Iron transport oxidases

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.)...

Figure 4.4 Comparison of oxidase-dependent iron transport in mammals and yeast. In mammals, the plasma glycoprotein cerulpolasmin mediates iron oxidation, facilitating iron export from the cells and delivery to other tissues throughout the body. In yeast, Fet3p, an integral membrane protein mediates iron oxidation, resulting in plasma membrane iron transport through the permease Ftrlp. Reprinted from Askwith and Kaplan, 1998. Copyright (1998), with permission from Elsevier Science.

Figure 8.3 A model of iron transport across the intestine. Reduction of ferric complexes to the ferrous form is achieved by the action of the brush border ferric reductase. The ferrous form is transported across the brush border membrane by the proton-coupled divalent cation transporter (DCT1) where it enters an unknown compartment in the cytosol. Ferrous iron is then transported across the basolateral membrane by IREG1, where the membrane-bound copper oxidase hephaestin (Hp) promotes release and binding of Fe3+ to circulating apotransferrin. Except for hephaestin the number of transmembrane domains for each protein is not shown in full. Reprinted from McKie et al., 2000. Copyright (2000), with permission from Elsevier Science.

Askwith, C., and Kaplan, J. (1997). An oxidase-permease-based iron transport system in... 

Buonaccorsi di Patti, M. C., Pascarella, S., Gatalucci, D., and Galabrese, L. (1999). Homology modeling of the multicopper oxidase Fet3 gives new insights in the mechanism of iron transport in yeast. Protein Eng. 12, 895-897. 

Spizzo, T, Byersdorfer, C., Duesterhoeft, S., and Eide, D. (1997). The yeast FET5 gene encodes a FET3-related multicopper oxidase implicated in iron transport. Mol. Gen. Genet. 256, 547-556. 

Urbanowski, J. L., and Piper, R. C. (1999). The iron transporter Fthlp forms a complex with the Fet5 iron oxidase and resides on the vacuolar membrane. J. Biol. Chem. 274, 38061-38070. 

A number of copper requiring enzymes are located at the cell surface or are exported into the extracellular milieu. Examples of such secretory Cu-enzymes include copper requiring ferroxidases that fimction in iron transport (e g. ceruloplasmin, CP), enzymes for neurotransmission (peptidyl amidating enzyme and dopamine hydroxylase), an extracellular superoxide dismutase (SOD) that fimctions in antioxidant defense and enzymes for formation of connective tissue (lysyl oxidase), and pigments (tyrosinase) (reviewed in ). En route to their designated location, each of these enzymes passes through a specialized compartment of the late Golgi where copper insertion takes place. 

FET3 gene product of S. cerevisiae is a multicopper oxidase and plays a key role in iron metabolism of this eukaryote has underpinned the function of ceruloplasmin in vertebrate iron transport. By virtue of its ferroxidase activity, ceruloplasmin converts Fe(II) into Fe(III), which binds to the iron-binding protein transferrin. Ceruloplasmin is critical for iron egress from some cell types. The transport system responsible for iron release into plasma has not been identified. ... 

Transcriptional control of iron metabolism has been described, and therefore IRP is imlikely to be the only iron biosensor in mammals. The serum protein ceruloplasmin is transcriptionally regulated by iron in the human hepatocellular carcinoma line HepG2 [48]. Ceruloplasmin is a ferrous iron oxidase (ferroxidase) involved in both transferrin-dependent and -independent iron transport, but appears to facilitate only the latter in the carcinoma cell line. The factor(s) that mediate(s) transcriptional control of ceruloplasmin by iron is not yet known. 

Similar to the occurence in animals, iron deprivation in the yeast Saccharomyces cerevisiae induces the expression of a high affinity iron transport system, and the mechanism by which the signal is transduced is partially understood. Ferric iron is reduced to Fe via Frel/Fre2, and subsequently reoxidized and transported into cells by Fet3 and Ftrl, respectively (reviewed in [49], see Chapter 4 and [50]). Fet3 is a multicopper oxidase homologous to ceruloplasmin it requires the protein Ccc2 for... 

Oxidase-permease based iron transport systems in other species 59... 

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... 

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. 

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. 

Copper occurs in cytochrome oxidase, a key protein in respiratory electron transport, and in plasto-cyanin, which substitutes for the iron protein cytochrome Cg in photosynthetic electron transport in oceanic phytoplankton. It is also an essential component of the high-affinity iron transport system of many eukaryotic algae. Because copper is needed for iron uptake and can metabolically substitute for iron, co-limitations can occur for Cu and Fe, as observed in some diatoms. 

Proton-coupled electron transfer (PCET) is known to play an important role in a variety of biological processes, including microbial iron transport by ferric enterobactin, enzyme catalysis in systems such as fumarate reductase and nitrate reducatase, and dioxygen binding by the non-heme iron protein hemerythrin. " As such, pH-dependent electrochemical studies can play an important role in unraveling these mechanisms. The most heavily studied biological system known to involve PCET is cytochrome c oxidase, the terminal electron-transfer complex of the mitochondrial respiratory chain, which catalyzes the reduction of molecular oxygen to water. ... 

The organization of xanthine oxidase appears to be quite complex. There is evidence that various substrates are not bound at the same site, and that the primary reaction of different substrates may occur with various ones of the cofactors. The oxidation of purines and aldehydes is inhibited by pteridyl aldehyde and by cyanide, but these reagents do not affect the oxidation of DPNH. It is possible that these inhibitors influence substrate binding sites and primary electron transport, respectively, and that the oxidation of DPNH involves a different binding site and avoids the cyanide-sensitive electron transport mechanism, which may well involve iron. Xanthine oxidase, and probably all flavoproteins, require —SH groups, but a definite function for these groups cannot be ascribed at this time. Similarly, various factors influence the reactions with oxidants differentially. Cyanide inhibits cytochrome reduction, but not the reactions with 0 or dyes. Reduction of either cytochrome c or nitrate depends upon the presence of molybdenum. These observations... 

In addition to these more-or-less well characterized proteins, iron is known to be bound to certain flavoproteins such as succinic dehydrogenase (20), aldehyde oxidase (27), xanthine oxidase (22) and dihydrooro-tate dehydrogenase (23). Iron is present and functional in non-heme segments of the electron transport chain but again no real structural information is at hand (24). 

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