Transferrin Iron Binding and Release
The iron binding sites of both the N- and C-lobes of transferrins are highly conserved Figure 5.5, Plate 6 consisting of four protein ligands together with a carbonate ion as a bidentate ligand to the metal. The iron binding sites in transferrins and hFBP have nearly ideal octahedral metal coordination, with tight yet reversible Fe3 binding affinity constant 1019-1020 M-1 . Whereas the iron binding site in transferrins is buried, the Fe3 binding site in hFBP is substantially more exposed to...
Regulation by Fe3 Siderophores
Synthesis of siderophores and synthesis of the Fe3 -siderophore transport systems are not only controlled by repressors such as Fur and DtxR, but might also be induced by Fe3 siderophores in the growth medium, which would have the advantage that the related transport system would only be formed when the cognate Fe3 siderophore is already present in the growth medium. In contrast, iron limitation derepresses gene transcription of all iron-transport systems regardless of which iron source is...
Siderophores
Figure 3.2 Chemical structures of selected siderophores to demonstrate the four major structural classes and the different solutions of microorganisms to scavenge iron. See for comparison the conformations of the Fe3 -complexes of ferrichrome and albomycin shown in Figure 3.5. pilosus ETH21748 after it had been recognized that ferrioxamine antagonized the killing of Staphylococcus aureus by ferrimycin from Streptomyces griseoflavus ETH 9578 Bickel et al., 1960 . We now know that the antagonism...
Intracellular Iron Metabolism
In the absence of a demonstrable yeast iron-storage protein ferritin , we have to recognize that, despite our substantial advances in understanding iron uptake in yeast, we know practically nothing about intracellular iron metabolism - except that is for iron transport into, and out of, mitochondria. Intracellular Iron Metabolism 4 4 1 Mitochondrial Iron Transport As we will see in Chapter 7, the mitochondria are not only the energy powerhouse of the cell, they is also the site of both haem...
Introduction 1
Bacteria have developed a large variety of iron-uptake systems, which probably reflect the type of iron sources in their particular environment at a given time. Since the environment of the bacteria can change, for example Escherichia coli cells in the gut can later be found in sewage, soil, blood and the meninges, several iron-transport systems exist in parallel in one organism. In E. coli K-12, the standard laboratory strain used for the elucidation of E. coli physiology, genetics, and...
The Labile Iron Pool
We have seen in Chapter 5 that extracellular iron can be taken up by various routes i from transferrin, via its receptor mediated pathway ii from transferrin, by the receptor independent pathway iii from ferritin iv from non-transferrin bound iron v from other sources like haem, in one of its many forms . Once it has been taken up by cells it enters a pool of labile, cytosolic iron. This iron is available for haem synthesis and for iron incorporation into iron-dependent enzymes and ferritin....
Iron Transfer Across the Mucosal Cell
The transfer step involves the passage of iron that has been taken up at the apical, brush border membrane across the mucosal epithelial cell to the basolateral membrane, where it is transferred to the circulation. However, not all the iron taken up from the lumen into the cell is transferred. As a function of the body's requirements for iron essentially determined by the rate of erythropoiesis a variable proportion could be sequestered within the mucosal cell, eventually to be discarded into...
Dicotyledons and Nongrass Monocotyledons Strategy I Ferrous Iron Transport
In the absence of iron stress, i.e. in iron-sufficient conditions, all plant roots reduce Fe III chelates, and transport Fe II through the plasma membrane by a constitutive plasma membrane-bound ferric reductase Briat et al., 1995 Bienfait, 1985 . However, physiological responses to iron-deficiency stress, which result in increased iron acquisition, can be classified into two different strategies Romheld and Marschner, 1986 . Dicotyledons and non-graminaceous monocotyledons utilize what is...
Iron Absorption in Disorders of Iron Metabolism
The mechanism of iron absorption, and the involvement of recently detected proteins in iron transport, is described in detail in Chapter 8. Our understanding of iron absorption at the molecular level is growing rapidly. This knowledge is essentially based on detailed quantitative information on iron absorption in normal subjects, in patients with iron deficiency, hereditary and secondary iron overload, and inflammation Heinrich et al., 1966 Marx, 1979a Werner and Kaltwasser, 1987 Wienk etal.,...
Activators of Molecular Oxygen
These haem enzymes include cytochrome oxidase, peroxidases, catalases and cytochrome P450S. They are characterized by a penta-coordinate geometry in which the sixth site of the metal centre can bind either molecular oxygen, hydrogen peroxide or, in the case of cytochrome P450s, form iron-carbon bonds with the substrate. Substrate specificity and oxygen activation is determined by the interaction between haem iron and ligands and the control of spin state Smith and Williams, 1970 . For all of...
IronSulfur Proteins
Fe-S proteins are presumed to be among the first catalysts that nature had to work with Huber and Wachtershauser 1998 . The second class of iron-containing proteins is those which contain iron atoms bound to sulfur, either forming a cluster linked to the polypeptide chain uniquely by the thiol groups of cysteine residues rubredoxins , or else with both inorganic sulfide and cysteine thiols as ligands. Although they have a very widespread distribution in all living organisms, their recognition...
