Recombinant contact factors. (A) Schematic diagrams of contact factors showing noncatalytic (white boxes) and catalytic (shaded boxes) domains. Positions of active site serine residues are indicated by black bars. Sites of proteolysis during activation are indicated by arrows, with black arrows indicating sites of cleavage required for full protease activity. FXII is an 80-kDa polypeptide that may be cleaved at 3 locations. Cleavage after Arg353 converts FXII to αFXIIa. Cleavage of αFXIIa after Arg334 separates the noncatalytic and catalytic domains, forming βFXIIa. The importance of cleavage after Arg343 is not clear. The FXII noncatalytic domains are the fibronectin type 2 (F2), epidermal growth factor (EGF), fibronectin type 1 (F1), and kringle (K) domains, and a proline-rich region (PRR). PK is a 93-kDa polypeptide that is cleaved after Arg371 to form α-kallikrein (α-Kal). A second cleavage after Arg140 produces β-kallikrein (β-Kal). FXI is a homodimer of 80-kDa polypeptides. It is converted to FXIa by cleavage after Arg369. The noncatalytic portions of PK and FXI contain 4 apple domains, designated A1 to A4. (B) Coomassie blue–stained nonreducing SDS-PAGE of purified FXII (left panel) and PK (right panel) containing ∼2-μg samples per lane. Positions of molecular mass standards in kilodaltons are shown to the left of the images.