The essential nature of iron usage and regulation

Abstract

The facile ability of iron to gain and lose electrons has made iron an important participant in a wide variety of biochemical reactions. Binding of ligands to iron modifies its redox potential, thereby permitting iron to transfer electrons with greater or lesser facility. The ability to transfer electrons, coupled with its abundance, as iron is the fourth most abundant mineral in the earth's crust, have contributed to iron being an element required by almost all species in the six kingdoms of life. Iron became an essential element for both Eubacteria and Archeabacteria in the early oxygen-free stages of the earth's evolution. With the advent of an oxygen-rich environment, the redox properties of iron made it extremely useful, as much of iron utilization in eukaryotes is focused on oxygen metabolism, either as an oxygen carrier or as an electron carrier that can facilitate oxygen-based chemistry.

Figure 1. Iron-containing moieties in proteins

(A) Iron–sulfur clusters within proteins can take several different forms with variations in the number of iron and sulfur atoms. 2Fe–2S and 4Fe–4S clusters bridged by cysteines are shown as examples. (B) Iron inserted into the porphyrin ring forms heme. (C) The oxo di-iron moiety from ribonucleotide reductase is shown (modified from Stubbe and Riggs-Gelasco (1998), Trends Biochem. Sci. 23, 438–443).

Figure 2. Iron acquisition mechanisms

(A) An acidic extracellular environment helps maintain the solubility of Fe³⁺. A reductase on the cell surface reduces Fe³⁺ to Fe²⁺. Fe²⁺ is transported into the cytoplasm by cell surface transporters, an example of which is the divalent metal transporter (DMT1). (B) Organisms secrete small organic molecules termed siderophores that bind soluble Fe³⁺ with high affinity. The Fe³⁺-containing siderophore binds to a siderophore transporter, which transports the Fe–siderophore complex into the cytoplasm. PM, plasma membrane.

Figure 3. Post-transcriptional regulation of the expression of proteins required for iron acqisition and storage

Many mRNAs encoding proteins involved in iron metabolism contain iron regulatory elements (IREs) in their 5’ or 3’ UTRs. When cytosolic iron is low, iron regulatory proteins (IRPs) bind to the 5’ IRE thereby preventing translation (in the case of ferritin or Fpn1) or to the 3’ IRE thereby stabilizing mRNA (as seen for TfR1 or DMT1). When cytosolic iron is high, IRPs are inactive and no longer bind IREs, allowing for either translation (ferritin or Fpn1) or mRNA degradation (TfR or DMT1).

Figure 4. Vertebrate systemic iron homeostasis

Total body iron in humans is about 3–4 g: 1–2 mg/day of iron is absorbed in the duodenum. Iron is then transported into plasma where it is bound to transferrin (Tf). Tf–Fe is delivered to developing tissues and erythrocytes. Aged erythrocytes are phagocytosed by macrophages, hemoglobin broken down, and iron (20–30 mg/day) transported back to plasma. When the amount of iron exceeds Tf-binding capacity, iron is deposited in parenchymal tissues (e.g. liver).

Figure 5. Tf–TfR1-mediated cellular iron uptake

Tf–Fe binds to the TfR1 on the plasma membrane and is internalized by endocytosis. The pH within the endosome drops due to H⁺ import, allowing Fe³⁺ to dissociate from Tf. Fe³⁺ is reduced by Steap3 and Fe²⁺ is transported from endosome to cytosol by DMT1. Iron-free Tf is recycled back to the cell surface where it is released at neutral pH and can rebind plasma Fe³⁺.