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Protein S and C are both molecules that have biological functions in the body. In a situation where a wound is present, for example, and the blood flow needs to be stopped, the body requires both Protein S and C to help control the extent of the clotting. Protein C needs Protein S to combine with in order to play its part in the clotting process. When people are deficient in one or both of the proteins, they have an increased risk of blood clots.
The process of blood clotting is complex and involves more than 20 different proteins. Each of these proteins interacts with another and forms only part of a cascade. The term cascade refers to a biological situation where one molecule affects another, then this interaction causes another substance to alter and so on in a waterfall effect, where the primary molecule sets lots of other reactions off to achieve a specific biological goal. In the case of blood clots, this cascade ends in a clumping together of blood cells to form a plug for a wound.
Each cascade needs certain molecules to drive the reaction and achieve the desired end result. If the cascade did not have regulation, however, the end result, such as blood clots, would be abnormally strong and dangerous to health. Therefore, every cascade is controlled at various points by other biological substances, so the clot is appropriately sized and the clotting stops when the job is done. Protein S and C are regulatory agents that perform this job.
Carried in the blood flow, Protein C moves around as an inactive molecule. Protein S also moves around in the blood and is uninterested in inactive Protein C. It is only when Protein C is activated that Protein S can combine with it.
Activation of Protein C happens when the levels of a substance known as the active form of thrombomodulin rises and acts on Protein C. Concentrations of activated thrombulin rise only when blood clotting is in progress. This is because thrombomodulin is part of the cascade of clotting and is turned on by yet another substance, called thrombin.
Fragments of cells known as platelets form the basic cell structure of a clot. These platelets are the locations where Protein S and C bind together. Protein C needs Protein S to bind in order to perform the necessary regulation.
This complex of the two proteins, sitting on the surface of the platelet, breaks down yet more proteins in the cascade. Factor Va and Factor VIIIa are molecules that are pro-clotting. They activate another molecule, which activates another molecule, which turns into thrombin. This is where the cascade returns to Protein S and C as thrombin is the substance that activates thrombomodulin, which in turn activates Protein C.
Regulation of the clotting process can therefore occur. As the level of activated Protein C is influenced by the level of thrombomodulin, it is indirectly influenced by the levels of all of the pro-blood clotting molecules. In a healthy person, this constant loop of influence keeps clotting to a reasonable and useful level and prevents blood clots from appearing in unwanted areas. When a person suffers from Protein C or Protein S deficiency, this regulation is disrupted, and dangerous blood clots can form in the circulatory system.