Overview
Transferrin is a glycoprotein (β₁-globulin, ~80 kDa) synthesized primarily by the liver that functions as the principal iron-transport protein in blood plasma. It binds ferric iron (Fe³⁺) with extraordinarily high affinity (Kd ~10⁻²³ M) and delivers it to cells via transferrin receptor-mediated endocytosis. In the context of nutritional immunity, elevated transferrin is a hallmark of host defense — the organism is actively sequestering circulating iron from pathogenic bacteria.
Transferrin is not itself an antimicrobial; it is a scavenging protein that renders iron biologically unavailable to iron-dependent pathogens in blood and tissues.
Mechanism
Transferrin binds Fe³⁺ at two high-affinity sites, each coordinated by two tyrosine residues, one histidine, and one aspartate, plus a synergistic carbonate ion. The protein exists in multiple iron-saturation states:
- Apo-transferrin (0% saturated): Iron-free; circulates looking for iron
- Monoferric transferrin (1 site occupied): Intermediate state
- Diferric transferrin (2 sites occupied): Fully loaded; transport form
Iron uptake and release are pH-dependent. At the acidic pH of endosomal compartments (pH ~5.5), Fe³⁺ dissociates; at physiological pH (~7.4), Fe³⁺ binds tightly. This pH gradient enables cellular iron loading while preventing uncontrolled iron loss during circulation.
The concentration of transferrin saturation (serum iron ÷ total iron-binding capacity × 100) is a key clinical metric. Normal values: 20–50% saturation; >50% is considered iron overload.
Role in Disease
Elevated serum transferrin occurs in:
- crohns disease: Nutritional immunity response to AIEC and other iron-dependent pathogens
- colorectal cancer: Inflammation-driven iron sequestration; tumor microenvironment hypoxia increases transferrin gene expression
- obesity: Chronic systemic inflammation triggers hepatic transferrin production
- endometriosis: Peritoneal inflammation; local iron sequestration in lesions
- Infection generally: Acute-phase response to bacterial and fungal pathogens
- Anemia of chronic disease: Iron is locked in transferrin but unavailable for erythropoiesis (functional anemia)
Important: Low serum iron with high transferrin saturation is not iron deficiency — it is iron sequestration and reflects successful nutritional immunity. Supplementing iron in this state feeds the pathogen.
Metal Connections
Transferrin is the primary delivery vehicle for iron (Fe³⁺) and also binds copper and other metals with lower affinity. In metallomics:
- Iron compartmentalization: Transferrin controls iron flux; hepcidin regulates transferrin-iron internalization by targeting ferroportin
- Iron-dependent pathogens: E. coli, H. pylori, B. fragilis, C. albicans all require ferric iron; transferrin sequestration blocks them
- Interlinking with lactoferrin: Both are iron-chelators; different anatomical compartments (transferrin = serum, lactoferrin = mucosal secretions)
Connections
Related proteins:
- lactoferrin — mucosal iron defense; higher affinity for Fe³⁺ than transferrin
- hepcidin — regulates iron absorption and recycling; increases transferrin synthesis during infection
- — iron exporter; hepcidin blocks it, driving iron intracellular sequestration
Related concepts:
- nutritional immunity — host defense via metal sequestration
- — bacterial iron-scavenging molecules competing with transferrin
- crohns disease, colorectal cancer, obesity — conditions with elevated transferrin
Metal entities:
- iron — the substrate; iron overload vs. iron sequestration distinction is clinically critical