Protein transferrin

General Iron ions also cycle easily between the ferrous and ferric forms, providing a handy tool for

manipulating individual electrons. The water filling cells and the oxygen in the air together conspire to convert iron ions to the

ferric state, which is highly insoluble, forming rust-like oxides. The cell must somehow shelter iron ions so that they may be stored and delivered in the

necessary quantities. This is the job of ferritin and transferrin.

Ferritin Inside cells, extra iron ions are locked safely in the protein shell of ferritin. Ferritin is composed of 24 identical protein subunits that form a hollow shell. After entering the ferritin shell, iron ions are converted into the ferric state, where they form

small crystallites along with phosphate and hydroxide ions. There is room to pack about 4500 iron ions inside.

Transferrin Iron ions are delivered in the blood by the protein transferrin. Each transferrin molecule can carry two iron ions, with each ion coupled with a carbonate

ion. The protein contains an array of amino acids that are perfectly arranged to form four bonds

to the iron ion, which locks it in place. Once it finds its iron atoms, transferrin flows through the blood until it finds a transferrin

receptor on the surface of a cell. Transferrin binds tightly to the receptor and is drawn into the cell in a small vesicle. The cell then acidifies the inside of this little pocket, which causes transferrin to release its

iron. Then, the receptor and empty transferrin are recycled back to the outside of the cell. Triggered by the neutral pH of the blood, the receptor releases the empty transferrin, and it

continues its job of gathering iron.

http://www.rcsb.org/pdb/101/motm.do?momID=35

Iron ions are transported in the blood by transferrin. Then, transferrin is [BW6]recognized by receptors on the surface of cells and transported inside. This structure includes transferrin with two iron ions.

General stepsUptake of the receptor-transferrin complex follows the general steps of receptor-mediated endocytosis, except that both the receptor and the transferrin ligand recycle to the plasma membrane once iron is released.

Diferric transferrin binds to the transferrin receptor Transferrin-receptor complexes concentrate in clathrin-coated pits The complexes are internalized in an endocytic vesicle, the clathrin coat is removed, and the

complexes are directed into an endosome The pH in the endosome drops to about 5.5-6.5 due to the action of the (H+)-ATPase pump The pH drop weakens the affinity of transferrin for iron, leading to Fe3+ dissociation from

the protein An oxidoreductase reduces Fe3+ to Fe2+ and DMT1 (divalent metal transporter 1) transports it

into the cytoplasm

As the pH drops, the affinity of the receptor for apotransferrin increases; as a result, apotransferrin remains bound to the receptor in the endosome

The apotransferrin-receptor complex recycles to the cell surface At the neutral surface pH of 7.4, the low affinity of apotransferrin for the receptor causes

the apotransferrin-receptor complex to dissociate The recycled receptor and transferrin are ready for another round of iron uptake

At both the N- and C-terminal region of transferrin, iron is directly co-ordinated to two tyrosines, one histidine and one aspartic acid, and indirectly co-ordinated to an arginine via the (bi)carbonate anion. The last co-ordination position of iron is occupied by a water molecule or a hydroxyl ion.

The general mechanism by which cells acquire iron from transferrin was apparently solved when the concept of receptor-mediated endocytosis (RME) was worked out. This model has been thoroughly discussed in several recent reviews [9,21-23]. Its main features are as follows.

Cellular uptake of iron from transferrin is initiated by the binding of transferrin to the transferrin receptor at the cell surface. Via coated pits and coated vesicles the transferrin-transferrin receptor complex becomes trapped within endocytic vesicles termed endosomes. Through the action of a proton-pumping ATPase of the endosomal membrane, the vesicle's lumen is rapidly acidified (pH 5-5.5). The low pH facilitates iron mobilization from transferrin and the iron is transported across the endosomal membrane into the cytosol. At the pH of the endosomal lumen the apotransferrin formed binds tightly to the transferrin receptor.

Through unknown processes the apotransferrin-transferrin receptor complex is sorted into exocytic vesicles, hence escaping lysosomal degradation. The exocytic vesicle fuses with the plasma membrane and the apotransferrin-transferrin receptor complex is exposed to the extracellular pH.

At this pH the apotransferrin has a very low affinity for the transferrin receptor. As a result apotransferrin dissociates from the receptor leaving it ready for another cycle of transferrin binding and endo-/exo-cytosis.

http://www.utm.utoronto.ca/~w3bio315/RME/iron.html

Ferric iron couples to transferrin only in the company of an anion (usually carbonate) that serves as a bridging ligand between metal and protein, excluding water from two coordination sites (Aisen and Listowsky, 1980); (Harris and Aisen, 1989); (Shongwe et al., 1992).