Graminaceous Plants Strategy II Ferric Iron Transport
Strategy II plants grasses respond to iron deficiency by releasing phytosiderophores Figure 4.1 , which can chelate poorly soluble iron from soils, in the form of phosphates or hydroxides, by the formation of Fe III -phytosiderophore complexes. The Fe-phytosiderophore complex is then taken up at the root plasma membrane via an as yet uncharacterized specific transporter Mihashi and Mori, 1989 Romheld and Marschner, 1986 . Phytosiderophores chelate and solubilize soil iron efficiently at high pH...
Oxygen Free Radicals
Molecular oxygen was not present when life began on Earth, with its essentially reducing atmosphere, and both the natural abundance of iron and its redox properties predisposed it to play a crucial role in the first stages of life on Earth. About 1 billion 109 years ago, photosynthetic prokaryotes cyanobacteria appeared and dioxygen was evolved into the Earth's atmosphere. It probably required 200-300 million years, a relatively short period on a geological time scale, for oxygen to attain a...
Chloroquine
The 4-aminoquinoline compound chloroquine is widely known as a classical anti-malarial and it is also used in clinical medicine to treat some autoimmune diseases such as rheumatoid arthritis and systemic lupus eryth-ematosus. The drug has been shown experimentally to be endowed with antiinfectious properties against selected intracellular microorganisms. It is likely that the mechanism whereby chloroquine inhibits the growth of pathogens in the macrophage is not uniform. Three mechanisms may be...
Siderophore Production
Many microorganisms, when in iron-depleted conditions, are able to mobilize environmental iron by secreting low-molecular weight, high-affinity iron-chelating compounds called siderophores, as well as cell-surface receptors for the ferric-siderophore complex and other proteins necessary for their uptake. Over the last three decades we have learned a lot about the chemical nature of these siderophores, their receptors, their genetics and the precise mechanism of iron assimilation by these...
Ferritin Distribution and Primary Structure
We begin with a definition of what constitutes ferritin - typically it is an oligomeric protein of 24 identical or similar subunits, each of molecular weight around 20 kD, forming a hollow protein shell with an external diameter of 12-13 nm, inside diameter of 7-8 nm, molecular weight about 500 kD, capable of storing up to 4500 iron atoms in a water-soluble, non-toxic, bioavailable form as ferric hydroxyphosphate micelles. There is one exception to this definition, namely the ferritin from the...
Hydrolysis of IronIII in Acid Media Formation of Polynuclear Species
Hydrolysis of ferric solutions is readily induced by addition of base. Initially, at rather acid pH, the purple ferric aqua-ion Fe H2O 63 , upon addition of base undergoes the first deprotonation step equation 5 , which is followed by reversible dimerization equation 6 , giving a yellow solution of mono- and dinuclear species 2Fe3 2H2O- gt 2FeOH2 2H 5 FeOH2 FeOH2 gt Fe2 OH 24 6 The equilibria leading to mono- and dinuclear hydrolysis products such as FeOH2 , Fe OH 2 and Fe2 OH 24 are...
Macrophages and Hepatocytes in Disorders of Iron Metabolism
After a lifetime of 100-120 days, effete red blood cells are recognized, phagocytosed and destroyed by macrophages, mainly in the spleen and the liver. A macrophage is capable of processing one erythrocyte, or over 200 million iron atoms, per hour. In man about 360 x 109 erythrocytes are processed per day Aisen, 1982 . As an normal adult human requires an average of 21 mg iron each day for haemoglobin synthesis, with only about 1 mg coming from absorption, it is easy to calculate that...
References Friedrichs Ataxia
The import of metals into the mitochondria is poorly understood. Clearly with the generation of ROS it is essential that the presence of low-molecular-weight iron be kept to a minimum to prevent Fenton chemistry occurring. Iron is required by mitochondria for the biosynthesis of haem Lange et al., 1999 and iron-sulfur proteins Lill et al., 1999 see Chapter 7 . Experiments in yeast Chapter 4 have shown that iron is not only imported into the mitochondrial matrix but is also transported out of...
Friedrichs Ataxia and Mitochondrial Iron Metabolism
Friedrich's ataxia, the most common of early-onset inherited ataxias accounts for about one half of all cases of hereditary ataxia reported in Europe and the USA. The gene, which has only five exons, codes for a small protein called frataxin, of 210 amino acids. The most common mutation is the expansion of a Ataxia - total or partial inability to coordinate voluntary bodily movements, particularly muscular movements. Friedrich's ataxia, an autosomal recessive trait with a prevalence of about 2...
Relationship Between NFKB and NO
One of the major sites of action of NFKB is the activation of iNOS. In 1993, it was shown that the murine gene coding iNOS contained two putative NFKB Figure 10.4 Life and death decisions in cells. Cellular stimulation with tumour necrosis factor-a TNFa simultaneously activates survival and death signalling pathways. Reprinted with permission from Nature from Pomerantz and Baltimore, 2000 . Copyright 2000 Macmillan Magazines Limited. p50 and p65 are subunits of NFKB Figure 10.4 Life and death...
