carbonic anhydrase from the marine diatom Thalassiosira weissflogii.

In many cases, the cofactor includes both an inorganic and organic component. One diverse set of examples is the heme proteins, which consist of a porphyrin ring coordinated to iron.

Ion	Examples of enzymes containing this ion
Cupric	Cytochrome oxidase
Ferrous or Ferric	Catalase Cytochrome (via Heme) Nitrogenase Hydrogenase
Magnesium	Glucose 6-phosphatase Hexokinase DNA polymerase
Manganese	Arginase
Molybdenum	Nitrate reductase Nitrogenase
Nickel	Urease
Zinc	Alcohol dehydrogenase Carbonic anhydrase DNA polymerase

A simple [Fe₂S₂] cluster containing two iron atoms and two sulfur atoms, coordinated by four protein cysteine residues.

Iron-sulfur clusters

Iron-sulfur clusters are complexes of iron and sulfur atoms held within proteins by cysteinyl residues. They play both structural and functional roles, including electron transfer, redox sensing, and as structural modules.

Organic

Organic cofactors are small organic molecules (typically a molecular mass less than 1000 Da) that can be either loosely or tightly bound to the enzyme and directly participate in the reaction. In the latter case, when it is difficult to remove without denaturing the enzyme, it can be called a prosthetic group. It is important to emphasize that there is no sharp division between loosely and tightly bound cofactors. Indeed, many such as NAD⁺ can be tightly bound in some enzymes, while it is loosely bound in others. Another example is thiamine pyrophosphate (TPP), which is tightly bound in transketolase or pyruvate decarboxylase, while it is less tightly bound in pyruvate dehydrogenase. Other coenzymes, flavin adenine dinucleotide (FAD), biotin, and lipoamide, for instance, are tightly bound. Tightly bound cofactors are, in general, regenerated during the same reaction cycle, while loosely bound cofactors can be regenerated in a subsequent reaction catalyzed by a different enzyme. In the latter case, the cofactor can also be considered a substrate or cosubstrate.

Vitamins can serve as precursors to many organic cofactors (e.g., vitamins B₁, B₂, B₆, B₁₂, niacin, folic acid) or as coenzymes themselves (e.g., vitamin C). However, vitamins do have other functions in the body. Many organic cofactors also contain a nucleotide, such as the electron carriers NAD and FAD, and coenzyme A, which carries acyl groups. Most of these cofactors are found in a huge variety of species, and some are universal to all forms of life. An exception to this wide distribution is a group of unique cofactors that evolved in methanogens, which are restricted to this group of archaea.

Vitamins and derivatives

Cofactor	Vitamin	Additional component	Chemical group(s) transferred	Distribution
Thiamine pyrophosphate	Thiamine (B1)	None	2-carbon groups, α cleavage	Bacteria, archaea, and eukaryotes
NAD+ and NADP+	Niacin (B3)	ADP	Electrons	Bacteria, archaea, and eukaryotes
Pyridoxal phosphate	Pyridoxine (B6)	None	Amino and carboxyl groups	Bacteria, archaea, and eukaryotes
Methylcobalamin	Vitamin B12	Methyl group	acyl groups	Bacteria, archaea, and eukaryotes
Cobalamine	Cobalamine (B12)	None	hydrogen, alkyl groups	Bacteria, archaea, and eukaryotes
Biotin	Biotin (H)	None	CO2	Bacteria, archaea, and eukaryotes
Coenzyme A	Pantothenic acid (B5)	ADP	Acetyl group and other acyl groups	Bacteria, archaea, and eukaryotes
Tetrahydrofolic acid	Folic acid (B9)	Glutamate residues	Methyl, formyl, methylene. and formimino groups	Bacteria, archaea, and eukaryotes
Menaquinone	Vitamin K	None	Carbonyl group and electrons	Bacteria, archaea, and eukaryotes
Ascorbic acid	Vitamin C	None	Electrons	Bacteria, archaea, and eukaryotes
Flavin mononucleotide	Riboflavin (B2)	None	Electrons	Bacteria, archaea, and eukaryotes
Flavin adenine dinucleotide	Riboflavin (B2)	ADP	Electrons	Bacteria, archaea, and eukaryotes
Coenzyme F420	Riboflavin (B2)	Amino acids	Electrons	Methanogensand some bacteria

