BIOCHEMISTRY TOPICS

The blood coagulation pathway

Intrinsic and extrinsic pathways. Common final pathway. Thrombin: (Yet another) serine protease.

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The activation of pancreatic serine proteases by zymogen processing is tightly controlled so that their proteolytic activity is confined to the digestive tract. A further example of this regulatory strategy of activation by proteolytic cleavage that is of obvious physiological importance is provided by the blood coagulation cascade. A series of proteolytic activation events, each of which is similar to the activation of the zymogen forms of the pancreatic serine proteases, leads to the formation of blood clots in response to injury or trauma. This blood clotting pathway has become the classic example of a zymogen activation cascade. A key feature of this cascade is the amplification of an initial triggering event into a major physiologic response.

Intrinsic pathway

* Triggered by the "non-physiological surface" of an injury
* Begins with activation of Factor XII (Hageman factor)

Extrinsic pathway

* Trauma triggers activation of Factor VII
* Release of lipoprotein called tissue factor follows

Final common pathway

* Intrinsic and extrinsic pathways converge, both activating Factor X
* The end result is the conversion of fibrinogen to fibrin by thrombin

Thrombin - yet another serine protease

In the blood clotting cascade, the active form of the serine protease thrombin [EC 3.4.21.5] is responsible for the conversion of fibrinogen to fibrin. A clot forms when fibrin monomers formed by the action of thrombin assemble into ordered fibrous arrays. Thrombin shows a specificity for cleavage of the peptide bond between Arg and Gly, suggesting a similarity to trypsin. Indeed, the B chain of thrombin shows sequence similarity to trypsin, and the X-ray structure of thrombin reveals all the hallmarks of a serine protease: the catalytic triad, oxyanion hole, and specificity pocket. The latter, like trypsin, has an aspartate residue at its bottom that interacts favorably with the Arg at P₁ of the substrate.

Thrombin arises from the processing of an inactive precursor, or zymogen, called prothrombin. Prothrombin is a 582-residue polypeptide whose first 274 residues constitute the large "pro" region. Processing (proteolytic cleavage) by Factor Xa (stimulated by Factor Va) corresponds to cleavage of the Arg274-Thr275 and Arg323-Ile324 peptide bonds. The pro region is released, while the two fragments of the mature polypeptide remain associated and are covalently joined by a disulfide bond.

The pro region of thrombin contains (near the N-terminus) a number of modified Glu residues that contain an extra carboxyl group. These residues, called γ-carboxyglutamate (three-letter abbreviation Gla), act as effective chelators of Ca²⁺ ions. This property of prothrombin is essential for the proper functioning of the clotting cascade. This is because the binding of calcium ions by prothrombin anchors it to phospholipid membranes derived from platelets following injury. This properly localizes prothrombin in proximity to its activating factors.