Identify the calcium-dependent protein that mediates an increase in the affinity of platelet membrane glycoproteins for fibrinogen.

• A. Calmodulin
• B. Thrombin
• C. Protein kinase C
• D. Protein S

Answer: (C)

When platelets are activated, calcium levels rise inside the cell and activate protein kinase C, a calcium-dependent enzyme that also requires diacylglycerol. PKC triggers intracellular signaling that reshapes the platelet surface, shifting the GPIIb/IIIa receptor into a high-affinity form for fibrinogen. With this high-affinity state, fibrinogen can bridge neighboring platelets, promoting aggregation. Calmodulin is a calcium-binding protein that regulates other enzymes but does not directly mediate this increase in GPIIb/IIIa affinity. Thrombin is a potent platelet activator, but the specific calcium-dependent mediator responsible for enhancing receptor affinity is PKC. Protein S is unrelated to this platelet receptor activation and relates to anticoagulation in the plasma.