Non-vitamins

Cofactor	Chemical group(s) transferred	Distribution
Adenosine triphosphate	Phosphate group	Bacteria, archaea,and eukaryotes
S-Adenosyl methionine	Methyl group	Bacteria, archaea. and eukaryotes
Coenzyme B	Electrons	Methanogens
Coenzyme M	Methyl group	Methanogens
Coenzyme Q	Electrons	Bacteria, archaea,and eukaryotes
Cytidine triphosphate	Diacylglycerols and lipid head groups	Bacteria, archaea,and eukaryotes
Glutathione	Electrons	Some bacteriaand most eukaryotes
Heme	Electrons	Bacteria, archaeaand eukaryotes
Lipoamide	Electrons, acyl groups	Bacteria, archaeaand eukaryotes
Methanofuran	Formyl group	Methanogens
Molybdopterin	Oxygen atoms	Bacteria, archaea and eukaryotes
Nucleotide sugars	Monosaccharides	Bacteria, archaeaand eukaryotes
3'-Phosphoadenosine-5'-phosphosulfate	Sulfate group	Bacteria, archaeaand eukaryotes
Pyrroloquinoline quinone	Electrons	Bacteria
Tetrahydrobiopterin	Oxygen atom and electrons	Bacteria, archaeaand eukaryotes
Tetrahydromethanopterin	Methyl group	Methanogens

Cofactors as metabolic intermediates

The redox reactions of nicotinamide adenine dinucleotide.

 

Metabolism involves a vast array of chemical reactions, but most fall under a few basic types of reactions that involve the transfer of functional groups. This common chemistry allows cells to use a small set of metabolic intermediates to carry chemical groups between different reactions. These group-transfer intermediates are the loosely bound organic cofactors, often called coenzymes.

Each class of group-transfer reaction is carried out by a particular cofactor, which is the substrate for a set of enzymes that produce it, and a set of enzymes that consume it. An example of this are the dehydrogenases that use nicotinamide adenine dinucleotide (NAD⁺) as a cofactor. Here, hundreds of separate types of enzymes remove electrons from their substrates and reduce NAD⁺ to NADH. This reduced cofactor is then a substrate for any of the reductases in the cell that require electrons to reduce their substrates.

Therefore, these cofactors are continuously recycled as part of metabolism. As an example, the total quantity of ATP in the human body is about 0.1 mole. This ATP is constantly being broken down into ADP, and then converted back into ATP. Thus, at any given time, the total amount of ATP + ADP remains fairly constant. The energy used by human cells requires the hydrolysis of 100 to 150 moles of ATP daily, which is around 50 to 75 kg. In typical situations, humans use up their body weight of ATP over the course of the day. This means that each ATP molecule is recycled 1000 to 1500 times daily.

Evolution

Organic cofactors, such as ATP and NADH, are present in all known forms of life and form a core part of metabolism. Such universal conservation indicates that these molecules evolved very early in the development of living things. At least some of the current set of cofactors may, therefore, have been present in the last universal ancestor, which lived about 4 billion years ago.

Organic cofactors may have been present even earlier in the history of life on Earth. The nucleotide adenosine is present in cofactors that catalyse many basic metabolic reactions such as methyl, acyl, and phosphoryl group transfer, as well as redox reactions. This ubiquitous chemical scaffold has, therefore, been proposed to be a remnant of the RNA world, with early ribozymes evolving to bind a restricted set of nucleotides and related compounds. Adenosine-based cofactors are thought to have acted as interchangeable adaptors that allowed enzymes and ribozymes to bind new cofactors through small modifications in existing adenosine-binding domains, which had originally evolved to bind a different cofactor. This process of adapting a pre-evolved structure for a novel use is known as exaptation. 

A computational method, IPRO, recently predicted mutations that experimentally switched the cofactor specificity of Candida boidinii xylose reductase from NADPH to NADH.

