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Iron-binding proteins are carrier proteins and metalloproteins which play many important roles in various organic processes, including metabolism, the immune response, and delivery pathways for small molecules such as oxygen (when bound to iron in heme). Iron is required by humans and bacteria for enzymes and metabolic pathways to function properly.

Iron-dependent enzymes catalyze a variety of different biochemical reactions and can be divided into three broad classes depending on the structure of their active site: mononuclear iron clusters, Rieske (Fe-S) centers, or heme centers. A well-known family of iron-dependent enzymes include oxygenases that facilitate hydroxyl group addition of one or both atoms from diatomic oxygen. Notable enzymes include tryptophan dioxygenase, ferredoxin, and 2-oxoglutarate dioxygenase (a major enzyme in flavonoid biosynthesis).

Iron Bio-availability
Iron is crucial for most biology. In humans, iron is a required nutrient, and deficiency can lead to diseases like anemia. In the environment, iron is highly abundant, but most of it is held in iron oxides or as poorly soluble Fe3+. Neither of these common forms is very bio-available. To be useful biologically, inorganic iron is usually reduced from Fe3+ to Fe2+ by ferric reductases. The Fe2+ species is much more soluble. This is helpful for loading it onto iron binding proteins, which keep the iron ions under regulation. Free iron is cytotoxic, because it participates in Fenton chemistry to produce reactive oxygen species that damage biomolecules. This means that an overabundance of iron can also be damaging, as demonstrated by the disease hemochromatosis.

Humans usually source most of their iron consumption from either plant or animal sources. This iron can be heme (in animal meat) or non-heme (in plants). Iron in heme is much more bio-available to humans (15-35% absorbed), so meats can be a powerful source of dietary iron. Non-heme iron is generally only 2-20% absorbed. However, most iron in an average human's diet comes from non-heme sources simply because higher amounts of plant based foods are consumed. Iron is held onto tightly by the body, and the only way for humans to expel extra iron is through blood loss. Therefore, iron is regulated at uptake, where a relatively low proportion of iron is absorbed. Hepcidin is a hormonal regulator of iron uptake in the intestine. On intestinal cell membranes, divalent metal transporter-1 actively transports iron into the cell. It is then captured in ferritin for further use. Through hepcidin, intestinal cell iron uptake can be tuned by physiological needs, such as iron shortage or abundance.

Heme Proteins
Heme proteins are proteins that contain a heme prosthetic group. The heme group consists of a porphyrin ring coordinated with an iron ion. Four nitrogen atoms in the porphyrin ring act as a ligand for the iron in the center. In many cases, the equatorial porphyrin is complemented by one or two axial ligands. An example of this is in hemoglobin, where the porphyrin works together with a histidine side chain and a bound O2 molecule, forming an octahedral complex.

Porphyrin synthesis (i.e., heme anabolism) is highly conserved across all domains of life. It begins with the production of D-aminolevulinic acid from glycine and succinyl-CoA, follows with formation of the tetrapyrrole ring, and ends with the addition of the iron atom into the chelating ring. Porphyrin synthesis is negatively regulated (i.e., by the absence/presence of a repressor) by increased glucose and heme concentrations. The presence of these inhibitors destabilize the mRNA-ribosome complex and also prevent its uptake into the mitochondria for mitochondrial protein biosynthesis.

Hemoglobin
Main article: Hemoglobin Hemoglobin is an oxygen-transport protein found in virtually all vertebrates. Hemoglobin A is the main type found in human adults. It is a tetramer consisting of two alpha and two beta subunits. Each of the four monomeric units contain a heme prosthetic group in which a ferric cation is bound between four nitrogen atoms of a porphyrin ring. Along with a histidine, the apo form has five ligands surrounding the iron atom. Oxygen binds to the empty sixth position to form an octahedral complex in the holo form. Oxygen binding is fully cooperative for each of the subunits because as the first oxygen binds to one of the four heme groups, the protein undergoes a drastic conformational change that sharply increases the oxygen affinity of the other three subunits.

