Protein S forms a reversible complex with which molecule (alternative wording)?

Protein S forms a reversible complex with which molecule (alternative wording)?

• A. C4b binding protein
• B. Protein C
• C. Fc receptors
• D. Total protein S

Answer: (A)

Protein S acts as a cofactor for activated protein C, helping to inactivate factors Va and VIIIa in the anticoagulant pathway. In plasma, most of Protein S is bound to C4b-binding protein, forming a complex that is strong but reversible. Only the free fraction of Protein S can act as a cofactor for APC, so the binding to C4BP effectively creates a reservoir of Protein S. Therefore, the molecule that forms a reversible complex with Protein S is C4b-binding protein. The other options don’t fit: Protein C is the enzyme that uses Protein S as a cofactor, Fc receptors are unrelated to this interaction, and total Protein S refers to the combined free and bound forms rather than the binding partner.

Protein S acts as a cofactor for activated protein C, helping to inactivate factors Va and VIIIa in the anticoagulant pathway. In plasma, most of Protein S is bound to C4b-binding protein, forming a complex that is strong but reversible. Only the free fraction of Protein S can act as a cofactor for APC, so the binding to C4BP effectively creates a reservoir of Protein S. Therefore, the molecule that forms a reversible complex with Protein S is C4b-binding protein. The other options don’t fit: Protein C is the enzyme that uses Protein S as a cofactor, Fc receptors are unrelated to this interaction, and total Protein S refers to the combined free and bound forms rather than the binding partner.