Transferrin

Transferrin is a blood plasma protein for iron ion delivery. Transferrin is a glycoprotein, which binds iron very tightly but reversibly. Although iron bound to transferrin is less than 0.1% (4 mg) of the total body iron, dynamically it is the most important iron pool, with the highest rate of turnover (25 mg/24 h). Transferrin has a molecular weight of around 80 kiloDaltons and contains 2 specific high affinity Fe(III) binding sites. The affinity of transferrin for Fe(III) is extremely high (10^23 M^-1 at pH 7.4) but decreases progressively with decreasing pH below neutrality.

When not bound to iron, it is known as "apotransferrin" (see also apoprotein).

Transport mechanism
When a transferrin protein loaded with iron encounters a transferrin receptor on the surface of a cell (importantly, to erythroid precursors in the bone marrow), it binds to it and is consequently transported into the cell in a vesicle. The cell will acidify the vesicle, causing transferrin to release its iron ions. The receptor is then transported through the endocytic cycle back to the cell surface, ready for another round of iron uptake. Each transferrin molecule has the ability to carry two iron ions in the ferric form (Fe3+).

The gene coding for transferrin in humans is located in chromosome band 3q21. Research on king snakes by Dessauer and Zwiefel in 1981 revealed the the inheritance of transferrin is a codominant trait.

Transferrin levels may be checked in iron deficiency, hemochromatosis and other iron overload disorders.

Immune system
Transferrin is also associated with the innate immune system. Transferrin is found in the mucosa and binds iron, thus creating an environment low in free iron, where few bacteria are able to survive.

A decrease in the amount of transferrin would result in hemosiderin in the liver.

Other effects
The metal binding properties of transferrin have a great influence on the biochemistry of plutonium in humans. Transferrin has a bacteriocidal effect on bacteria, in that it makes Fe3+ unavailable to the bacteria.

Pathology
A deficiency is associated with atransferrinemia.