A simple and rapid method to study the association of the contact proteins of blood coagulation

Structure/function analysis of human factor XII using recombinant deletion mutants. Evidence for an additional region involved in the binding to negatively charged surfaces

The binding site of human factor XII (FXII) for negatively charged surfaces has been proposed to be localized in the N-terminal region of factor XII. We have generated two recombinant factor XII proteins that lack this region: one protein consisting of the second growth-factor-like domain, the kringle domain, the proline-rich region and the catalytic domain of FXII (rFXII-U-like), and another consisting of only 16 amino acids of the proline-rich region of the heavy-chain region and the catalytic domain (rFXII-1pc). Each recombinant truncated protein, as well as recombinant full-length FXII (rFXII), were produced in HepG2 cells and purified by immunoaffinity chromatography. The capability of these recombinant proteins to bind to negatively charged surfaces and to initiate contact activation was studied. Radiolabeled rFXII-U-like and, to a lesser extent, rFXII-lpc bound to glass in a concentration-dependent manner, yet with lower efficiency than rFXII. The binding of the recombinant proteins was inhibited by a 100-fold molar excess of non-labeled native factor XII. On native polyacrylamide gel electrophoresis, both truncated proteins appeared to bind also to dextran sulfate, a soluble negatively charged compound. Glass-bound rFXII-U-like was able to activate prekallikrein in FXII-deficient plasma (assessed by measuring the generation of kallikrein-C1-inhibitor complexes), but less efficiently than rFXII, rFXII-U-like and rFXII-lpc exhibited coagulant activity, but this activity was significantly lower than that of rFXII. These data confirm that the N-terminal part of the heavy-chain region of factor XII contains a binding site for negatively charged activating surfaces, and indicate that other sequences, possibly located on the second epidermal-growth-factor-like domain and/or the kringle domain, contribute to the binding of factor XII to these surfaces.

The low pH stability of human coagulation factor XII (Hageman factor) is due to reversible conformational transitions

Factor XII undergoes autoactivation when bound to negatively charged surfaces. To gain insight into the mechanism of factor XII autoactivation and stability at low pH, structural studies in the presence and absence of a soluble surface, dextran sulfate, at pH 5.3 and pH 8.3 were carried out. The circular dichroism data indicate that the secondary structure at pH 5.3 is only modestly different from that at pH 8.3. However, large changes in the secondary structure are found to occur when factor XII is exposed to pH 5.3 in the presence of surface. Changes in tertiary structure at low pH are also evident from the difference in tryptophan fluorescence and chemical reactivity of the histidine residues. Factor XII binds to the surface even at pH 5.3 though it is inactive at this pH. It is concluded that factor XII adopts a different conformation at pH 5.3 and causes it to interact differently with dextran sulfate. This results in an obstructed cleavage site that accounts for its stability at low pH.