The 99 and 170 Loop-modified Factor VIIa Mutants Show Enhanced Catalytic Activity without Tissue Factor

To elucidate the functions of the surface loops of VIIa, we prepared two mutants, VII-30 and VII-39. The VII-30 mutant had all of the residues in the 99 loop replaced with those of trypsin. In the VII-39 mutant, both the 99 and 170 loops were replaced with those of trypsin. The k cat / K m value for hydrolysis of the chromogenic peptidyl substrate S-2288 by VIIa-30 (103 m m − 1 s − 1 ) was 3-fold higher than that of wild-type VIIa (30.3 m m − 1 s − 1 ) in the presence of soluble tissue factor (sTF). This enhancement was due to a decrease in the K m value but not to an increase in the k cat value. On the other hand, the k cat / K m value for S-2288 hydrolysis by VIIa-39 (17.9 m m − 1 s − 1 ) was 18-fold higher than that of wild-type (1.0 m m − 1 s − 1 ) in the absence of sTF, and the value was almost the same as that of wild-type measured in the presence of sTF. This enhancement was due to not only a decrease in the K m value but also to an increase in the k cat value. These results were in good agreement with their susceptibilities to a subsite 1-directed serine protease inhibitor. In our previous paper (Soejima, K., Mizuguchi, J., Yuguchi, M., Nakagaki, T., Higashi, S., and Iwanaga, S. (2001) J. Biol. Chem. 276, 17229–17235), the replacement of the 170 loop of VIIa with that of trypsin induced a 10-fold enhancement of the k cat value for S-2288 hydrolysis as compared with that of wild-type VIIa in the absence of sTF. These results suggested that the 99 and the 170 loop structures of VIIa independently affect the K m and k cat values, respectively. Furthermore, we studied the effect of mutations on proteolytic activity toward S -alkylated lysozyme as a macromolecular substrate and the activation of natural macromolecular substrate factor X.