A genome-wide CRISPR-Cas9 knockout screen identifies FSP1 as the warfarin-resistant vitamin K reductase.

Vitamin K is a vital micronutrient implicated in a variety of human diseases. Warfarin, a vitamin K antagonist, is the most commonly prescribed oral anticoagulant. Patients overdosed on warfarin can be rescued by administering high doses of vitamin K because of the existence of a warfarin-resistant vitamin K reductase. Despite the functional discovery of vitamin K reductase over eight decades ago, its identity remained elusive. Here, we report the identification of warfarin-resistant vitamin K reductase using a genome-wide CRISPR-Cas9 knockout screen with a vitamin K-dependent apoptotic reporter cell line. We find that ferroptosis suppressor protein 1 (FSP1), a ubiquinone oxidoreductase, is the enzyme responsible for vitamin K reduction in a warfarin-resistant manner, consistent with a recent discovery by Mishima et al. FSP1 inhibitor that inhibited ubiquinone reduction and thus triggered cancer cell ferroptosis, displays strong inhibition of vitamin K-dependent carboxylation. Intriguingly, dihydroorotate dehydrogenase, another ubiquinone-associated ferroptosis suppressor protein parallel to the function of FSP1, does not support vitamin K-dependent carboxylation. These findings provide new insights into selectively controlling the physiological and pathological processes involving electron transfers mediated by vitamin K and ubiquinone.

Naturally occurring UBIAD1 mutations differentially affect menaquinone biosynthesis and vitamin K-dependent carboxylation.

UbiA prenyltransferase domain-containing protein-1 (UBIAD1) is responsible for the biosynthesis of menaquinone-4 (MK-4), a cofactor for extrahepatic carboxylation of vitamin K-dependent (VKD) proteins. Genetic variations of UBIAD1 are mainly associated with Schnyder corneal dystrophy (SCD), a disease characterized by abnormal accumulation of cholesterol in the cornea. Results from in vitro studies demonstrate that SCD-associated UBIAD1 mutations are defective in MK-4 biosynthesis. However, SCD patients do not exhibit typical phenotypes associated with defects of MK-4 or VKD carboxylation. Here, we coupled UBIAD1's biosynthetic activity of MK-4 with VKD carboxylation in HEK293 cells that stably express a chimeric VKD reporter protein. The endogenous Ubiad1 gene in these cells was knocked out by CRISPR-Cas9-mediated genome editing. The effect of UBIAD1 mutations on MK-4 biosynthesis and VKD carboxylation was evaluated in Ubiad1-deficient reporter cells by determining the production of MK-4 or by measuring the efficiency of reporter-protein carboxylation. Our results show that the hot-spot mutation N102S has a moderate impact on MK-4 biosynthesis (retained ˜ 82% activity) but does not affect VKD carboxylation. However, the G186R mutation significantly affected both MK-4 biosynthesis and VKD carboxylation. Other mutations exhibit varying degrees of effects on MK-4 biosynthesis and VKD carboxylation. These results are consistent with in vivo results obtained from gene knock-in mice and SCD patients. Our findings suggest that UBIAD1's MK-4 biosynthetic activity does not directly correlate with the phenotypes of SCD patients. The established cell-based assays in this study provide a powerful tool for the functional studies of UBIAD1 in a cellular milieu.

The Function of extravascular coagulation factor IX in haemostasis.

