I explained how Factor V helped to speed up clotting in my last post. I compared it to pouring petrol on a fire. In this post, I will look at how Factor V is controlled.
Clotting is complex but quite precisely controlled. It only takes place when and where you need it. You want the bleeding to stop at the site of the injury when you cut your finger, but you don’t want clotting to spread to deeper tissues. What is needed is a short burst of activity to form the initial blood clot, which is then rapidly switched off to stop further clotting.
If we go back to our fire analogy, factor V is the petrol to make the burst of activity, but we immediately need a fire extinguisher to prevent the fire from spreading.
Protein C is a protein in the blood that helps to slow down clotting; it is the blood’s fire extinguisher. Protein C is often referred to as a natural anticoagulant. It works by inactivating factor V and factor VIII, the two petrol sources for the fire, and puts it out.
Protein C usually is inactive, but as soon as blood starts to clot, it is converted to an activated form (APC). Activated Protein C works like a pair of chemical scissors; it chops up factor V and factor VIII and stops them from working. The picture below shows the three places where APC cuts up Factor V.
The Factor V heavy chain comprises 709 amino acids linked together in a line. Activated protein C cuts up the molecule at three arginine amino acids at positions 306, 506 and 579. Once cut up, it no longer works, and clotting slows down. The cut at arginine 306 is the most important and only occurs at the injury site on the lining of the blood vessel. This is important because it means the fire extinguisher only works at the site of the clot.
In my next post, we will examine what is different about Factor V Leiden.
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