Amino acid sequence comparison of human thrombin and human PC. The amino acid sequence of thrombin (residues 328-622 of PT, pir:tbhu) was compared with that of PC (residues 43-461 of the PC precursor, pir:kxhu) using the Gap program. PC is a two-chain molecule, consisted of a light chain (residues 43-197) and a heavy chain (residues 200-461). The dots (.) are introduced to maximize the similarity between the two sequences. The heavy chains (beginning with I364 in thrombin and D200 in PC, respectively) and trypsin/thrombin-like serine protease domains (encompassing residues 364-613 in thrombin and 212-445 in PC, respectively) in both proteins are marked. The pipe characters (∣) are put between amino acids that are the same; colons (:) between amino acids whose comparison values are greater or equal to 0.5; and periods (.) between amino acids whose comparison values are greater or equal to 0.1. Four regions of the most homologous amino acid sequence are underlined.