History

The first organic cofactor to be discovered was NAD⁺, which was identified by Arthur Harden and William Youndin 1906. They noticed that adding boiled and filtered yeast extract greatly accelerated alcoholic fermentation in unboiled yeast extracts. They called the unidentified factor responsible for this effect a coferment. Through a long and difficult purification from yeast extracts, this heat-stable factor was identified as a nucleotide sugar phosphate by Hans von Euler-Chelpin. Other cofactors were identified throughout the early 20th century, with ATP being isolated in 1929 by Karl Lohmann, and coenzyme A being discovered in 1945 by Fritz Albert Lipmann.

The functions of these molecules were at first mysterious, but, in 1936, Otto Heinrich Warburg identified the function of NAD⁺ in hydride transfer. This discovery was followed in the early 1940s by the work of Herman Kalckar, who established the link between the oxidation of sugars and the generation of ATP. This confirmed the central role of ATP in energy transfer that had been proposed by Fritz Albert Lipmann in 1941. Later, in 1949, Morris Friedkin and Albert L. Lehninger proved that NAD⁺ linked metabolic pathways such as the citric acid cycle and the synthesis of ATP.

Protein-derived cofactors

In a number of enzymes, the moiety that acts as a cofactor is formed by post-translational modification of a part of the protein sequence. This often replaces the need for an external binding factor, such as a metal ion, for protein function. Potential modifications could be oxidation of aromatic residues, binding between residues, cleavage or ring-forming. These alterations are distinct from other post-translation protein modifications, such as phosphorylation, methylation, or glycosylation in that the amino acids typically acquire new functions. This increases the functionality of the protein; unmodified amino acids are typically limited to acid-base reactions, and the alteration of resides can give the protein electrophilic sites or the ability to stabilize free radicals. Examples of cofactor production include tryptophan tryptophylquinone (TTQ), derived from two tryptophan side chains, and 4-methylidene-imidazole-5-one (MIO), derived from an Ala-Ser-Gly motif. Characterization of protein-derived cofactors is conducted using X-ray crystallography and mass spectroscopy; structural data is necessary because sequencing does not readily identify the altered sites.

Non-enzymatic cofactors

The term is used in other areas of biology to refer more broadly to non-protein (or even protein) molecules that either activate, inhibit, or are required for the protein to function. For example, ligands such as hormones that bind to and activate receptor proteins are termed cofactors or coactivators, whereas molecules that inhibit receptor proteins are termed corepressors. One such example is the G protein-coupled receptor family of receptors, which are frequently found in sensory neurons. Ligand binding to the receptors activates the G protein, which then activates an enzyme to activate the effector. In order to avoid confusion, it has been suggested that such proteins that have ligand-binding mediated activation or repression be referred to as coregulators.

Human iron metabolism

From Wikipedia, the free encyclopedia

Diagram showing a generalized view of cellular iron homeostasis in humans. Iron import can occur via endocytosis of transferrin receptor 1 or via ferrous iron importers DMT1 and ZIP14, which require the activity of iron reductases such as STEAP2, SDR-2 and Dcytb. Intracellular iron can be stored in ferritin, used for protein biosynthesis, generate reactive oxygen species (ROS) and regulate transcription via iron-responsive element-binding proteins (IRP1/2). Export occurs through ferroportin, often aided by hephaestin (Hp) and/or ceruloplasmin (Cp), and repressed by hepcidin.

 

Human iron metabolism is the set of chemical reactions that maintain human homeostasis of iron at the systemic and cellular level. Iron is both necessary to the body and potentially toxic, and controlling iron levels in the body is a critically important part of many aspects of human health and disease. Hematologists have been especially interested in systemic iron metabolism because iron is essential for red blood cells, where most of the human body's iron is contained. Understanding iron metabolism is also important for understanding diseases of iron overload, such as hereditary hemochromatosis, and iron deficiency, such as iron deficiency anemia.

Importance of iron regulation

Structure of Heme b; "Fe" is the chemical symbol of iron, "II" indicates its oxidation state.

 

Iron is an essential bioelement for most forms of life, from bacteria to mammals. Its importance lies in its ability to mediate electron transfer. In the ferrous state, iron acts as an electron donor, while in the ferric state it acts as an acceptor. Thus, iron plays a vital role in the catalysis of enzymatic reactions that involve electron transfer (reduction and oxidation, redox). Proteins can contain iron as part of different cofactors, such as iron-sulfur clusters (Fe-S) and heme groups, both of which are assembled in mitochondria.

