Regulation of Pathways

Initiation of the Extrinsic Blood Clotting Cascade

Similarities between Pathway Regulation and the Blood Coagulation Cascade

The extrinsic blood clotting is the system that begins in response to tissue damage. It is important to remember here that there are a variety of factors in the blood serum that are always present. Most of the factors are proteolytic enzymes that are synthesized as a single inactive polypeptide. They are "activated" by a limited number of amide bond cleavages within this polypeptide. A small change in their structural characteristics (from POOR to GOOD) then makes the active site more accessible to substrate or alters the structure of the active site such that it is now an efficient catalyst. The small structural change in the protein to make a better catalyst is just like that described on the previous gazillion pages for the glycolysis and glycogen pathways.

What is different is that for the blood coagulation cascade the change in protein structure is not very reverisble. The termodynamics of hydrolysis of an amide bond to make an organic acid and an amine heavily favor the acid/amine products. To regenerate an amide requires ATP. However, the amount of ATP in blood serun available for amide bond synthesis is ZERO.

Since each "factor" is a proteolytic protein it is not hard to envision the cascade itself. Once the first enzyme of the pathway is activated it can now act on the next one in the sequence, by hydrolyzing one or more amide bond(s) which activates that one etc. etc....... What's missing is what begins the cleavage of the first one?! Here we have to bring in a different kind of protein.

Initialization of the Cascade by Thromboplastin

Thromboplastin is a protein that is ubiquitously present in organelles of most cell types (except for some of those in the blood such as platelets, red blood cells). Thromboplastin can be found in vessel wall cells, skin cells, brain, kidney etc. It turns out that Thromboplastin can form a specific and special kind of noncovalent complex with factor VII. This complex infers a small amount of activity to factor VII even without a proteolytic cleavage event.

The initiation of the clotting events are prevented pretty much by keeping the initiator (Thromboplastin) physically away from the clotting factors by making sure that they are not in the same medium until some tissue damage has occurred. Upon tissues damage the cells which contain Thromboplastin release some of their contents thus mixing the factors with the initiator.

This initiation then is very much like allosteric regulation. Binding of some compound (A protein called Thromboplastin in this case) stabilizes the efficient catalyic structural form of factor VII. This allows it to become a good catalyst, but only so long as there is thromboplastin binding to it.

Keeping the two proteins apart - in separate containers = until you need them seems like the old style epoxy glues. There are two tubes of stuff: the resin and the hardner. Both can be stored indefinitely until they are mixed. Once mixing occurs, there is a limited amount of time before the epoxy glue becomes hard. In the clotting system fibrinogen plus the blood born factors are the resin and the hardner is Thromboplastin.

I presented a simplified version above requiring just thromboplastin and factor VII. It is not quite that simple. The single isolated peptide of Thromboplastin actually does NOT have activity toward initiating the clotting process. It turns out that thromboplastin is not just a protein, but a protein with significant amount of phospholipids (component lipids of membrane). IF Thromboplastin AND these phospholipids are added to factor VII THEN it is very instrumental in initiating the cascade. The phospholipids form membrane like vessicles that form a structure on which the Thromboplastin - factor VII complex can associate together. Then in the Thromboplastin - factorVII -phospholipids complex, factor VII has some proteolytic activity. It can now begin the process of hydrolyzing an amide bond in factor X and in factor IX the next proteins in the cascade.