The b2m Mouse as a Model for Human Hereditary Haemochromatosis
Although many animal models for iron overload exist, some mimicking certain aspects of HH, the b2-microglobulin knockout mouse is of special interest as it revealed for the first time crucial aspects of the pathogenesis of human HH in an animal model, and also because it underlines the important links between iron metabolism and the immune system. Hepatic iron overload in b2-microglobulin b2m -deficient mice appeared to be similar to that found in HH, with pathological iron depositions...
Oxidative Stress in Plants
Atmospheric pollutants represent an important source of oxidative and nitrosative stress to terrestrial plants. In plants, salicylic acid is known to be the mediator for systemic acquired resistance. The salicylic-binding protein shows catalase activity, which decreases significantly upon ligand binding, thereby resulting in the accumulation of hydrogen peroxide Chen etal., 1993 . The plant immune system involves the generation of ROS messengers by ligand-induced inhibition of an...
References
Behrenfeld, M.J., Bale, A.J., Kolber, Z.S., Aiken, J. and Falkowski, P.G. 1996 . Nature, 383, 508-10. Boyd, P.W., Watson, A.J., Law, C.S., Abraham, E.R. et al. 2000 . Nature, 407, 695-702. Braun, V., Crichton, R.R. and Braunitzer, G. 1968 . Hoppe Zeyler's Z. Physiol. Chem., 349, 197-210. Butler, A. 1999 . Science, 281,207-10. Coale, K.H., Johnson, K.S., Fitzwater, S.E., Gordon, R.M. et al. 1996 . Nature, 383, 495-501. Crichton, R.R. 1991 . Inorganic Biochemistry of Iron Metabolism, Ellis...
The Fur Regulon
Genes regulated by Fur code for proteins that function in iron transport and iron metabolism under aerobic conditions, iron metabolism is associated with oxidative stress. In addition, some virulence factors are regulated by Fur. Table 3.2 lists examples and functions of Fur- and iron-regulated genes in E. coli, including pathogenic E. coli strains. Table 3.2 Proteins encoded by Fur-regulated genes in E. coli strains. Table 3.2 Proteins encoded by Fur-regulated genes in E. coli strains....
Aluminium Chemistry and Biochemistry
The chemistry of aluminium combines features in common with two other groups of elements, namely i divalent magnesium and calcium, and ii trivalent chromium and iron Williams, 1999 . It is likely that the toxic effects of aluminium are related to its interference with calcium directed processes, whereas its access to tissues is probably a function of its similarity to ferric iron Ward and Crichton, 2001 . The effective ionic radius of Al3 in sixfold coordination 54 pm is most like that of Fe3...
Structure of Transferrins
Determination of the amino-acid sequence of human serum transferrin MacGillivray et al., 1983 and of human lactoferrin Metz-Boutique et al., 1984 revealed an internal two-fold sequence repeat. The amino-terminal half has approximately 40 sequence identity with the carboxyl-terminal half. Similar results have subsequently been found for a number of other transferrins Baldwin, 1993 , suggesting that the modern transferrins have evolved by a gene duplication event from an ancestral gene coding for...
Haem Biosynthesis
Mitochondria are not only the powerhouse of the cell, supplying abundant ATP through the coupling of active proton pumping across their inner membrane with the transfer of electrons along the respiratory chain, they are also the alpha and omega of haem biosynthesis. Before the reader starts searching for the Four Horsemen of the Apocalypse, a quick look at Figure 7.1 should prove reassuring. The overall pathway of haem biosynthesis begins in the mitochondria, with the condensation of succinyl...
Iron Metabolism
While much is known about siderophore-mediated ferric-iron transport, very little is known about ferrous-iron transport and iron metabolism inside the cell. It is generally assumed that Fe3 chelated to the siderophore must be reduced to allow removal from the strong claws of the chelator. Indeed, in some cases the siderophore transported iron was found 30 minutes later in the intracellular Fe2 pool of the cells Matzanke et al., 1991 . The ent-fes-fep gene cluster is necessary for the synthesis...
Other Ironcontaining Proteins37
2.5.1 Mononuclear Non-haem Iron Enzymes 40 Dioxygenases 40 Hydroxylases 41 a-Ketoacid-dependent Enzymes 42 Isopenicillin N Synthase 43 Superoxide Dismutases 43 2.5.2 Dinuclear Non-haem Iron Enzymes 44 m-Carboxylato diiron Proteins 45 3.2.1 FhuA-mediated Ferrichrome Transport Across the Outer Membrane-of E. coli 54 3.2.2 FhuA as an Antibiotic Transporter 58 3.2.3 Transport of Ferrichrome Across the Cytoplasmic Membrane 58 3.2.4 Variety of Fe3 Transport Systems in Bacteria 63 3.3 Ferrous Iron...
Preface
Two roads diverged in a yellow wood, And sorry I could not travel both And be one traveller, long I stood And looked down one as far as I could Then took the other as just as fair, Oh, I kept the first for another day Yet knowing how way leads on to way, I doubted if I should ever come back. I shall be telling this with a sigh Somewhere ages and ages hence Two roads diverged in a wood, and I -I took the one less traveled by, And that has made all the difference. Robert Frost 'The road not...