Iron is fundamental to nearly all life processes, including energy production and oxygen handling. However, unbound iron is highly reactive. Free iron ions generate unstable molecules called reactive oxygen species, which damage cellular components like DNA, proteins, and lipids. To neutralize this toxicity and manage its poor solubility, the body uses an intricate system where virtually all iron is chaperoned by specific proteins. These specialized iron-containing molecules are metalloproteins, which utilize a metal ion as a functional part of their structure.
Iron Transport in the Body
Iron transport safely through the circulation is handled by the protein Transferrin. This glycoprotein is synthesized mainly in the liver and acts as the body’s dedicated iron delivery vehicle within the blood plasma. Transferrin possesses two distinct binding sites, each capable of tightly securing one atom of ferric iron (Fe³⁺).
Transferrin shuttles the mineral from sites of absorption, such as the intestines and liver, to areas with high demand, primarily the bone marrow, where iron is incorporated into new red blood cells. The amount of iron bound to this transporter is routinely measured in a clinical test known as Transferrin Saturation. This measurement indicates the percentage of Transferrin molecules currently carrying iron, offering insight into the body’s iron status and helping diagnose deficiency or overload.
The Oxygen Carriers
The most abundant iron-containing proteins manage oxygen throughout the body: Hemoglobin and Myoglobin. Both rely on Heme, a ring-shaped chemical group that centrally holds a single atom of ferrous iron (Fe²⁺). This iron atom is the precise site where oxygen reversibly attaches, allowing for its transport or storage.
Hemoglobin is found exclusively inside red blood cells and is the primary method of oxygen distribution. It is a large protein made up of four individual chains, allowing a single molecule to bind up to four oxygen molecules. This structure allows for cooperative binding, where the attachment of one oxygen molecule increases the protein’s affinity for the next. This makes it highly efficient at picking up oxygen in the lungs and releasing it in the tissues.
Myoglobin, conversely, is a much smaller protein composed of only a single chain, limiting it to binding just one oxygen molecule. It is concentrated primarily within cardiac and skeletal muscle cells, serving as an oxygen reservoir. Myoglobin has a higher attraction for oxygen than Hemoglobin, allowing it to efficiently extract and store oxygen within the muscle tissue. This stored oxygen provides an immediate supply for muscle cells during intense activity when blood-borne delivery may be temporarily insufficient.
Cellular Iron Storage
A third set of proteins focuses on storing iron reserves within cells, acting as a protective mechanism against overload. The primary storage protein is Ferritin, found in nearly all cell types but especially concentrated in the liver, spleen, and bone marrow. Ferritin forms a hollow, spherical nanocage composed of 24 protein subunits.
This cage sequesters thousands of iron atoms, converting the reactive iron ions into a mineral form called ferrihydrite. A single Ferritin molecule can hold up to 4,500 atoms of iron, though it typically stores around 2,000. Ferritin functions as a buffer, releasing iron when needed and storing it when intake is high.
A related, less accessible iron complex is Hemosiderin, which forms when Ferritin aggregates and partially breaks down during high iron accumulation. Hemosiderin is a long-term, relatively insoluble iron deposit from which the iron is not easily released for use. The concentration of Ferritin circulating in the blood (serum ferritin) is often measured as an indirect indicator of the body’s total iron stores.