Ferrous Iron Transport Systems

A second iron-transport system may be used by various cells. This system is thought to involve ceruloplasmin, a small peptide, and a membrane-bound transporter, A fraction of the iron in the bloodstream occurs, in the ferrous form, bound to a small peptide. Ceruloplasmin is a pJasma protein that can catalyj e the oxidation of ferrous iron to ferric iron. Evidence suggests that the cemloplasmin in the bloodstream catalyzes the oxidation of iron, and the coincident transfer of the iron... 

Iron transport systems must maintain a balance between the amount of iron needed within the cell and the amoimt which is toxic. Iron transport is complicated by the fact that iron in the extracellular environment is generally present as bio-imavailable insoluble ferric chelates [3]. Cells have devised several mechanisms to overcome this insolubility. Soluble carrier molecules, such as siderophores and transferrin, may be used to chelate the ferric iron and make it soluble [4]. Alternatively, cells may reduce the insoluble ferric iron to the more soluble ferrous form. Cells may use an enzymatic cell surface ferrireductase or may secrete organic molecules that accelerate the ferric to ferrous conversion [5]. The ferrous iron is then transported into the cell by elemental iron transport systems. 

In order to identify a eukaryotic iron transporter, we chose to work with the yeast Saccharomyces cerevisiae because of its tractable genetic system and the simplicity and redundancy of its iron transporters. S. cerevisiae employs two main methods to obtain iron from the environment. One, they possess a siderophore-dependent iron transport system [10]. While S. cerevisiae is able to use siderophores secreted by other microorganisms, it does not make or secrete siderophores [11]. Two, in laboratory conditions S. cerevisiae must rely on elemental iron transport which depends on cell surface ferrireductases to convert extracellular ferric chelates to ferrous iron [12]. Two yeast ferrireductase genes FREl and FRE2 are transcriptionally induced by iron need and have been shown to play a role in iron transport [13, 14]. The ferrireductases possess multiple transmembrane domains and potential FAD and NADPH binding domains. These ferrireductases use intracellular NADPH as an electron donor for the conversion of ferric iron to ferrous (Figure 4-1) [15]. The ferrireductases also require heme biosynthesis for function and bind two heme molecules in a maimer similar to the B-type cytochromes [16],... 

The ferrous iron produced by the ferrireductases can be transported through either a high affinity or a low afiinity iron transport system. The low affinity iron transport system has a Kta for iron of 30 pM and transports other metals in addition to iron including the potentially toxic metals cadmium and cobalt [25]. The FET4... 

The high affinity iron transport system in yeast is composed of an oxidase and a permease. The oxidase converts extracellular ferrous iron to ferric which crosses the... 

The accumulation of iron is dependent on its transport into the cell. Askwith and Kaplan (Chapter 4) discuss iron transport mechanisms in eukaryotic cells, developing models based on studies carried out in the yeast, Saccharomyces cerevisiae. These cells possess both siderophore-dependent and elemental iron transport systems. The latter system relies on cell surface ferrireductases to convert extracellular ferric chelates to ferrous iron, which can be transported through either a high or low affinity iron transport system. Studies on a high affinity ferrous iron transporter (FET3) revealed that the multicopper oxidase will oxidize ferrous to ferric iron, which is then mobilized across the membrane by a ferric transmembrane permease (Ftrlp). This is a highly specific transport system in yeast it only transports iron. In humans, the copper enzyme, ceruloplasmin, is responsible for the radical-free oxidase activity. This plasma protein oxidizes the ferrous iron that is excreted from cells into the transferrin-usable ferric form. 

These different systems come into operation under different conditions both environmental and in terms of growth requirements. As we will see later in this chapter, yeasts do not appear to have a mechanism for iron excretion, so that their cellular iron homeostasis, as in E. coli, relies on tight control of uptake and eventually storage. As we will see when we examine these iron uptake systems in detail, most of them require ferrous iron, rather than ferric. This implies that the first step required for iron transport is the reduction of Fe3+ to Fe2+ by membrane-bound reductases. 

Based on experimental data and analysis of sequences available from the databases, we can conclude that different routes for the translocation of iron across the cytoplasmic membrane are possible in bacteria. They can mediate the importation of ferrous iron, and of ferric iron, both in its ionic form and coupled to siderophores or haem. Three of the transport systems represent members of the binding protein-dependent type (a subfamily of ABC transporters or traffic ATPases) (see Section 6.3.2). 

The fact that ferrous salts are in general much more effective than ferric salts in treating anemia must be taken into account in constructing a theory for iron transport in biological systems. In fact, most workers have assumed that iron can pass through intestinal membranes only in the ferrous form, and that ferric iron would have to be reduced before it could be utilized by the organism. This has recently been shown not to be the case, as will be detailed below in section IV A. There is an obvious... 

TfR-mediated endocytosis is a well-known uptake system Tf binds one or two Fe atoms, but only diferric Tf (Fe2Tf) has a high affinity for TfR to be taken up by the receptor-mediated endocytosis. This system uses a mobilization pathway that involves endosomal acidification, reduction of ferric Fe, and ferrous Fe transport [8]. Recently, it was clarified that divalent cation/ metal ion transporter (DCT1) or Nramp2 involves iron transport from the endosome to the cytosol [9, 10]. Al resembles Fe in chemical characteristics ionic radius, charge density, and coordination number [11]. Therefore, Al binds with Tf to form di—Al—Tf. Al bound to Tf even passes through the blood-brain barrier to enter the neuronal cells via Tf receptor-mediated endocytosis [12]. 

Hepatic mitochondria isolated from copper-deficient animals were found to be deficient in the cytochrome oxidase activity which correlated well with hem synthesis (57). Failure to synthesize hem from ferric iron and protoporphyrin could be enhanced by succinate or inhibited by cyanide, which suggests that the reduction from ferric to ferrous requires an intact electron transport system in order for hem synthesis to go into completion. 

Postma, D., and Appelo, C. A. J., 2000, Reduction of Mn-oxides by ferrous iron in a flow system Column experiment and reactive transport modeling Geochimica et Cosmochimica Acta, v. 64, p. 1237-1247. 

If both MT-1 and HO-1 mRNA induction by heme-hemopexin involves a copper-redox enzyme in both heme transport (and consequent induction of HO-1 mRNA) and the signaling pathway for MT-1 expression, a plausible working model can be formulated by analogy with aspects of the yeast iron uptake processes and with redox reactions in transport (Figure 5-6). First, the ferric heme-iron bound to hemopexin can act as an electron acceptor, and reduction is proposed to be required for heme release. The ferrous heme and oxygen are substrates for an oxidase, possibly NADH-dependent, in the system for heme transport. Like ferrous iron, ferrous heme is more water soluble than ferric heme and thus more suitable as a transport intermediate between the heme-binding site on hemopexin and the next protein in the overall uptake process. The hemopexin system would also include a copper-redox protein in which the copper electrons would be available to produce Cu(I), either as the copper oxidase or for Cu(I) transport across the plasma membrane to cytosolic copper carrier proteins for incorporation into copper-requiring proteins [145]. The copper requirement for iron transport in yeast is detectable only under low levels of extracellular copper as occur in the serum-free experimental conditions often used. 

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