Mechanism of action of thrombin on fibrinogen. Direct evidence for the involvement of phenylalanine at position P9.

The following peptides were synthesized by classical methods in solution: Ac-Phe-Leu-Ala-Glu-Gly-Gly-Gly-Val-Arg-Gly-Pro-NHCH3 (F-6) and Ac-Leu-Ala-Glu-Gly-Gly-Gly-Val-Arg-Gly-Pro-NHCH3 (F-7). The rates of hydrolysis of the Arg-Gly bond in these peptides by thrombin were measured, and the rate for the Phe-containing peptide F-6 was found to be much larger than that for F-7. Previous work [van Nispen, J. W., Hageman, T. C., & Scheraga, H. A. (1977) Arch. Biochem. Biophys. 182, 227] has demonstrated the importance of Phe-Leu at positions P9-P8 of the A alpha chain of fibrinogen for the thrombin-fibrinogen interaction. This work demonstrates that the presence of Leu (P8) alone is insufficient to account for the enhanced hydrolysis rates and that the presence of Phe (P9) is essential for normal action of thrombin on the A alpha chain of fibrinogen.