The structural features of thrombin. A stereo view of the surface thrombin in the standard orientation reveals the main features of the protease. The surface is colored according to electrostatic potential, with red and blue representing negatively and positively charged patches, respectively. In this orientation, the active site is centered so that a peptide substrate will run from the left to the right, from its N- to C-termini. Here, the P4 to P2′ residues of antithrombin are depicted as found in the structure of the Michaelis complex, with the reactive center bond (P1-P1′) indicated by the yellow arrow. The active site cleft is flanked above by the 60-loop, and below by the γ-loop making the cleft unusually deep. The 2 major anion binding exosites (ABEI and ABEII) are indicated by the ovals. The adjacent Na⁺ binding site is also indicated.