Active site properties of thrombin and APC. (A) Electrostatic surface representations of human thrombin (1PPB⁴² ) and human APC (from 1AUT⁴⁴ ), with the small molecule inhibitor PPACK bound into the active site cleft (sticks). (B) Zoomed-in view of the active sites of human thrombin (3B9F³⁴ ) and APC (from 1AUT), with modeled KRK sequence corresponding to the P2, P1, and P1′ positions found in the crystal structure of the PCI-thrombin complex.³⁴