Please be patient while the structures in the left frame load. In order to display all of the structures in the tour properly, press 'View' buttons below in order (from 1 to the end).
Transferrins are the proteins responsible for
the transport of iron into the cells of higher organisms. The members of this
family are serum transferrin (found in blood serum), ovotransferrin (found in
eggs) and lactotransferrin (a constituent of milk). All of them have remarkably
similar overall structure with a mass of about 80 kDa and approximately 700
amino acid residues. They are glycoproteins but the sugar group is not shown in
the structure of this tour. Transferrin not only acts as an iron carrier, but
also functions as an Fe scavenger keeping the concentration of free iron at a
low value. It therefore functions to deprive bacteria of Fe so they cannot
Serum transferrins shuttle iron from the liver to bone marrow cells where
hemoglobin is produced. The structure of porcine serum transferrin is shown on
coordinate to the iron. The water molecule HOH59 (only the oxygen is shown)
hydrogen-bonds to the oxygen of Tyr94.
3)2- and the
residues located in the NII domain keep the protein wrapped tightly around the
iron. For simplicity, the side chain residues that coordinate the Fe are
represented by their donor atoms only. The O1 atom from the carbonate ligand is
H-bonded to the -OH group on Thr119 and to the NH group on Gly126. Ala 125
hydrogen bonds to the O2 atom of the (CO3)2-. The
remaining oxygen, O1, of the (CO3)2- forms two O..H-N
hydrogen bonds with Arg123.
3+ form while the other is in the
neutral, NH2 form and these form an N-H..N hydrogen bond.
The iron-loaded transferrin enters the cell by endocytosis. Once inside,
this endosome is acidified to pH 5, and
the transferrin releases the iron ions. The unprotonated amino group of the
lysine trigger is now protonated and the lysine ammonium groups repel each
other, causing an opening of the protein. This allows the protonation of the
(CO3)2- group and His253 and the subsequent release of
the iron ion.
(Interestingly, this "dilysine trigger" does not exist in the C lobe.
This might explain the fact that the N lobe releases its Fe more easily than
the C lobe.)
3)2- (magenta) are also on the opposite sides.
Transferrin picks up the iron and carbonate outside of the cell at the
pH value of 7.4 as Fe(II). Oxygen then oxidizes Fe(II) to Fe(III): 
[(Fe3+)Tf(CO3)2- ]+ O2-
(Tf = transferrin)