Protein transferrin

Other non-haem proteins, distinct from the above iron-sulfur proteins are involved in the roles of iron transport and storage. Iron is absorbed as Fe" in the human duodenum and passes into the blood as the Fe protein, transferrin, The Fe is in a distorted octahedral environment consisting of 1 x N, 3x0 and a chelating carbonate ion which... 

Figure 50-4. Absorption of iron. is converted to Fe + by ferric reductase, and Fe " is transported into the enterocyte by the apicai membrane iron transporter DMTl. Fieme is transported into the enterocyte by a separate heme transporter (HT), and heme oxidase (FiO) reieases Fe from the heme. Some of the intraceiiuiar Fe + is converted to Fe + and bound by ferritin. The remainder binds to the basoiaterai Fe + transporter (FP) and is transported into the biood-stream, aided by hephaestin (FiP). in piasma, Fe + is bound to the iron transport protein transferrin (TF). (Reproduced, with permission, from Ganong WF Review of Medical Physiology, 21 st ed. McGraw-Hill, 2003.)...

Iron transport agents may belong to the protein or non-protein class. In the former group are found the animal proteins transferrin (25), lactoferrin (26) and conalbumin (27). The low molecular weight iron carrying compounds from microorganisms, the siderochromes, may occur with or without a bound metal ion. Typically, severe repression of biosynthesis of these substances can be expected to set in at an iron concentration of ca. 2 x 10-5 g atoms/liter (28). Most, but not all, of these substances can be described as phenolates or hydroxamates (4). 

Meanwhile chromodulin, stored in apo form in the cell cytosol and nucleus, is metallated by transfer of Cr(III) from the blood to insulin-dependent cells, perhaps by the metal transport protein transferrin. 

It is known that part of this process involves the 80-kDa blood serum protein transferrin that tightly binds and transports two ferric iron ions. Because the iron binding uses only 30% of transferrin s metal binding capacity, it has long been thought to bind and transfer other metal ions (including perhaps chromium) in vivo, although this has not been demonstrated by experiment. 

This receptor-mediated endocytotic pathway has been especially well studied in the uptake of iron from blood plasma. Iron, because of its very low-solubility product (< 1(T17 at pH 7.4), is transported in plasma bound to the iron-binding protein transferrin. Two Fe3+ ions bind to each transferrin molecule. Entry into... 

Other metal complexes also have promising anticancer activity. Two Ti(IV) complexes are on clinical trial, an acetylacetonate derivative (budotitane) and titanocene dichloride, and the antimetastic activity of octahedral Ru(III) complexes is attracting attention, one of which is now on clinical trial. Ru(III), like several other metal ions, can be delivered to cells via the iron transport protein transferrin. 

In the extracellular fluids of mammals, the iron-binding protein transferrin is usually only 30% saturated, so that it has 70% of its metal-binding capacity free. If Al3+ could get... 

Another example is uptake of the iron-containing protein, transferrin, which circulates in the blood. It binds to its receptor to form a complex that enters the cell via endocytosis. The iron is then released from the endosome for use in the cell (e.g. haemoglobin formation for erythrocyte production or cytochrome production in proliferating cells). The number of transferrin receptors in the plasma membrane increases in proliferating cells and the number in the liver is increased by cytokines during infection. This results in a lower concentration of iron in the blood which decreases the proliferation of invading pathogens (Chapters 15 and 18). 

Iron ion The protein transferrin binds ferric ions and transports them in the semm aronnd the body. The ions are taken np by cells via a transferrin receptor which is present in the plasma membrane. The receptor binds transferrin and the complex enters the cell where transferrin releases... 

Iron (Fe) is quantitatively the most important trace element (see p. 362). The human body contains 4-5 g iron, which is almost exclusively present in protein-bound form. Approximately three-quarters of the total amount is found in heme proteins (see pp. 106,192), mainly hemoglobin and myoglobin. About 1% of the iron is bound in iron-sulfur clusters (see p. 106), which function as cofactors in the respiratory chain, in photosynthesis, and in other redox chains. The remainder consists of iron in transport and storage proteins (transferrin, ferritin see B). 

