Tethered Processivity of the Vitamin K-Dependent Carboxylase:  Factor IX Is Efficiently Modified in a Mechanism Which Distinguishes Gla's from Glu's and Which Accounts for Comprehensive Carboxylation in Vivo

Abstract

The vitamin K-dependent (VKD) carboxylase binds VKD proteins via their propeptide and converts Glu's to gamma-carboxylated Glu's, or Gla's, in the Gla domain. Multiple carboxylation is required for activity, which could be achieved if the carboxylase is processive. In the only previous study to test for this capability, an indirect assay was used which suggested processivity; however, the efficiency was poor and raised questions regarding how full carboxylation is accomplished. To unequivocally determine if the carboxylase is processive and if it can account for comprehensive carboxylation in vivo, as well as to elucidate the enzyme mechanism, we developed a direct test for processivity. The in vitro carboxylation of a complex containing carboxylase and full-length factor IX (fIX) was challenged with an excess amount of a distinguishable fIX variant. Remarkably, carboxylation of fIX in the complex was completely unaffected by the challenge protein, and comprehensive carboxylation was achieved, showing conclusively that the carboxylase is processive and highly efficient. These studies also showed that carboxylation of individual fIX/carboxylase complexes was nonsynchronous and implicated a driving force for the reaction which requires the carboxylase to distinguish Glu's from Gla's. We found that the Gla domain is tightly associated with the carboxylase during carboxylation, blocking the access of a small peptide substrate (EEL). The studies describe the first analysis of preformed complexes, and the rate for full-length, native fIX in the complex was equivalent to that of the substrate EEL. Thus, intramolecular movement within the Gla domain to reposition new Glu's for catalysis is as rapid as diffusion-limited positioning of a small substrate, and the Gla domain is not sterically constrained by the rest of the fIX molecule during carboxylation. The rate of carboxylation of fIX in the preformed complex was 24-fold higher than for fIX modified by free carboxylase, which supports carboxylase processivity and which indicates that binding and/or release is the rate-limiting step in protein carboxylation. These data indicate a model of tethered processivity, in which the VKD proteins remain bound to the carboxylase throughout the reaction via their propeptide, while the Gla domain undergoes intramolecular movement to reposition new Glu's for catalysis to ultimately achieve comprehensive carboxylation.

Binding of the Factor IX γ-Carboxyglutamic Acid Domain to the Vitamin K-dependent γ-Glutamyl Carboxylase Active Site Induces an Allosteric Effect That May Ensure Processive Carboxylation and Regulate the Release of Carboxylated Product

“…This suggests that for the first 10 carboxylations, the effective concentration of Glu does not change because the observed rate remains the same, whereas the actual concentration of Glu decreases. In agreement with Stenina et al (9), this indicates the enzyme distinguishes between Glu and Gla and binds Glu preferentially, i.e. Gla does not compete with Glu for the active site.…”

“…In the only other study using concentrations that approximate single turnover, Stenina et al (9) reported that the rates of carboxylation for profactor IX and that of the small substrate EEL are similar. The k cat value they report for EEL was about f-FIXproGla41 (Fig.…”

“…Previous studies indicate that all or nearly all of the Glu residues to be carboxylated are modified during a single substrate binding event (9,10). The loss of as few as three carboxylations can markedly decrease the activities of vitamin K-dependent proteins (8).…”

“…A tethered model has been proposed to account for how carboxylase accomplishes full carboxylation of pro-factor IX (9,11). In this hypothesis, the primary interaction between carboxylase and its substrates is mediated by a propeptide sequence of 18 amino acids.…”

The Vitamin K Oxidoreductase Is a Multimer That Efficiently Reduces Vitamin K Epoxide to Hydroquinone to Allow Vitamin K-dependent Protein Carboxylation

“…These considerations raise the possibility that VKORC1 reduces KO to KH 2 by a processive mechanism. The carboxylase is processive, with VKD protein binding resulting in the conversion of multiple Glu to Gla (35). A processive VKORC1 mechanism that maintains high local concentrations of vitamin K is attractive, as vitamin K levels in the diet and in tissue are low.…”