Carboxylase processivity and disruption in disease. (A) Processivity depends upon the relative rates of catalysis and release. Wild-type (WT) carboxylase completed carboxylation of the FIX Gla domain at a rate fivefold faster than that of the overall reaction. The rates of catalysis and the overall reaction were similar with the V255M mutant, which generated partially carboxylated FIX. (B) WT carboxylase shields a VKD protein undergoing carboxylation from external VKD proteins, which allows full carboxylation. Shielding implicates a transition after Glu carboxylation to an open conformation that allows exit of the carboxylated VKD protein and entrance of an uncarboxylated substrate. A conformational change in the V255M mutant may disrupt this normal process to result in partially carboxylated VKD proteins. Alternatively, weakened interaction between the V255M mutant and VKD proteins could lead to premature release, accounting for partial carboxylation.