INTRODUCTION: The majority of clotting factor IX (FIX) resides extravascularly, in the subendothelial basement membrane, where it is important for haemostasis. AIM: We summarize preclinical studies demonstrating extravascular FIX and its role in haemostasis and discuss clinical observations supporting this. We compare the in vivo binding of BeneFIX® and the extended half-life FIX, Alprolix® , to extravascular type IV collagen (Col4). METHODS: Three mouse models of haemophilia were used: the FIX knockout as the CRM- model and two knock-in mice, representing a CRM+ model of a commonly occurring patient mutation (FIXR333Q ) or a mutation that binds poorly to Col4 (FIXK5A ). The murine saphenous vein bleeding model was used to assess haemostatic competency. Clinical publications were reviewed for relevance to extravascular FIX. RESULTS: CRM status affects recovery and prophylactic efficacy. Prophylactic protection decreases ~5X faster in CRM+ animals. Extravascular haemostasis can explain unexpected breakthrough bleeding in patients treated with some EHL-FIX therapeutics. In mice, both Alprolix® and BeneFIX® bind Col4 with similar affinities (Kd~20-40 nM) and show dose-dependent recoveries. As expected, the concentration of binding sites in the mouse calculated for Alprolix® (574 nM) was greater than for BeneFIX® (405 nM), due to Alprolix® binding to both Col4 and the endothelial cell neonatal Fc receptor. CONCLUSION: Preclinical and clinical results support the interpretation that FIX plays a role in haemostasis from its extravascular location. We believe that knowing the CRM status of haemophilia B patients is important for optimizing prophylactic dosing with less trial and error, thereby decreasing clinical morbidity.

γ-Glutamyl carboxylase mutations differentially affect the biological function of vitamin K-dependent proteins.

γ-Glutamyl carboxylase (GGCX) is an integral membrane protein that catalyzes posttranslational carboxylation of a number of vitamin K-dependent (VKD) proteins involved in a wide variety of physiologic processes, including blood coagulation, vascular calcification, and bone metabolism. Naturally occurring GGCX mutations are associated with multiple distinct clinical phenotypes. However, the genotype-phenotype correlation of GGCX remains elusive. Here, we systematically examined the effect of all naturally occurring GGCX mutations on the carboxylation of 3 structure-function distinct VKD proteins in a cellular environment. GGCX mutations were transiently introduced into GGCX-deficient human embryonic kidney 293 cells stably expressing chimeric coagulation factor, matrix Gla protein (MGP), or osteocalcin as VKD reporter proteins, and then the carboxylation efficiency of these reporter proteins was evaluated. Our results show that GGCX mutations differentially affect the carboxylation of these reporter proteins and the efficiency of using vitamin K as a cofactor. Carboxylation of these reporter proteins by a C-terminal truncation mutation (R704X) implies that GGCX's C terminus plays a critical role in the binding of osteocalcin but not in the binding of coagulation factors and MGP. This has been confirmed by probing the protein-protein interaction between GGCX and its protein substrates in live cells using bimolecular fluorescence complementation and chemical cross-linking assays. Additionally, using a minigene splicing assay, we demonstrated that several GGCX missense mutations affect GGCX's pre-messenger RNA splicing rather than altering the corresponding amino acid residues. Results from this study interpreted the correlation of GGCX's genotype and its clinical phenotypes and clarified why vitamin K administration rectified bleeding disorders but not nonbleeding disorders.

A novel vitamin K derived anticoagulant tolerant to genetic variations of vitamin K epoxide reductase.

BACKGROUND: Vitamin K antagonists (VKAs), such as warfarin, have remained the cornerstone of oral anticoagulation therapy in the prevention and treatment of thromboembolism for more than half a century. They function by impairing the biosynthesis of vitamin K-dependent (VKD) clotting factors through the inhibition of vitamin K epoxide reductase (VKOR). The challenge of VKAs therapy is their narrow therapeutic index and highly variable dosing requirements, which are partially the result of genetic variations of VKOR. OBJECTIVES: The goal of this study was to search for an improved VKA that is tolerant to the genetic variations of its target enzyme. METHODS: A series of vitamin K derivatives with benzyl and related side-chain substitutions at the 3-position of 1,4-naphthoquinone were synthesized. The role of these compounds in VKD carboxylation was evaluated by mammalian cell-based assays and conventional in vitro activity assays. RESULTS: Our results showed that replacing the phytyl side-chain with a methylene cyclooctatetraene (COT) moiety at the 3-position of vitamin K1 converted it from a substrate to an inhibitor for VKD carboxylation. Strikingly, this COT-vitamin K derivative displayed a similar inhibition potency in warfarin-resistant VKOR mutations whose warfarin resistance varied more than 400-fold. Further characterization of COT-vitamin K for the inhibition of VKD carboxylation suggested that this compound targets multiple enzymes in the vitamin K redox cycle. Importantly, the anticoagulation effect of COT-vitamin K can be rescued with high doses of vitamin K1 . CONCLUSION: We discovered a vitamin K analogue that functions as a VKA and is tolerant to genetic variations in the target enzyme.

