The structure of factor XII. Proteolysis at arginines 334, 343, and 353 (see arrows) results in activated factor XII (β-factor XIIa). The catalytic triad of factor XIIa consists of histidine₃₉₃ , aspartic acid₄₄₂ , and serine₅₄₄ . A circle with a shaded background represents the artificial surface binding domains on factor XII's heavy chain. A circle with horizontal lines represent two of factor XII's zinc binding domains. Adapted from Cool and MacGillivray.119