The role of the Gla domain in the activation of bovine coagulation factor X by the snake venom protein XCP

The activation by XCP of coagulation factor X and a factor X species lacking the Gla-domain was studied in the presence and absence of Ca 2+. Both proteins could be activated at low rates in the absence of Ca 2+. The activation of the unmodified factor X was stimulated by the addition of Ca 2+, whereas GD factor X activation was insensitive to Ca 2+. The stimulatory effect of Ca 2+ seen with the unmodified factor X correlated strongly with a calcium-dependent change in intrinsic protein fluorescence. This conformational change required the Gla-domain as the fluorescence emission of GD factor X was the same with or without Ca 2+. Fluorescence changes which accompanied activation were the same for both factor X and GD factor X. This suggests that the Gla-domain does not participate in the structural changes which accompany activation.