Cellular respiration

Human cells require iron in order to obtain energy as ATP from a multi-step process known as cellular respiration, more specifically from oxidative phosphorylation at the mitochondrial cristae. Iron is present in the iron-sulfur clusters and heme groups of the electron transport chain proteins that generate a proton gradient that allows ATP synthase to synthesize ATP (chemiosmosis). 

Heme groups are part of hemoglobin, a protein found in red blood cells that serves to transport oxygen from the lungs to the tissues. Heme groups are also present in myoglobin to store and diffuse oxygen in muscle cells.

Oxygen transport

The human body needs iron for oxygen transport. Oxygen (O₂) is required for the functioning and survival of nearly all cell types. Oxygen is transported from the lungs to the rest of the body bound to the heme group of hemoglobin in erythrocytes. In muscles cells, iron binds myoglobin, which regulates its release.

Toxicity

Iron is also potentially toxic. Its ability to donate and accept electrons means that it can catalyze the conversion of hydrogen peroxide into free radicals. Free radicals can cause damage to a wide variety of cellular structures, and ultimately kill the cell.

Iron bound to proteins or cofactors such as heme is safe. Also, there are virtually no truly free iron ions in the cell, since they readily form complexes with organic molecules. However, some of the intracellular iron is bound to low-affinity complexes, and is termed labile iron or "free" iron. Iron in such complexes can cause damage as described above.

To prevent that kind of damage, all life forms that use iron bind the iron atoms to proteins. This binding allows cells to benefit from iron while also limiting its ability to do harm. Typical intracellular labile iron concentrations in bacteria are 10-20 micromolar, though they can be 10-fold higher in anaerobic environment, where free radicals and reactive oxygen species are scarcer. In mammalian cells, intracellular labile iron concentrations are typically smaller than 1 micromolar, less than 5 percent of total cellular iron.

Bacterial protection

Electron micrograph of E. coli. Most bacteria that cause human disease require iron to live and to multiply.

 

In response to a systemic bacterial infection, the immune system initiates a process known as iron withholding. If bacteria are to survive, then they must obtain iron from their environment. Disease-causing bacteria do this in many ways, including releasing iron-binding molecules called siderophores and then reabsorbing them to recover iron, or scavenging iron from hemoglobin and transferrin. The harder they have to work to get iron, the greater a metabolic price they must pay. That means that iron-deprived bacteria reproduce more slowly. So our control of iron levels appears to be an important defense against most bacterial infections; there are some exceptions however. TB causing bacterium can reside within macrophages which are an iron rich environment and Borrelia burgdorferi utilises manganese in place of iron. People with increased amounts of iron, like people with hemochromatosis, are more susceptible to some bacterial infection.

Although this mechanism is an elegant response to short-term bacterial infection, it can cause problems when inflammation goes on for longer. Since the liver produces hepcidin in response to inflammatory cytokines, hepcidin levels can increase as the result of non-bacterial sources of inflammation, like viral infection, cancer, auto-immune diseases or other chronic diseases. When this occurs, the sequestration of iron appears to be the major cause of the syndrome of anemia of chronic disease, in which not enough iron is available to produce enough hemoglobin-containing red blood cells.

Body iron stores

Illustration of blood cell production in the bone marrow. In iron deficiency, the bone marrow produces fewer blood cells, and as the deficiency gets worse, the cells become smaller.

 

Most well-nourished people in industrialized countries have 4 to 5 grams of iron in their bodies (∼38 mg iron/kg body weight for women and ∼50 mg iron/kg body for men). Of this, about 2.5 g is contained in the hemoglobin needed to carry oxygen through the blood, and most of the rest (approximately 2 grams in adult men, and somewhat less in women of childbearing age) is contained in ferritin complexes that are present in all cells, but most common in bone marrow, liver, and spleen. The liver's stores of ferritin are the primary physiologic source of reserve iron in the body. The reserves of iron in industrialized countries tend to be lower in children and women of child-bearing age than in men and in the elderly. Women who must use their stores to compensate for iron lost through menstruation, pregnancy or lactation have lower non-hemoglobin body stores, which may consist of 500 mg, or even less. 

