New Perspectives on Iron Uptake in Eukaryotes

Abstract

All eukaryotic organisms require iron to function. Malfunctions within iron homeostasis have a range of physiological consequences, and can lead to the development of pathological conditions that can result in an excess of non-transferrin bound iron (NTBI). Despite extensive understanding of iron homeostasis, the links between the "macroscopic" transport of iron across biological barriers (cellular membranes) and the chemistry of redox changes that drive these processes still needs elucidating. This review draws conclusions from the current literature, and describes some of the underlying biophysical and biochemical processes that occur in iron homeostasis. By first taking a broad view of iron uptake within the gut and subsequent delivery to tissues, in addition to describing the transferrin and non-transferrin mediated components of these processes, we provide a base of knowledge from which we further explore NTBI uptake. We provide concise up-to-date information of the transplasma electron transport systems (tPMETSs) involved within NTBI uptake, and highlight how these systems are not only involved within NTBI uptake for detoxification but also may play a role within the reduction of metabolic stress through regeneration of intracellular NAD(P)H/NAD(P)⁺ levels. Furthermore, we illuminate the thermodynamics that governs iron transport, namely the redox potential cascade and electrochemical behavior of key components of the electron transport systems that facilitate the movement of electrons across the plasma membrane to the extracellular compartment. We also take account of kinetic changes that occur to transport iron into the cell, namely membrane dipole change and their consequent effects within membrane structure that act to facilitate transport of ions.

Keywords: electron transfer; iron; non-transferrin bound iron; plasma membrane oxidoreductase system; redox; transferrin; transplasma membrane electron transport systems (tPMETS).

Figure 1

Transport of dietary iron. Ferrous iron can enter the enterocyte cell in the form of haem. The exact method of haem uptake is yet to be elucidated, but Haem Carrier Protein 1 (HCP1) has been suggested to fill this role. Non-haem ferrous iron can enter the enterocyte cell in the form of ferritin via receptor mediated endocytosis, although a specific receptor is yet to be identified (Theil, ; Kalgaonkar and Lönnerdal, 2009). The main route of iron uptake in the duodenum is of “free” ferric iron, via a transferrin-independent mechanism. Iron is first reduced into a soluble form via a ferric reductase enzyme at the brush border of duodenal enterocytes. Transport into the enterocyte is then facilitated by the divalent metal transport 1 (DMT1). Once inside the enterocyte cell ferrous iron can be utilized for intracellular processes such as haem and Fe-S cluster biosynthesis, or stored in the molecule ferritin. Export primarily occurs, which is facilitated by ferroportin, at this stage iron is oxidized back to its ferric form by hephaestin. Most ferric iron passes through the fenestrated capillary endothelium of the intestine before being bound by the glycoprotein transferrin, where it is circulated within the plasma. Here transferrin bound iron can pass via transcytosis (Williams et al., 1984) across the capillary endothelium and into the interstitial fluid where it is delivered to tissues via transferrin receptor 1 (TfR1) mediated uptake (under normal physiological conditions). In cases of iron overload a pool of non-transferrin bound iron (NTBI) is formed, in which case iron uptake into most mammalian cells can also occur via NTBI uptake (Brissot et al., 2012).

Figure 2

The transferrin bound cycle of iron uptake. Ferric iron binds to transferrin to form holo-transferrin (Tf). This di-ferric complex then binds to the transferrin receptor 1 (TfR1). Upon binding to the TfR1, the whole complex is internalized via receptor-mediated endocytosis. An ATP driven H⁺ pump drives a pH reduction within the endosome, which causes dissociation of ferric iron from transferrin. The unbound ferric iron is reduced via a transmembrane ferri-reductase to form ferrous iron. The ferrous iron is then transported into the cytoplasm by the DMT1 transporter. The receptor and transferrin protein are then recycled to the surface, ready for the cycle to repeat (not shown). Adapted from Lawen and Lane (2013).

Figure 3

(A). Generalized, traditional model of non-transferrin bound iron uptake. A reversible redox couple provides electrons to the ferri-reductase, through its oxidation from a reduced to an oxidized form. Facilitated by the reductase, these electrons are used to reduce two molecules of ferric ion to form ferrous ion in the extracellular compartment. This ferrous ion is co-transported across the cell membrane with H⁺ ions, into the intracellular space; this is facilitated by the DMT1 transporter. The DMT1 transporter also acts as a uniporter, and “leaks” H⁺ ions to the intracellular compartment from the extracellular compartment. (B) Ubiquinone shuttle of NADH-ferri-reductases. It has been shown that coenzyme Q can facilitate electron transport through the membrane of cells for the NADH-ferri-reductases (Oakhill et al., 2008). This schematic proposes a method of how this can occur, based upon the mechanisms of coenzyme Q electron shuttling within complex I and II of the electron transport chain. (C) Schematic of the favored electron transfer pathway in a general transplasma membrane electron transport system according to the (in some cases approximated) standard redox potential at pH 7.

Figure 4

Current ascorbate cycling model for NTBI reduction and uptake. Ascorbate provides the necessary reducing equivalents to reduce ferric iron to ferrous iron, by becoming oxidized to DHA. Fe²⁺ is then transported, putatively by DMT1, across the membrane. DHA is now cycled back to the intracellular phase via glucose transporters such as GLUT1. Internal regeneration of DHA to ascorbate occurs via direct two-electron reduction using glutathione/glutathione disulfide couple (GSH/GSSG). Reduction also occurs enzymatically using NADPH and GSH dependent enzymes. The regenerated ascorbate crosses the membrane via anion channels and the process repeats.

Figure 5

Membrane occlusion and intramembrane dipole potentials. (A) In the non-occluded state, the membrane is in a state of thinner (compared to B) hydrophobic thickness leading to a structural changes in the membrane protein. This in turn produces a low lipid packing density and thus low membrane potential. Panel (B) illustrates the opposite, where the occluded state leads to high packing density and thus high membrane potential. The increased hydrophobic thickness resulting from the occluded state causes these changes. Diagram adapted from Mares et al. (2014).