Fibrin Meshwork Formation :Role of phosphatidylserine and calcium

The presence of the negatively charged phospholipid phosphatidylserine (PS) and positively charged calcium ions (Ca²⁺) accelerates the rate of some steps in the clotting cascade.
1. Phosphatidylserine: PS is located primarily on the intracellular (cytosolic) face of the plasma membrane. [Note: Flippases create the asymmetry .] Its exposure signals injury to the endothelial cells that line blood vessels. PS is also exposed on the surface of activated platelets.
2. Calcium ions: Ca²⁺ binds the negatively charged γ-carboxyglutamate (Gla) residues present in four of the serine proteases of clotting (FII, FVII, FIX, and FX), facilitating the binding of these proteins to exposed phospholipids (Fig. 1). The Gla residues are good chelators of Ca²⁺ because of their two adjacent negatively charged carboxylate groups (Fig. 2). [Note: The use of chelating agents such as sodium citrate to bind Ca2+ in blood-collecting tubes or bags prevents the blood from clotting.]

Figure 1:  Ca²⁺ facilitates the binding of γ-carboxyglutamate (Gla)-containing factors to membrane phospholipids. F = factor.

Figure 2:  Gla residue.