II. General Structure
Activated protein C is made up of two distinct protein
chains joined by a
. One chain is a
formed from three domains, two of which are epidermal growth factor
(EGF)-like. The first EGF-like domain is modified, and
characterized by a seven residue insert, causing a lateral
bulge, and an additional disulfide. The third domain is a
glutamic acid (Gla)-domain. The four disulfide bridges present
in the first domain involve Cys residues
,
,
, and
. These disulfide bridges form three loops, with the first one being
larger than normal. Connecting the two EGF domains is a
peptide forming an
including residues 83-85 of EGF-1 and 91-93 of the connecting peptide.
The amino-terminal sequence of the light chain is
Ala-Asn-Ser-Phe-Leu-.
The other chain is a
consisting of an active site serine residue and has an amino-terminal
sequence of Asp-Pro-Glu-Asp-Gln. The heavy chain consists of a
catalytic domain, including the protein active site,
, and a
.
III. Activation by thrombin
Protein C becomes activated by means of various
interactions between thrombin, thrombomodulin, endothelial cell
protein C receptor (ECPR), and inactive protein C. Thrombin is a
molecule that plays a role in coagulation, fibrinolysis, wound
healing, chemotaxis of inflammatory cells, mitogenesis, and
brain development. In fibrinolysis, thrombin is responsible for
converting fibrinogen to fibrin and activating various blood
factors. Incubation with alpha thrombin causes the cleavage of a
peptide bond between Arg-Leu. in the amino terminal region of
the heavy chain. Thrombin binds to thrombomodulin on the
endothelial cell membrane, which interacts with ECPR and causes
a conformational change, allowing protein C to bind to the
receptor. ECPR binds to protein C at its
. This then allows the newly activated protein C to form a complex with
protein S and go on to regulate fibrinolysis.
IV. Active Site
Protein C has been found to have a structurally open
active site, allowing both substrates and inhibitors to easily
access it. Although there are not many loops within the active
site, there are several surrounding it. The 37, 60, and 97
loops form an almost continuous chain of residues around one
side of the active site, creating a depression on its surface.
Inside the active site are side chains of
that divide the primed area of the active site from the unprimed area.
These residues also create a groove within the active site,
which allows the substrates to bind.
The differences between the primed and unprimed sections of the
active site allow for binding specificity in those areas. In the
primed side of the active site, there are multiple deep cavities.
One of these cavities is polar, characterized by
, while the other cavity is very non-polar, including
and
. The non-primed region of the active site is notable for the
hydrophilic residue Thr and lack of obstruction within its S2
pocket. This allows for protein C to bind to larger residues than
other proteolytic proteins.
V. References
Fukudome, K., and Esmon C.T. (1994).
Indendification, cloning, and regulation of a nvoel endothelial
cell protein C/activated protein C receptor. The Journal of
Biological Chemistry 269(42):26486-26491.
Kisiel, W. (1979). Human plasma protein
C: isolation, characterization, and mechanism of activation by
thrombin Journal of Clinical Investigation
64(3):761-769.
Mather, T., Oganessyan, V., Hof, P.,
Huber, R., Foundling, S., Esmon., C., Bode, W. (1996). The 2.8
angstrom crystal structure of Gla-domainless activated protein
C. The EMBO Journal 15(24):6822-6831.
Narayanan, S. (1999). Multifunctional
roles of thrombin. Annals of Clinical and Laboratory
Science 29(4):275-280.
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