Figure 53-7 The major coagulation reactions are subdivided and depicted in schematic form to emphasize their similarity. They all rely on the formation of surface-bound enzyme-cofactor complexes. Abbreviations: PK, prekallikrein; K, kallikrein; HMWK, high-molecular-weight kininogen; TF, tissue factor; Ca2+, calcium; PT, prothrombin; Thr, thrombin. By convention, other coagulation factors are indicated by roman numerals, with a lowercase “a” appended to indicate their active form. The >>> is used to indicate the Gla (di-γ-carboxyglutamic acid)–containing domains of factors VII, IX, X, Xa, and PT, which bind calcium and phospholipid. Hatching is used to indicate proteins that adhere to surfaces by hydrophobic interaction.