Replacement of a single residue changes the primary specificity of thrombin.

Background: Thrombin prefers substrates carrying Arg at the site of cleavage (P1) because of the presence of D189 in the primary specificity (S1) pocket but can also cleave substrates carrying Phe at P1. The structural basis of this property is unknown.

Objective: Solve the X-ray structure of thrombin bound to a ligand carrying Phe at P1 and investigate the effects of replacing D189.