Of the body's total iron content, about 400 mg is devoted to cellular proteins that use iron for important cellular processes like storing oxygen (myoglobin) or performing energy-producing redox reactions (cytochromes). A relatively small amount (3–4 mg) circulates through the plasma, bound to transferrin. Because of its toxicity, free soluble iron is kept in low concentration in the body. 

Iron deficiency first affects the storage iron in the body, and depletion of these stores is thought to be relatively non-symptomatic, although some vague and non-specific symptoms have been associated with it. Since iron is primarily required for hemoglobin, iron deficiency anemia is the primary clinical manifestation of iron deficiency. Iron-deficient people will suffer or die from organ damage well before cells run out of the iron needed for intracellular processes like electron transport. 

Macrophages of the reticuloendothelial system store iron as part of the process of breaking down and processing hemoglobin from engulfed red blood cells. Iron is also stored as a pigment called hemosiderin which is an ill-defined deposit of protein and iron, created by macrophages where excess iron is present, either locally or systemically for example among people with iron overload due to frequent blood cell destruction and transfusions. If the systemic iron overload is corrected, over time the hemosiderin is slowly resorbed by macrophages.

Mechanisms of iron regulation

Human iron homeostasis is regulated at two different levels. Systemic iron levels are balanced by the controlled absorption of dietary iron by enterocytes, the cells that line the interior of the intestines, and the uncontrolled loss of iron from epithelial sloughing, sweat, injuries and blood loss. In addition, systemic iron is continuously recycled. Cellular iron levels are controlled differently by different cell types due to the expression of particular iron regulatory and transport proteins.

Systemic iron regulation

Humans use 20 mg of iron each day for the production of new red blood cells, much of which is recycled from old red blood cells.

Dietary iron uptake

The absorption of dietary iron is a variable and dynamic process. The amount of iron absorbed compared to the amount ingested is typically low, but may range from 5% to as much as 35% depending on circumstances and type of iron. The efficiency with which iron is absorbed varies depending on the source. Generally the best-absorbed forms of iron come from animal products. Absorption of dietary iron in iron salt form (as in most supplements) varies somewhat according to the body’s need for iron, and is usually between 10% and 20% of iron intake. Absorption of iron from animal products, and some plant products, is in the form of heme iron, and is more efficient, allowing absorption of from 15% to 35% of intake. Heme iron in animals is from blood and heme-containing proteins in meat and mitochondria, whereas in plants, heme iron is present in mitochondria in all cells that use oxygen for respiration. 

Like most mineral nutrients, the majority of the iron absorbed from digested food or supplements is absorbed in the duodenum by enterocytes of the duodenal lining. These cells have special molecules that allow them to move iron into the body. To be absorbed, dietary iron can be absorbed as part of a protein such as heme protein or iron must be in its ferrous Fe²⁺ form. A ferric reductase enzyme on the enterocytes’ brush border, duodenal cytochrome B (Dcytb), reduces ferric Fe³⁺ to Fe²⁺. A protein called divalent metal transporter 1 (DMT1), which can transport several divalent metals across the plasma membrane, then transports iron across the enterocyte’s cell membrane into the cell. It the iron is bound to Heme it is instead transported across the apical membrane by Heme carrier protein 1 (HCP1).

These intestinal lining cells can then either store the iron as ferritin, which is accomplished by Fe³⁺ binding to apoferritin (in which case the iron will leave the body when the cell dies and is sloughed off into feces), or the cell can release it into the body via the only known iron exporter in mammals, ferroportin. Hephaestin, a ferroxidase that can oxidize Fe²⁺ to Fe³⁺ and is found mainly in the small intestine, helps ferroportin transfer iron across the basolateral end of the intestine cells. In contrast, ferroportin is post-translationally repressed by hepcidin, a 25-amino acid peptide hormone. The body regulates iron levels by regulating each of these steps. For instance, enterocytes synthesize more Dcytb, DMT1 and ferroportin in response to iron deficiency anemia. Iron absorption from diet is enhanced in the presence of vitamin C and diminished by excess calcium, zinc, or manganese.