Secretion of nonglycosylated macromolecules in the presence of tunicamycin has also been investigated in a number of other cells. Rat-liver cell-secretion ol albumin (a carbohydrate-free protein), transferrin, and a-acid glycoprotein was not inhibited, and, in chick-liver cells, only a decrease by 10-25% in the secretion of transferrin and the apoprotein B chain of very-low-density lipoprotein was noted.463,464 The secretion of ovalbumin (a glycoprotein) from hen oviduct was not blocked by tunicamycin.465... 

A few substances are so large or impermeant that they can enter cells only by endocytosis, the process by which the substance is bound at a cell-surface receptor, engulfed by the cell membrane, and carried into the cell by pinching off of the newly formed vesicle inside the membrane. The substance can then be released inside the cytosol by breakdown of the vesicle membrane. Figure 1-5D. This process is responsible for the transport of vitamin B12, complexed with a binding protein (intrinsic factor) across the wall of the gut into the blood. Similarly, iron is transported into hemoglobin-synthesizing red blood cell precursors in association with the protein transferrin. Specific receptors for the transport proteins must be present for this process to work. 

In the context of iron coordination by oxygen/nitrogen, the iron transport and storage proteins, transferrin and ferritin, which are the essential parts of iron homeostasis in cells, are also treated here. 

Some proteins are imported into cells from the surrounding medium examples in eukaryotes include low-density lipoprotein (LDL), the iron-carrying protein transferrin, peptide hormones, and circulating proteins destined for degradation. The proteins bind to receptors in invaginations of the membrane called coated pits, which concentrate endocytic receptors in preference to other cell-surface proteins. The pits are coated on their cytosolic side with a lattice of the protein clathrin, which forms closed polyhedral structures (Fig. 27-40). The clathrin lattice grows as more recep-... 

A solution of the iron-transport protein, transferrin (Figure 7-4), can be titrated with iron to measure the transferrin content. Transferrin without iron, called apotransferrin, is colorless. Each molecule, with a molecular mass of 81 000, binds two Fe3+ ions. When iron binds to the protein, a red color with an absorbance maximum at a wavelength of 465 nm develops The absorbance is proportional to the concentration of iron bound to the protein. Therefore, the absorbance may be used to follow the course of a titration of an unknown amount of apotransferrin with a standard solution of Fe3+. 

Microequilibrium constants for binding of metal to a protein. The iron-transport protein, transferrin, has two distinguishable metal-binding sites, designated a and b. The microequilibrium formation constants for each site are defined as follows ... 

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. 

Iron for biosynthesis is transported through the bloodstream by the protein transferrin. The following procedure measures the Fe content of transferrin 10 This analysis requires only about 1 p,g for an accuracy of 2-5%. Human blood usually contains about 45 vol% cells and 55 vol% plasma (liquid). If blood is collected without an anticoagulant, the blood clots, and the liquid that remains is called serum. Serum normally contains about 1 pg of Fe/mL attached to transferrin. 

Obtain a precast SDS-polyacrylamide slab gel or prepare one according to instructions in Experiment 4. The recommended gel is 12°/o acrylamide with a thickness of 0.75 mm. Protein samples are prepared as follows Purified proteins (transferrin, bovine serum albumin, a, -antitrypsin, a-lactalbumin from Experiment 4, and molecular weight standards) are supplied in Tris buffer, pH 6.8 solutions at a concentration of 1 mg/mL. Sera samples have been diluted and are ready for use. Prepare protein samples for electrophoresis in 0.5-mL microcentrifuge tubes with attached caps. Label the tubes from 1 to 5 as below or per your Instructor s directions. 

In an adult human some 65% of the total iron is found in hemoglobin and myoglobin, and the bulk of the remainder is found in the storage proteins ferritin and hemosiderin. A small amount is utilized in iron enzymes at any one time. An account will be given of ferritin and the transport protein transferrin, prior to a general discussion of iron transport and storage. 

Apoferritin is synthesized in response to several factors, including the presence of iron. Added iron is taken up by the iron transport protein transferrin, which then binds to receptor sites on several types of cells which store iron.1114,1115... 

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