Iron Uptake, Storage and Homeostasis

Introduction

Iron is essential for haemoglobin, cytochromes and many enzymes, yet it presents a double problem. In the presence of oxygen at neutral pH, iron is almost insoluble, so it is hard to obtain. And when loosely bound, it catalyses the formation of highly damaging radicals. Living organisms therefore never leave iron "free": it is always carried, stored and delivered by proteins. This page follows iron from the gut to storage and examines how the body controls the total amount.

Core explanation

The solubility problem. In aerobic water at pH 7, iron exists as Fe³⁺, whose hydroxide is extremely insoluble (Ksp of Fe(OH)₃ around 10⁻³⁸). The equilibrium concentration of free aqueous Fe³⁺ is therefore on the order of 10⁻¹⁸ M — far too low to supply a cell. Organisms must bind iron with strong ligands or keep it reduced.

The toxicity problem. Loosely bound Fe²⁺ reacts with hydrogen peroxide in the Fenton reaction , Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + •OH. The hydroxyl radical attacks lipids, proteins and DNA almost indiscriminately. Reductants such as superoxide or ascorbate can regenerate Fe²⁺, making iron a catalyst of oxidative damage.

Uptake in the gut. In humans, dietary non-heme iron is mostly Fe³⁺. At the intestinal surface a reductase converts it to Fe²⁺, which enters cells through the divalent metal transporter DMT1. Heme iron is absorbed by a separate route. Iron leaves intestinal cells through ferroportin and is reoxidised to Fe³⁺ by a copper-containing ferroxidase (hephaestin, or ceruloplasmin in plasma) — a neat link between copper and iron metabolism.

Transport: transferrin. In blood, Fe³⁺ is carried by transferrin, which has two lobes, each binding one Fe³⁺ with two tyrosinate oxygens, one histidine nitrogen, one aspartate oxygen and a bidentate carbonate anion. The hard phenolate donors suit hard Fe³⁺, and the binding constant is about 10²⁰ M⁻¹ at pH 7.4. Diferric transferrin binds a receptor on the cell surface and is taken into an endosome. The endosome is acidified to about pH 5.5; protonation of the tyrosinates and carbonate lowers the affinity, Fe³⁺ is released, reduced to Fe²⁺ and exported to the cytoplasm. The empty transferrin returns to the blood for reuse.

Storage: ferritin. Ferritin is a shell of 24 subunits enclosing a cavity about 8 nm across that can hold up to about 4500 iron atoms. Fe²⁺ enters through channels and is oxidised at ferroxidase sites (dinuclear iron centres) by O₂. The Fe³⁺ then moves into the cavity and forms a mineral core resembling ferrihydrite, a hydrated iron(III) oxide. Iron is safe here: it is solid, shielded and unavailable for Fenton chemistry, yet can be mobilised by reduction when needed.

Whole-body control. Humans have no regulated pathway for iron excretion, so control is exerted at absorption and release. The liver hormone hepcidin binds ferroportin and triggers its internalisation and breakdown, trapping iron in gut cells and macrophages. High iron raises hepcidin; iron deficiency or increased red-cell production lowers it. Inside cells, iron-regulatory proteins sense iron and adjust the translation of ferritin and transferrin receptor messenger RNAs in opposite directions.

Step-by-step reasoning

To trace iron from diet to storage:

1. Fe³⁺ in food is reduced to Fe²⁺ and imported by DMT1. 2. Fe²⁺ is exported by ferroportin and oxidised to Fe³⁺. 3. Fe³⁺ binds transferrin in plasma. 4. Transferrin is internalised; acidification releases Fe³⁺. 5. Iron is used or stored in ferritin as an iron(III) oxide core.

Visual explanation

Sketch a flow diagram: gut cell → ferroportin → plasma transferrin (two iron-filled lobes) → receptor on a target cell → acidified endosome → cytoplasm → ferritin sphere with a dark mineral core. Add hepcidin as a "tap" that closes ferroportin.

Real-world analogy

Iron handling resembles the transport of a valuable but hazardous cargo. It travels only in sealed armoured vans (transferrin), is unloaded in a secure depot by changing the lock conditions (acidification), and is kept in a vault (ferritin). The central bank (hepcidin) controls how much enters circulation.

Real-world example

In hereditary haemochromatosis, mutations reduce hepcidin signalling, so ferroportin stays active and too much iron is absorbed. Iron accumulates in the liver, heart and pancreas, where Fenton chemistry damages tissue. Treatment by regular removal of blood lowers the iron load.

Why?

Why does transferrin release iron in the endosome but not in the blood? Its tyrosinate and carbonate donors are basic; at pH 5.5 they are protonated and bind Fe³⁺ much more weakly. The pH switch lets the same protein bind iron very tightly in transport and release it at the destination.

Common misconception

"Iron in the body exists as free Fe²⁺ and Fe³⁺ ions in solution." Almost all body iron is protein-bound: in heme, iron–sulfur clusters, transferrin and ferritin. The pool of loosely bound "labile" iron is tiny and tightly regulated because it is harmful.

Worked example

Question: A ferritin molecule contains 2500 iron atoms. What mass of iron is this, and how many such molecules would store 1.0 mg of iron?

Reasoning: Moles of Fe per molecule = 2500 ÷ 6.02 × 10²³ = 4.15 × 10⁻²¹ mol; mass = 4.15 × 10⁻²¹ × 55.8 = 2.32 × 10⁻¹⁹ g. Molecules for 1.0 × 10⁻³ g = 1.0 × 10⁻³ ÷ 2.32 × 10⁻¹⁹ = 4.3 × 10¹⁵.

Answer: About 2.3 × 10⁻¹⁹ g of iron per molecule; about 4.3 × 10¹⁵ molecules store 1.0 mg.

Quick check

1. Why must ferritin oxidise Fe²⁺ to Fe³⁺ before depositing it in its core? Answer: Fe³⁺ forms the insoluble hydrated oxide mineral, which locks iron safely away from Fenton chemistry.

Exam focus

Explain both the solubility and the toxicity problems, give transferrin's donor set including carbonate and the pH-triggered release, describe ferritin as a protein shell with an iron(III) oxide core, and state that hepcidin controls iron by degrading ferroportin.

Advanced insight

Transferrin's carbonate is called a synergistic anion: without it, iron binding is weak. The anion completes the octahedral coordination and bridges to positively charged residues. Acidification protonates it first, which is one reason release is fast and controlled, and this design principle has inspired pH-responsive delivery systems in medicine.

Summary

Iron is essential but scarce (insoluble Fe³⁺ hydroxide) and dangerous (Fenton radicals). Transferrin carries Fe³⁺ with tyrosinate, histidine, aspartate and carbonate donors and releases it on acidification; ferritin stores thousands of iron atoms as an iron(III) oxide core. Hepcidin regulates body iron by degrading the exporter ferroportin.

Practice questions

1. Write the equation for the Fenton reaction and explain why it is harmful. Answer: Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + •OH; the hydroxyl radical damages lipids, proteins and DNA. 2. Why are tyrosinate donors well suited to binding Fe³⁺? Answer: They are hard, negatively charged oxygen donors, which match the hard, highly charged Fe³⁺ ion and form strong bonds. 3. Explain how raised hepcidin affects plasma iron levels. Answer: Hepcidin causes ferroportin to be removed from cell surfaces, so iron is not exported from gut cells and macrophages and plasma iron falls. 4. Suggest why the body relies on controlling absorption rather than excretion of iron. Answer: Humans have no regulated iron-excretion pathway; losses occur only through shed cells and bleeding, so absorption is the main point of control.