The human body’s rate of iron absorption appears to respond to a variety of interdependent factors, including total iron stores, the extent to which the bone marrow is producing new red blood cells, the concentration of hemoglobin in the blood, and the oxygen content of the blood. The body also absorbs less iron during times of inflammation, in order to deprive bacteria of iron. Recent discoveries demonstrate that hepcidin regulation of ferroportin is responsible for the syndrome of anemia of chronic disease.

Iron recycling and loss

Most of the iron in the body is hoarded and recycled by the reticuloendothelial system, which breaks down aged red blood cells. In contrast to iron uptake and recycling, there is no physiologic regulatory mechanism for excreting iron. People lose a small but steady amount by gastrointestinal blood loss, sweating and by shedding cells of the skin and the mucosal lining of the gastrointestinal tract. The total amount of loss for healthy people in the developed world amounts to an estimated average of 1 mg a day for men, and 1.5–2 mg a day for women with regular menstrual periods. People with gastrointestinal parasitic infections, more commonly found in developing countries, often lose more. Those who cannot regulate absorption well enough get disorders of iron overload. In these diseases, the toxicity of iron starts overwhelming the body's ability to bind and store it.

Cellular iron regulation

Iron import

Most cell types take up iron primarily through receptor-mediated endocytosis via transferrin receptor 1 (TFR1), transferrin receptor 2 (TFR2) and GAPDH. TFR1 has a 30-fold higher affinity for transferrin-bound iron than TFR2 and thus is the main player in this process. The higher order multifunctional glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) also acts as a transferrin receptor. Transferrin-bound ferric iron is recognized by these transferrin receptors, triggering a conformational change that causes endocytosis. Iron then enters the cytoplasm from the endosome via importer DMT1 after being reduced to its ferrous state by a STEAP family reductase.

Alternatively, iron can enter the cell directly via plasma membrane divalent cation importers such as DMT1 and ZIP14 (Zrt-Irt-like protein 14). Again, iron enters the cytoplasm in the ferrous state after being reduced in the extracellular space by a reductase such as STEAP2, STEAP3 (in erythrocytes), Dcytb (in enterocytes) and SDR2.

The labile iron pool

In the cytoplasm, ferrous iron is found in a soluble, chelatable state which constitutes the labile iron pool (~0.001 mM). In this pool, iron is thought to be bound to low-mass compounds such as peptides, carboxylates and phosphates, although some might be in a free, hydrated form (aqua ions). Alternatively, iron ions might be bound to specialized proteins known as metallochaperones. Specifically, poly-r(C)-binding proteins (PCBPs) appear to mediate transfer of free iron to ferritin (for storage) and non-heme iron enzymes (for use in catalysis). The labile iron pool is potentially toxic due to iron's ability to generate reactive oxygen species. Iron from this pool can be taken up by mitochondria via mitoferrin to synthesize Fe-S clusters and heme groups.

The storage iron pool

Iron can be stored in ferritin as ferric iron due to the ferroxidase activity of the ferritin heavy chain. Dysfunctional ferritin may accumulate as hemosiderin, which can be problematic in cases of iron overload. The ferritin storage iron pool is much larger than the labile iron pool, ranging in concentration from 0.7 mM to 3.6 mM.

Iron export

Iron export occurs in a variety of cell types, including neurons, erythrocytes, macrophages and enterocytes. The latter two are especially important since systemic iron levels depend upon them. There is only one known iron exporter, ferroportin. It transports ferrous iron out of the cell, generally aided by ceruloplasmin and/or hephaestin (mostly in enterocytes), which oxidize iron to its ferric state so it can bind ferritin in the extracellular medium. Hepcidin causes the internalization of ferroportin, decreasing iron export. Besides, hepcidin seems to downregulate both TFR1 and DMT1 through an unknown mechanism. Another player assisting ferroportin in effecting cellular iron export is GAPDH. A specific post translationally modified isoform of GAPDH is recruited to the surface of iron loaded cells where it recruits apo-transferrin in close proximity to ferroportin so as to rapidly chelate the iron extruded.

