Figure 2. Pathways for the proteolytic conversion of prothrombin to thrombin.

The conversion of prothrombin to thrombin results from cleavages following Arg²⁷¹ and Arg³²⁰. Initial cleavage following Arg²⁷¹ yields the pathway on the left and produces the zymogen, prethrombin 2 (P2) and the propiece, fragment 1.2 (F12) as intermediates. P2 requires further processing at Arg³²⁰ to yield thrombin. The pathway on the right arises from initial cleavage following Arg³²⁰, which produces the proteinase meizothrombin (mIIa) as an intermediate. Further cleavage following Arg²⁷¹ is required to yield IIa and the propiece, F12. The Arg¹⁵⁵ site (Red Arrow) is susceptible to thrombin cleavage and separates the fragment 1 region from fragment 2 within F12.