Mechanism of action of thrombin on fibrinogen. Size of the A alpha fibrinogen-like peptide that contacts the active site of thrombin.

The following peptides were synthesized by classical methods in solution: Ac-Phe-Leu-Ala-Glu-Gly-Gly-Gly-Val-Arg-Gly-Pro-Arg-Val-Val-Glu-NHCH3 (F-4), Ac-Phe-Leu-Ala-Glu-Gly-Gly-Gly-Val-Arg-Gly-Pro-Arg-Val-NHCH3 (F-5), and Ac-Phe-Leu-Ala-Glu-Glv-Gly-Gly-Gly-Val-Arg-Gly-Pro-NHCH3 (F-6). The rates of hydrolysis of the Arg-Gly bond in these peptides by thrombin were measured, and the values of the specificity constant, kcat/KM, were all found to be approximately 2 X 10(-7) [(NIH unit/L)s]-1, similar to that for a peptide (F-3) having an additional Arg residue between Glu- and -NHCH3 of F-4. The difference between this value and that for the A alpha chain of bovine fibrinogen is attributed to slight conformational differences arising from long-range interactions present in fibrinogen but not in the synthetic peptides. In addition to the requirement for the Phe residue, demonstrated earlier, it is shown here that no residues on the C-terminal side of Pro are required for interaction between thrombin and fibrinogen. The active site of thrombin thus appears to interact with a peptide of the size of F-6, with the Phe residue possibly being in close spatial proximity to the Val-Arg-Gly moiety.