Piyali Guha Thakurta, Debi Choudhury, Rakhi Dasgupta and J. K. Dattagupta
Crystallography and Molecular Biology Division,Saha Institute of Nuclear Physics, 1/AF Bidhanagar. Kolkata-700064, India (piyali@cmb2.saha.ernet.in)
Serum transferrin is the major
iron-transport protein in vertebrates having molecular mass ~80 kDa. Its
function depends on its ability to bind iron with very high affinity, yet to
release this bound iron at the low intracellular pH. The three dimensional
structure of diferric hen serum transferrin (hST) has been determined by X-ray
crystallography at 2.8 resolution. The overall fold of the polypeptide chain
of hST is similar to those of hen ovo transferrin (hOT), rabit serum
transferrin (rST) and human lactoferrin (hLF), being delineated into two
homologous lobes, each containing two dissimilar domains with one Fe3+
and one CO32- ion
bound at a specific site in each interdomain cleft. However, the relative
orientations of the two lobes, which may be related to the class specificity of
transferrins to recepters, is different in hST from that of hOT, rST and hLF. A
number of additional hydrogen bonds between the two domains in the N- and C-
lobes have been identified in this structure that might indicate a more compact
structure of hST than that of hOT and hence may be correlated with its iron
transport function. A pair of hydrogen bonded lysine residues, Lys209 and
Lys301, close to the N-lobe iron binding site in the serum and ovo transferrins
is termed as a dilysine trigger. The NZ atoms of two lysine residues in hST
are 2.45 apart which is comparable to other serum transferrins but is different
from that of hOT. One carbohydrate
binding site has been identified in the N- lobe at Asn52 of hST which is
different from the carbohydrate binding site Asn473 of hOT present in the C-
lobe. A fucose molecule has been modelled at this site. We also have a 3.45 diffraction data
set of apo- form of hST, the Cα chain trace of which has already been
completed. The apo- and the holo- forms of chicken serum transferrins have been
compared with each other as also with other proteins of the same family.