Blood Clotting: The Basics

 
When blood vessels are damaged the leak must be sealed as rapidly as possible to prevent excess blood loss. The first response is formation of a blood clot at the site of damage. The clot is made up of cross-linked fibers made from a protein called fibrin.

The fibrin network is formed from a precursor of fibrin called fibrinogen [hear it]. Fibrinogen is a large protein that circulates freely in the blood stream. The key to understanding the mechanism of blood clot formation is in understanding how fibrinogen is converted to fibrin and why this only occurs at the site of damage to the lining of the blood vessel.

The activation mechanism is very complicated and highly regulated. The disruption of a blood clot when it is no longer needed is also complicated and highly regulated.

We'll start by looking at the basics of clot formation and dissolution.

Fibrinogen is composed of three different polypeptide chains or subunits. Each one is present in two copies (α₂β₂γ₂). The α, β, and γ chains wrap around each other to form a coiled coil triple helix. Two of these coiled coil complexes are joined head-to-head at the N-terminal ends of the polypeptides to make the complete molecule.


The complete fibrinogen molecule, which is very large as far as proteins go, consists of two domains. The central region where the N-terminal ends (N) are located forms the E domain. The outside ends where the C-terminal (C) ends are found are called the D domains.

Fibrinogen is soluble in blood plasma and the molecules show very little tendency to aggregate to form blood clots. Aggregation is prevented in large part by the N-terminal tails of the α (red) and β (blue) subunits projecting out of the central E domain. Blood clotting is initiated when these tails are chopped off by a specific protein-cutting enzyme (protease) called thrombin. Thrombin converts fibrinogen to fibrin and fibrin spontaneously aggregates to form a clot.

The activation takes place in two stages. In the first stage thrombin cleaves the α subunit releasing fibrinogen peptide A (FpA) and creating fibrin. The resulting fibrins can interact through their E domains to form filaments.

In the slower second step, the β subunit is cleaved releasing fibrinogen peptide B (FpB) and this permits aggregation of filaments to form complex networks. The resulting clot is called a soft clot. It is converted to a hard clot by Factor XIIIa (the "a" stands for "activated"). FXIIIa catalyzes the formation of covalent cross-links between fibrin molecules. The activated cross-link enzyme (FXIIIa) is formed from an inactive precursor (FXIII) by the action of thrombin. Thrombin not only cleaves fibrinogen, it also cleaves a number of clotting factors, like FXIII, to create active forms.

The initiation of clotting depends on thrombin activity. Thrombin is formed by proteolytic cleavage of inactive prothrombin to create the active protease (thrombin). This activation of prothrombin takes place at the site of injury and it's the way clotting is regulated. We'll cover it later on. You're probably getting the idea—blood clotting is controlled and regulated by a cascade of protein cleavages.

Once a clot is formed it eventually has to be dissolved once the injury is healed. This step is called fibrinolysis. The enzyme that dissolves clots is called plasmin. It chops aggregated fibrin fibers in the coiled coil region thus breaking up the clot. Can you guess how active plasmin is formed?

That's right. It's formed from a precursor called plasminogen by proteolytic cleavage. The enzyme that activates plasminogen is called tissue plasminogen activator (TPA). Plasminogen has a high affinity for fibrin clots but not for free fibrinogen. TPA also binds to fibrin and it only cleaves plasminogen when a complex of fibrin clot+plasminogen+TPA forms. The scheme on the right is a summary of what we've covered so far.

You may have heard of TPA. It's an enzyme that's given to heart attack patients but it must be delivered as soon as possible in order to prevent death. Here's what The American Heart Association says about TPA.

We strongly urge people to seek medical attention as soon as possible if they believe they're having a stroke or heart attack. The sooner tPA or other appropriate treatment is begun, the better the chances for recovery.

Tissue plasminogen activator (tPA) is a thrombolytic agent (clot-busting drug). It's approved for use in certain patients having a heart attack or stroke. The drug can dissolve blood clots, which cause most heart attacks and strokes.

Studies have shown that tPA and other clot-dissolving agents can reduce the amount of damage to the heart muscle and save lives. However, to be effective, they must be given within a few hours after symptoms begin. Administering tPA or other clot-dissolving agents is complex and is done through an intravenous (IV) line in the arm by hospital personnel.

There are very few textbooks that do a good job of summarizing and simplifying blood clotting. One of the better ones is Textbook of Biochemistry with Clinical Correlations 6th ed. edited by Thomas H. Devlin. This is an excellent book for those interested in biochemistry with a medical slant. I recommend it very highly as a reference text. It ain't cheap.

Devlin, T.H. (ed.) (2006) Textbook of Biochemistry with Clinical Correlations 6th ed., Wiley-Liss, Hoboken, N.J. (USA)

Wolberg, A.S. (2007) Thrombin generation and fibrin clot structure. Blood Reviews Jan. 5 2007. [PubMed]