Enhanced γ-carboxylation of recombinant factor X using a chimeric construct containing the prothrombin propeptide

Factor Xa is the serine protease component of prothrombinase, the enzymatic complex responsible for thrombin generation. Production of recombinant factor X/Xa has proven to be difficult because of inefficient gamma -carboxylation, a critical post-translational modification. The affinities of the vitamin K-dependent propeptides for the gamma -carboxylase vary over 2 logs, with the propeptide of factor X having the highest affinity followed by the propeptides of factor VII, protein S, factor LX, protein C, and prothrombin [Stanley, T. B. (1999) J. Biol. Chem. 274, 16940-16944], On the basis of this observation, it was hypothesized that exchanging the propeptide of factor X with one that binds the gamma -carboxylase with a reduced affinity would enhance gamma -carboxylation by allowing greater substrate turnover, A chimeric cDNA consisting of the human prothrombin signal sequence and propeptide followed by mature human factor:X was generated and stably transfected into HEK 293 cells, and modified factor X was purified from conditioned medium, The results indicate that on average 85% of the total factor X produced with the prothrombin propeptide was fully gamma -carboxylated, representing a substantial improvement over a system that employs the native factor X propeptide, with which on average only 32% of the protein is fully gamma -carboxylated. These results indicate that the affinity of the gamma -carboxylase for the propeptide greatly influences the extent of gamma -carboxylation. It was also observed that regardless of which propeptide sequence is directing gamma -carboxylation (factor X or prothrombin), two pools of factor X are secreted; one is uncarboxylated and a second is fully gamma -carboxylated, supporting the notion that the gamma -carboxylase is a processive enzyme.