Binding site for C4b-binding protein in vitamin K-dependent protein S fully contained in carboxy-terminal laminin-G-type repeats. A study using recombinant factor IX-protein S chimeras and surface plasmon resonance.

The interaction between vitamin K-dependent protein S and the C4b-binding protein (C4BP) was studied using surface plasmon resonance and genetic engineering. The affinity, as well as association and dissociation rates of the complex, was measured for human and bovine protein S at five different calcium concentrations. The binding to C4BP of six protein hybrids containing different parts of coagulation factor IX and protein S was studied in the absence and presence of calcium. The results show that dissociation of the human protein S−C4BP complex is extremely slow in the presence of ≥10 μM calcium (koff = 7 × 10-6 s-1) and the association rate constant is kon = 7 × 4 M-1 s-1. Human and bovine protein S were found to bind to human C4BP with the same affinity, KD = 0.1 nM, but the rates of association and dissociation were higher for the bovine protein S (kon = 2 × 5 M-1 s-1, koff = 2 × 10-5 s-1). In the absence of calcium, the affinity for C4BP was reduced by a factor of 65 for human protein S and by a fact...