Without the anion cofactor, iron binding to transferrin is negligible. With it, ferric transferrin is resistant to all but the most potent chelators. The remaining four coordination sites are provided by the transferrin protein –

o a histidine nitrogen, o an aspartic acid carboxylate oxygeno two tyrosine phenolate oxygens (Bailey et al., 1988); (Anderson et al., 1989).

Available evidence suggests that anion-binding takes place prior to iron-binding. Iron release from transferrin involves protonation of the carbonate anion, loosening the metal-protein bond.

Under normal circumstances, about one-third of transferrin iron-binding pockets are filled. Consequently, with the exception of iron overload where all the transferrin binding sites are occupied, non-transferrin-bound iron in the circulation is virtually nonexistent.

Distribution of plasma and tissue iron can be traced using 59Fe as a radioactive tag.

Such radioactive tracer studies indicate that at least eighty percent of the iron bound to circulating transferrin is delivered to the bone marrow and incorporated into newly formed erythrocytes (Jandl and Katz, 1963); (Finch et al., 1982); Fig. 1).

Other major sites of iron delivery include the liver, which is a primary depot for stored iron, and the spleen.

Hepatic iron is found in both reticuloendothelial cells and hepatocytes. Reticuloendothelial cells acquire iron primarily by phagocytosis and breakdown of aging red cells. These cells extract the iron from heme and return it to the circulation bound to transferrin.

Hepatocytes take up iron by at least two different pathways. The first involves receptor-mediated endocytosis of transferrin. In addition, hepatocytes can take up ionic iron by a process independent of transferrin (Inman and Wesling-Resnick, 1993).

The concept that the iron-transferrin complex is internalized by receptor-mediated endocytosis

Iron is taken into cells by receptor-mediated endocytosis of monoferric and diferric transferrin (Karin and Mintz, 1981); (Klausner et al., 1983); (Iacopetta and Morgan, 1983)

The dissociation constant (Kd) for bound diferric transferrin ranges from 10 -7 M to 10-9 M at physiologic pH, depending on the species and tissue assayed (Stein and Sussman, 1983); (Sawyer and Krantz, 1986). The Kd of monoferric transferrin is approximately 10-6 M.

The concentration of circulating transferrin is about 25 M. Therefore, cellular transferrin receptors ordinarily are fully saturated.

After binding to its receptor on the cell surface, transferrin is rapidly internalized by invagination of clathrin-coated pits with formation of endocytic vesicles

An ATP-dependent proton pump lowers the pH of the endosome to about 5.5 (Van Renswoude et al., 1982); (Dautry-Varsat et al., 1983); (Paterson et al., 1984); (Yamashiro et al., 1984). The acidification of the endosome weakens the association between iron and transferrin. Even at pH 5.5, Fe3+ would not normally dissociate from transferrin in the several minutes between its endocytosis and the return of transferrin apoprotein to the cell surface (Ciechanover et al., 1983). A plasma membrane oxidoreductase reduces transferrin bound iron from the Fe3+ state to Fe2+, directly or indirectly facilitating the removal of iron from the protein (Low et al., 1987); (Thorstensen and Romslo, 1988); (Nunez et al., 1990). Conformational changes in the transferrin receptor also play a role in iron release (Bali et al., 1991); (Sipe and Murphy, 1991).

Rather than entering lysosomes for degradation, as do ligands in other receptor-mediated

endocytosis pathways, intact receptor-bound apotransferrin recycles to the cell surface, where neutral pH promotes detachment into the circulation (Zak and Aisen, 1990). Thus the preservation and re-use of transferrin are accomplished by pH-dependent changes in the affinity of transferrin for its receptor (Van Renswoude et al., 1982); (Klausner et al., 1983); (Dautry-Varsat et al., 1983). Exported apotransferrin binds additional iron and undergoes further rounds of iron delivery to cells. The average transferrin molecule, with a half-life of eight days, may be used up to one hundred times for iron delivery (Harford et al., 1994)

Once inside the cell cytoplasm, iron appears to be bound by a low molecular weight carrier molecule, which may assist in delivery to various intracellular locations including mitochondria (for heme biosynthesis) and ferritin (for storage). The identity of the intracellular iron carrier molecule(s) remains unknown.

Fe2+ oxidation state, is the biologically active form of the element. http://sickle.bwh.harvard.edu/iron_transport.html

A reduction in pH induces the release of iron from transferrin in a process that involves a conformational change in the protein from a closed to an open form

http://www.ncbi.nlm.nih.gov/pmc/articl es/PMC1299617/pdf/9635730.pdf