Protein S forms a reversible complex with which molecule?

Protein S forms a reversible complex with which molecule?

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

Answer: (A)

Protein S acts as a cofactor for Activated Protein C to help inactivate factors Va and VIIIa, dampening coagulation. In circulation, it exists as free protein S and as a reversible complex with C4b-binding protein. This binding sequesters protein S and reduces those molecules available to assist Activated Protein C, while still allowing protein S to dissociate when needed. So the molecule that protein S forms a reversible complex with is C4b-binding protein. The other options don’t fit because protein S does not form a functional complex with Protein C; protein C and protein S work together but do not bind to form a complex, total protein S is just the amount of the protein itself, and Fc receptors relate to antibody interactions, not the protein S–C4BP interaction.

Protein S acts as a cofactor for Activated Protein C to help inactivate factors Va and VIIIa, dampening coagulation. In circulation, it exists as free protein S and as a reversible complex with C4b-binding protein. This binding sequesters protein S and reduces those molecules available to assist Activated Protein C, while still allowing protein S to dissociate when needed. So the molecule that protein S forms a reversible complex with is C4b-binding protein.

The other options don’t fit because protein S does not form a functional complex with Protein C; protein C and protein S work together but do not bind to form a complex, total protein S is just the amount of the protein itself, and Fc receptors relate to antibody interactions, not the protein S–C4BP interaction.