The Transferrin Family
(Sero, Lacto, Ovo & Melano)
Organisms have developed elaborate mechanisms to control the flux of
iron. In one of the most important among these mechanisms, the transferrins
(TF) sequester Fe(III) avidly (Kd~10-22 M). Thus, Fe(III) is protected
from hydrolysis at physiological pH and is unavailable for catalysis of
superoxide radical formation via Fenton reactions or for the growth of
many pathogens. Serum TFs (STF) bind iron that is absorbed through the
gut and transported to various cells. (Note:
All actively dividing cells in an organism require iron for the transport
of oxygen by hemoglobin, electron transport by cytochromes and the function
of ribonucleotide reductase, the rate limiting enzyme in DNA synthesis.)
Another family member, lactoferrin (LTF), is found in milk and other bodily
secretions. Due to the high affinity with which LTFs bind iron, they are
believed to deprive potential pathogens of iron which they require for
growth and proliferation. Ovotransferrins (oTF), found in egg whites,
may serve the same function, although chicken serum transferrin which
shares the same amino acid sequence as OTF also is involved in iron transport.
Melanotransferrin (MTF) is found on the surface of melanocytes and its
function is unknown.
All TFs feature a bilobal structure with
each lobe putatively binding a single iron atom. It has been hypothesized
that this bilobal structure is the result of gene duplication. All TFs
which have been experimentally shown to bind iron feature the same four
amino acid ligands, namely two tyrosines, one histidine and an aspartic
acid as well as a synergistic anion which is carbonate. Metal binding
has an absolute requirement for the anion which contributes two additional
To date, the only confirmed receptors for
TFs are those found for serum TFs. The receptors bind diferric-TF, cluster
with other receptor complexes, and can be endocytosed in a clathrin coated
vesicle. Once internalized, the pH is believed to fall to around 5.6 where
an unknown chelator probably assists in the removal of the two iron atoms.
The vesicle is then returned to the cell surface where the complexes dissociate
and the receptors are free to bind more diferric-TF.
of a "dilysine trigger" in the hSTF N-lobe came from the crystal
structure of the proteolytically derived oTF N-lobe and was offered as
an explanation for the pH dependence observed for iron release in this
lobe. Two lysines, lying in opposite domains, are part of the second shell (defined as
residues which interact with liganding residues).
In the iron loaded human STF N-lobe structure, the e
-amino groups of Lys206 and Lys296 are 3.14 Å apart. This pair is
stabilized by a low pKa of one of the Lys-residues, which permits formation
of a low energy hydrogen bond. When the pH is lowered, protonation causes
the two lysines to repulse each other, providing a positive force that
opens the cleft and releases iron. In the apo-protein the lysines are
separated by 9 Å. Confirmation of the critical role of this dilysine
pair in the mechanism of iron release has been provided by mutagenesis
studies. In contrast, although bovine LTF has lysines at equivalent positions,
its crystal structure shows that they do not share a hydrogen bond. Why-are
they too far apart or misorientated? This structural difference may provide
a partial explanation for the slower rate of iron release from LTF.
It is important to note that the C-lobe
of human STF does not have an equivalent lysine pair. Instead, a triad
of C-lobe residues appear to serve as the pH-sensitive release mechanism.
Mutation of any of these three residues to alanine drastically slows iron
release or prevents iron release altogether from the C-lobe (our unpublished
None provided. Search the databases for STF, LTF, oTF, MTF and any homologs!
Biology Workbench (http://workbench.sdsc.edu)
GeneDoc (for PCs only:http://www.psc.edu/biomed/genedoc/)
SwissPDB Viewer (http://us.expasy.org/spdbv/)
- Did a gene duplication occur that forms
the N- & C-lobes? If so, when?
- When did the STF, oTF, MTF & LTF paralogs
diverge and become specialized and why?
- Can you identify other potential members
of the transferrin family?
- What distinguishes an STF, oTF or LTF
from one another?
- What are the physiological functions
of oTF & LTF?
- When did the transferrin receptor arrive
on the scene?
- Can you identify other proteins which
use a dilysine trigger as a pH sensitive mechanism to bind and/or
- Identify definable questions that can
be approached using phylogenetics
- Find the data & use the tools provided
to explore these questions
- Share your experiments and insights
with the group
- Design and execute experiments to approach these larger questions