Hemoglobin has various affinities, depending on pH, structure, and CO2 partial pressure. Fetal hemoglobin is a variant containing two gamma subunits instead of two beta subunits. Fetal hemoglobin is the predominant form up until the infant is several months old, and it has a greater oxygen affinity to compensate for the low oxygen tension of supplied maternal blood during pregnancy. Hemoglobin has a lower oxygen affinity at low pH. This allows for rapid dissociation as oxygenated hemoglobin is transported to cells throughout the body. Because of the CO2 production and aqueous formation of carbonic acid in respiring cells, oxygenated hemoglobin dissociates in order to deliver the necessary oxygen to the cells. Hemoglobin has a binding affinity for carbon monoxide that is 250 times greater than for oxygen. This is the basis of carbon monoxide poisoning, as hemoglobin can no longer transport oxygen to cells.

Cytochromes
Main article: Cytochrome

Cytochromes are heme-containing enzymes that act as single-electron transporters, most notably as electron shuttles in oxidative phosphorylation and photosynthesis. Types of well-studied cytochromes include cytochromes a-c, cytochrome oxidase, and cytochrome P450. These proteins act as electron shuttles by switching the oxidation state of the heme iron atom between ferrous (Fe2+) and ferric (Fe3+). Various cytochromes in combination with other redox-active molecules form a gradient of standard reduction potentials that increases the efficiency of energy coupling during electron-transfer events.

Transferrin
Main article: Transferrin

Transferrin is found in human plasma, and it is used to traffic and import non-heme iron. It travels freely in the extracellular space. When its iron is needed by the cell, it is brought into the cytosol by a transferrin receptor. Transferrin can bind two Fe(III) ions, along with an anion (usually carbonate). To release the iron, the carbonate anion is protonated. This changes the carbonate's interaction with the protein, changing the conformation and allowing Fe(III) to be transferred.

Transferrin has a molecular weight of about 80 kDa. It is a glycoprotein, meaning that it has sugars attached to its amino acid chain.

Lactoferrin
Main article: Lactoferrin

Lactoferrin is a member of the transferrin family and is the predominant protein found in mammal exocrine secretions, such as tears, milk, and saliva. It is composed of approximately 700 residues and exists mainly as a tetramer, with the monomer:tetramer ratio being 1:4 at 10 µM protein concentrations. The tertiary structure is composed of two lobes, termed N and C lobes, each containing one iron-binding pocket. Each pocket contributes four amino acids (two tyrosines, one histidine, and one aspartate) and, along with two carbonate or bicarbonate anions, forms a six-membered coordinate around the iron cation. It is this specific combination that makes lactoferrin's iron affinity 300 times greater than transferrin. Lactoferrin has significant antimicrobial properties. It is found in the highest concentration of 150 ng/mL in human colostrum (the type of milk produced at the end stages of pregnancy), providing much needed immune support to newly born infants. It was widely believed that lactoferrin was only a bacteriostatic agent due to its high iron affinity and its ability to sequester free iron atoms from pathogenic microbes. It is now known, however, that the major antimicrobial driving force lies in the bactericidal properties of its iron-bound pocket and a specific peptide lactoferricin located at the N-lobe. Lactoferrin is able to bind to the LPS layer of bacteria, and in its holo form the iron atom oxidizes the lipopolysaccharides to lyse the outer membrane and simultaneously produce toxic hydrogen peroxide. Additionally, upon cleavage of lactoferrin by trypsin, the peptide lactoferricin is produced which binds to H+-ATPase, disrupting proton translocation and ultimately killing the cell.

Ferritin
Main article: Ferritin

Ferritin is an iron reservoir for an individual cell. It is found in all cells types and localized in the cytosol. Ferritin is a large protein composed of 24 subunits surrounding a core full of iron atoms. It is capable of holding 0-4500 iron atoms, which can be used as a reservoir for cellular needs. Iron is stored when there is excess, and retrieved when iron is needed again. The subunits are a mixture of H (heavy or heart) and L (light or liver). The subunits form a cluster 70-80 Angstroms wide, which is then filled with iron ferrihydrite.

Ferritin is a highly conserved protein through all domains of life. It is so conserved that subunits from horses and humans can assemble together into a functional protein. Each subunit is composed of five alpha helices.

Ferritin is used to diagnose low iron levels in humans. It can be used to indicate the level of bioavailable iron, which is helpful for diagnosing anemia. The usual range for men is 18-270 ng/mL and the range for women is 18-160 ng/mL.