Iron protein release from transferrin

A biological example of E° is the reduction of Fe(III) in the protein transferrin, which was introduced in Figure 7-4. This protein has two Fe(III)-binding sites, one in each half of the molecule designated C and N for the carboxyl and amino terminals of the peptide chain. Transferrin carries Fe(III) through the blood to cells that require iron. Membranes of these cells have a receptor that binds Fe(III)-transferrin and takes it into a compartment called an endosome into which H is pumped to lower the pH to —5.8. Iron is released from transferrin in the endosome and continues into the cell as Fe(II) attached to an intracellular metal-transport protein. The entire cycle of transferrin uptake, metal removal, and transferrin release back to the bloodstream takes 1-2 min. The time required for Fe(III) to dissociate from transferrin at pH 5.8 is —6 min, which is too long to account for release in the endosome. The reduction potential of Fe(IH)-transferrin at pH 5.8 is E° = —0.52 V, which is too low for physiologic reductants to reach. 

The bulk of transferrin iron is delivered to immature erythroid cells for utilization in heme synthesis. Iron in excess of this requirement is stored as ferritin and hemosiderin. Unloading of iron to immature erythroid cells is by receptor-mediated endocytosis. The process begins in the clathrin-coated pits with the binding of diferric transferrin to specific plasma membrane transferrin receptors that are associated with the HFE protein complex. The next step is the internalization of the transferrin-transferrin receptor-HFE protein complex with formation of endosomes. The iron transporter DMTl present in the cell membrane is also internalized into the endosomes. In the endosomes, a proton pump acidifies the complex to pH 5.4, and by altering conformation of proteins, iron is released from transferrin bound to transferrin receptor... 

The method of choice for serum iron analysis has been recommended by the International Committee for Standardization in Haematology (ICSH) [42]. This reference procedure obviates problems that are inherent in convenient automated methods that are precise but inaccurate [43]. In the recommended reference method, iron is released from transferrin with trichloroacetic acid (TCA), which precipitates apotransferrin and other serum proteins that are removed by centrifugation. Fe in the supernatant fluid is reduced to Fe " with thioglycolic (mercaptoacetic) acid, which is then complexed with a chromogenic iron-chelating reagent whose color is proportional to iron concentration. 

Once in the serum, aluminium can be transported bound to transferrin, and also to albumin and low-molecular ligands such as citrate. However, the transferrrin-aluminium complex will be able to enter cells via the transferrin-transferrin-receptor pathway (see Chapter 8). Within the acidic environment of the endosome, we assume that aluminium would be released from transferrin, but how it exits from this compartment remains unknown. Once in the cytosol of the cell, aluminium is unlikely to be readily incorporated into the iron storage protein ferritin, since this requires redox cycling between Fe2+ and Fe3+ (see Chapter 19). Studies of the subcellular distribution of aluminium in various cell lines and animal models have shown that the majority accumulates in the mitochondria, where it can interfere with calcium homeostasis. Once in the circulation, there seems little doubt that aluminium can cross the blood-brain barrier. 

Iron (Fe " ) released from hemoglobin in the histiocytes is bound to ferritin and then transported in the blood by transferrin, which can deliver it to tissues for synthesis of heme. Important proteins in this context are ... 

The protein that transports iron around the body in blood and lymph, and indeed within the cell, is transferrin (500 kDa). It has two binding sites for iron (Fe " ) when no iron is bound, it is known as apotransferrin. Transferrin picks up not only the iron absorbed from the intestine but also that released from the macrophages and then transports it to the cells that require it, which is primarily the cells in the bone marrow but also other cells that are proliferating. For uptake into the cells, the transferrin binds to a receptor on the plasma membrane and then the complex enters the cell by endocytosis. The iron is released from the complex in the cytosol where it is bound by the intra-... 

Transferrin, which is the major iron-transport protein, holds two Fe(III) atoms per molecule, and it accounts for nearly all the iron in plasma, where its concentration is usually 2-5x 10-5 M [149]. In cells and tissues, the iron release from transferrin would be controlled by local pH variations in the presence of Fe(III) chelators [149]. Conflicting reports have been published on the ability of superoxide to initiate transferrin-promoted Fenton reactions [154]. 

Internal exchange of iron is accomplished by the plasma protein transferrin. This 76 kDa /Ij-glycoprotein has 2 binding sites for ferric iron. Iron is delivered from transferrin to intracellular sites by means of specific transferrin receptors in the plasma membrane. The iron-transferrin complex binds to the receptor, and the ternary complex is taken up by receptor-mediated endocytosis. Iron subsequently dissociates in the acidic, intracellular vesicular compartment (the endosomes), and the receptor returns the apotransferrin to the cell surface, where it is released into the extracellular environment. Cells regulate their expression of transferrin receptors and intracellular ferritin in response to the iron supply. Apoferritin synthesis is regulated post-transcriptionally by 2 cytoplasmic binding proteins (IRP-1 and lRP-2) and an iron-regulating element on its mRNA (IRE). 

Regulation of transcription by iron. A cell s ability to acquire and store iron is a carefully controlled process. Iron obtained from the diet is absorbed in the intestine and released into the circulation, where it is bound by transferrin, the iron transport protein in plasma. When a cell requires iron, the plasma iron-transferrin complex binds to the transferrin receptor in the cell membrane and is internalized into the cell. Once the iron is freed from transferrin, it then binds to ferritin, which is the cellular storage protein for iron. Ferritin has the capacity to store up to 4,000 molecules of iron per ferritin molecule. Both transcriptional and translational controls work to maintain intracellular levels of iron (see Figs. 16.23 and 16.24). When iron levels are low, the iron response element binding protein (IRE-BP) binds to specific looped structures on both the ferritin and transferrin receptor mRNAs. This binding event stabilizes the transferrin receptor mRNA so that it can be translated and the number of transferrin receptors in the cell membrane increased. Consequently, cells will take up more iron, even when plasma transferrin/iron levels are low. The binding of IRE-BP to the ferritin mRNA, however, blocks translation of the mRNA. With low levels of intracellular iron, there is little iron to store and less need for intracellular ferritin. Thus, the IRE-BP can stabilize one mRNA, and block translation from a different mRNA. 

Figure 14 Iron release from transferrin. Iron is coordinated through four protein residues (D63, H249, Y95, and Y188) and the synergistic bidentate carbonate anion. The lobe continually samples the open and closed conformations. Partial loss of the carbonate, aspartate, and histidine are the first steps in the process. Coordination of the anion or chelator (A/C) forms a quaternary complex between the protein, iron, carbonate and the chelator/anion. Decay of the quaternary complex yields apo transferrin.

Figure 11.1 Schematic representation of iron uptake mechanisms, (a) The transferrin-mediated pathway in animals involves receptor-mediated endocytosis of diferric transferrin (Tf), release of iron at the lower pH of the endocytic vesicle and recycling of apoTf. (b) The mechanism in H. influenzae involves extraction of iron from Tf at outer membrane receptors and transport to the inner membrane permease system by a periplasmic ferric binding protein (Fbp). From Baker, 1997. Reproduced by permission of Nature Publishing Group.

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