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Structure

In humans, transferrin consists of a polypeptide chain containing 679 amino acids and two carbohydrate chains. The protein is composed of alpha helices and beta sheets that form the secondary structure of the protein. Transferrin is divided into two homogeneous lobes: the C and N lobes. The amino acid identity of the two lobes is 45% similar, suggesting that transferrin is the product of a duplication and fusion of DNA which coded for a singly iron binding globular protein. The C and N lobes are further divided into two smaller domains. These domains are connected via two antiparallel β-strands which act as a backbone and a hinge. The vacant region which separates the two domains is referred to as the interdomain clef, this clef is the location of the Fe³⁺ binding site. The domains take on two configurations; the open state and the closed state. The closed state occurs when the lobe contains Fe³⁺; the two domains hinge at their connecting β-strands and move towards each other to strongly contain and bind the Fe³⁺. The open state occurs when there is no Fe³⁺ in the protein; the two domains hinge at their connecting β-strands and move away from each other to allow Fe³⁺ to enter the protein. The open and closed states may be compared to a lobster claw when it is open and closed, respectively.

The binding sites within the clefs of the C and N lobes are very similar in structure; there are three anionic ligands, two tyrosines residues and an aspartic acid residue, as well as one neutral histidine residue. Other structures of importance within the clef include an N terminus and an arginine side chain. Transferrin's incredibly high affinity to iron is in part due to the presence of carbonate in the binding site. The carbonate anion neutralizes the positive charges on the N terminus of the α-helix and the arginine side chain which could otherwise repel the positively charged metal ion. The carbonate anion has the additional functionality of priming the apoprotein by providing two addition potential ligands to which the incoming Fe³⁺ may bind. After the carbonate is bound to the active site within the clef, there is a total anionic charge of three which is then perfectly countered by the incoming Fe³⁺.

Release of the Iron

Though the complete mechanism for the release of the Fe³⁺ from transferrin is not known, there are certain well identified factors and interactions that are considered to play an important role in the mechanism. The serum transferrin binds to the transferrin receptor, which results in the entire complex being internalized by the cell via endocytosis. The release of Fe³⁺ is different in the N lobe than it is in the C lobe. To provide an example of this, the presence of chlorine, or other like anions, have a different effect on the release of Fe³⁺ for each lobe. In the C lobe, these anions accelerate the release of Fe³⁺, but the release is slowed in the N lobe by the presence of these ions. The kinetically active site or sites to which bind these anions is currently not known for either lobe. The release of Fe³⁺ is triggered by a reduction in pH and that the release likely has to do with interdomain interactions involving the amino acids. This triggering reduction of pH is due to a process of intravesicular acidification, which brings the pH below 6. The exact interdomain interactions which cause the opening of the two domains is not known.