The expression of hepcidin, which only occurs in certain cell types such as hepatocytes, is tightly controlled at the transcriptional level and it represents the link between cellular and systemic iron homeostasis due to hepcidin's role as "gatekeeper" of iron release from enterocytes into the rest of the body. Erythroblasts produce erythroferrone, a hormone which inhibits hepcidin and so increases the availability of iron needed for hemoglobin synthesis.

Translational control of cellular iron

Although some control exists at the transcriptional level, the regulation of cellular iron levels is ultimately controlled at the translational level by iron-responsive element-binding proteins IRP1 and especially IRP2. When iron levels are low, these proteins are able to bind to iron-responsive elements (IREs). IREs are stem loop structures in the untranslated regions (UTRs) of mRNA.

Both ferritin and ferroportin contain an IRE in their 5' UTRs, so that under iron deficiency their translation is repressed by IRP2, preventing the unnecessary synthesis of storage protein and the detrimental export of iron. In contrast, TFR1 and some DMT1 variants contain 3' UTR IREs, which bind IRP2 under iron deficiency, stabilizing the mRNA, which guarantees the synthesis of iron importers.

Pathology

Iron deficiency

Iron is an important topic in prenatal care because women can sometimes become iron-deficient from the increased iron demands of pregnancy.

 

Functional or actual iron deficiency can result from a variety of causes. These causes can be grouped into several categories:

• Increased demand for iron, which the diet cannot accommodate.
• Increased loss of iron (usually through loss of blood).
• Nutritional deficiency. This can result due to a lack of dietary iron or consumption of foods that inhibit iron absorption. Absorption inhibition has been observed caused by phytates in bran, calcium from supplements or dairy products, and tannins from tea, although in all three of these studies the effect was small and the authors of the studies cited regarding bran and tea note that the effect will probably only have a noticeable impact when most iron is obtained from vegetable sources.
• Acid-reducing medications: Acid-reducing medications reduce the absorption of dietary iron. These medications are commonly used for gastritis, reflux disease, and ulcers. Proton pump inhibitors (PPIs), H2 antihistamines, and antacids will reduce iron metabolism.
• Damage to the intestinal lining. Examples of causes of this kind of damage include surgery involving the duodenum, or diseases like Crohn's or celiac sprue which severely reduce the surface area available for absorption.
• Inflammation leading to hepcidin-induced restriction on iron release from enterocytes (see above).

Iron overload

The body is able to substantially reduce the amount of iron it absorbs across the mucosa. It does not seem to be able to entirely shut down the iron transport process. Also, in situations where excess iron damages the intestinal lining itself (for instance, when children eat a large quantity of iron tablets produced for adult consumption), even more iron can enter the bloodstream and cause a potentially deadly syndrome of iron overload. Large amounts of free iron in the circulation will cause damage to critical cells in the liver, the heart and other metabolically active organs. 

Iron toxicity results when the amount of circulating iron exceeds the amount of transferrin available to bind it, but the body is able to vigorously regulate its iron uptake. Thus, iron toxicity from ingestion is usually the result of extraordinary circumstances like iron tablet over-consumption rather than variations in diet. The type of acute toxicity from iron ingestion causes severe mucosal damage in the gastrointestinal tract, among other problems. 

Excess iron has been linked to some cancers. Of note, a recent study showed that breast cancer patients with low ferroportin expression (leading to higher concentrations of intracellular iron) survive for a shorter period of time on average. Conversely, high ferroportin expression in breast cancer predicts 90% 10-year survival.

Chronic iron toxicity is usually the result of more chronic iron overload syndromes associated with genetic diseases, repeated transfusions or other causes. In such cases the iron stores of an adult may reach 50 grams (10 times normal total body iron) or more. Classic examples of genetic iron overload includes hereditary hemochromatosis (HH) and the more severe disease juvenile hemochromatosis (JH) caused by mutations in either the gene RGMc gene, a member of a three gene repulsive guidance molecule family, (also called hemojuvelin (HJV), and HFE2), Hemojuvelin, or the HAMP gene that encodes (an iron regulatory peptide). The exact mechanisms of most of the various forms of adult hemochromatosis, which make up most of the genetic iron overload disorders, remain unsolved. So while researchers have been able to identify genetic mutations causing several adult variants of hemochromatosis, they now must turn their attention to the normal function of these mutated genes.