Download Targeting the Blood Clotting Thrombin in Blood

Argatroban
Stefan Kreida
- Targeting the Blood Clotting Thrombin in
Blood Clotting Treatment
Blood clotting can occur in the cardiovascular system, potentially with lethal consequences. Today, the drug heparin is used with
great success as an anticoagulant drug, working as an enhancer to
antithrombin, an innate anticoagulant protein. In some occasions
however, an immune response to heparin occurs, making the drug
ineffective. To treat these patients, another drug – argatroban -
Thrombin - argatroban complex
can be used.[1] It acts as an inhibitor on thrombin - a key protein
in the cardiovascular repair system. The chemical and geometrical
properties of argatroban is very suited for this task, enabling intimate interactions with the thrombin active site cleft, effectively
blocking it from performing its catalytic activity.[2]
T
he blood coagulation system is complex and for this
reason there are of course many candidates for drug
targeting. Perhaps it is the central role of thrombin
that has made it a suitable drug target candidate. Leaches,
for example inject the peptide hirudin that act as an inhibitor on thrombin to prevent coagulation. The commonly used
heparin, instead targets one of the thrombin degrading enzymes as an enhancer, speeding up its catalytic rate.
Active Site Close up in
Argatroban Inhibition
Heparin-induced thrombocytopenia
0,5 % percent of patients treated with heparin develop an
immune reaction with IgG. This leads to clusters containing heparin and blood platelets, resulting in fewer platelets
(thrombocytopenia) aswell as imobilization of heparin. With
heparin not able to function properly, another drug is needed to counter blood clotting.[1]
Argatroban
Agratroban (grey) act as an active site inhibitor, blocking
the three key catalytic units (green) of thrombin. (PDB entry
1DWC)
With heparin not able to enhance the anticoagulation in heparin-induced thrombocytopenia, argatroban, which inhibits
thrombin directly can be used as a treatment.[1] The drug
is rather small, perfectly fitting into the catalytic cleft of
thrombin (top left figure). The “arginine arm” is a characteristic property of the compund, aswell as the carboxy-terminal piperidine. Important
for argatroban inhibition
are the numerous oxygen
and nitrogen atoms with
the potential to perform
hydrogen-bonding with a
suitable acceptor or donor (right figure).[2]
Argatroban (orange) interacts primarily via hydrogen bonding (dotted green lines) in the active site cleft, blocking the
three key catalytic amino acids (green) - Ser195, Asp102 and
His57.The inhibitors long “arginine arm” (middle) interacts
by H-bonding with Asp 189. Gly216 is an important interaction point with several H-bonds between the amidino-phenylalanine of the inhibitor backbone. Also note the H-bonding between the carboxylic group of argatroban and one of
the catalytic residues - Ser 195.[2] (PDB entry 1DWC)
[3]
Coagulation pathway & regulation point logic
1
If a blood vessel i broken, the surface under the vascular endothelial cells reveals collagen and von Willebrand factors (vWF). In a receptor-mediated process, a reaction starts
which makes the platelets aggregate to the location. To prevent this from happening
in healthy, unbroken cells, endothelial cells themselves emit PGI2 and nitric oxide (NO) that
prevents agregation. [1]
2
Besides collagen and vWF, tissue factors (TF) are revealed by a broken blood vessel.
These act as cofactors for the plasma protein coagulation factor VII (VII). With the bound
cofactor, VII activates with great speed X and IX. Antithrombin is a native plasma protein
which “switches of” X and VII, thus preventing the activation of thrombin. It is however quite
slow - which is were heparin comes in - it enhances the inhibitory activity of antithrombin
and makes it a so called suicide inhibitor. The peptide tail of heparin blocks the exit route
of the compound bound to the anithropin active site, preventing it from releasing from the
complex.[1]
3
Thrombin is formed by proteolytic cleavage from prothrombin by X and is a key enzyme
in the coagulation system: it activates its own cofactor V, which increases the rate of
thrombin formation, more importantly, it cleaves fibrinogen to fibrin - a glycosylated
peptide used to plug holes as well as catalyzing the activation of XIII which cross linkes fibrin,
forming a fibrin mesh.[1]
References
[1] Weitz Jeffrey I, “Chapter 30. Blood Coagulation and Anticoagulant, Fibrinolytic, and Antiplatelet Drugs” (Chapter). Brunton LL, Chabner BA, Knollmann BC: Goodman &
Gilman’s The Pharmacological Basis of Therapeutics, 12e: http://www.accessmedicine.com/content.aspx?aID=16668944.
[2] Brandstetter H, Turk D, Hoeffken H W, Grosse D, Stürzebecher J, Martin P D, Edwards B F P, Bode W. Refined 2.3 Å X-ray Crystal Structure of Bovine Thrombin Complexes
Formed with the Benzamidine and Arginine-based Thrombin Inhibitors NAPAP, 4-TAPAP and MQPA. J. Mol. Biol. (1992) 226, 1085-1099
[3] http://en.wikipedia.org/wiki/File:Argatroban.svg