A cell-based high-throughput screen identifies drugs that cause bleeding disorders by off-targeting the vitamin K cycle.

Drug-induced bleeding disorders contribute to substantial morbidity and mortality. Antithrombotic agents that cause unintended bleeding of obvious cause are relatively easy to control. However, the mechanisms of most drug-induced bleeding disorders are poorly understood, which makes intervention more difficult. As most bleeding disorders are associated with the dysfunction of coagulation factors, we adapted our recently established cell-based assay to identify drugs that affect the biosynthesis of active vitamin K-dependent (VKD) coagulation factors with possible adverse off-target results. The National Institutes of Health (NIH) Clinical Collection (NCC) library containing 727 drugs was screened, and 9 drugs were identified, including the most commonly prescribed anticoagulant warfarin. Bleeding complications associated with most of these drugs have been clinically reported, but the pathogenic mechanisms remain unclear. Further characterization of the 9 top-hit drugs on the inhibition of VKD carboxylation suggests that warfarin, lansoprazole, and nitazoxanide mainly target vitamin K epoxide reductase (VKOR), whereas idebenone, clofazimine, and AM404 mainly target vitamin K reductase (VKR) in vitamin K redox cycling. The other 3 drugs mainly affect vitamin K availability within the cells. The molecular mechanisms underlying the inactivation of VKOR and VKR by these drugs are clarified. Results from both cell-based and animal model studies suggest that the anticoagulation effect of drugs that target VKOR, but not VKR, can be rescued by the administration of vitamin K. These findings provide insights into the prevention and management of drug-induced bleeding disorders. The established cell-based, high-throughput screening approach provides a powerful tool for identifying new vitamin K antagonists that function as anticoagulants.

Vitamin K-dependent carboxylation of coagulation factors: insights from a cell-based functional study.

Vitamin K-dependent carboxylation is a post-translational modification essential for the biological function of coagulation factors. Defects in carboxylation are mainly associated with bleeding disorders. With the discovery of new vitamin K-dependent proteins, the importance of carboxylation now encompasses vascular calcification, bone metabolism, and other important physiological processes. Our current knowledge of carboxylation, however, comes mainly from in vitro studies carried out under artificial conditions, which have a limited usefulness in understanding the carboxylation of vitamin K-dependent proteins in native conditions. Using a recently established mammalian cell-based assay, we studied the carboxylation of coagulation factors in a cellular environment. Our results show that the coagulation factor's propeptide controls substrate binding and product releasing during carboxylation, and the propeptide of factor IX appears to have the optimal affinity for efficient carboxylation. Additionally, non-conserved residues in the propeptide play an important role in carboxylation. A cell-based functional study of naturally occurring mutations in the propeptide successfully interpreted the clinical phenotype of warfarin's hypersensitivity during anticoagulation therapy in patients with these mutations. Unlike results obtained from in vitro studies, results from our cell-based study indicate that although the propeptide of osteocalcin cannot direct the carboxylation of the coagulation factor, it is required for the efficient carboxylation of osteocalcin. This suggests that the coagulation factors may have a different mechanism of carboxylation from osteocalcin. Together, results from this study provide insight into efficiently controlling one physiological process, such as coagulation without affecting the other, like bone metabolism.

Dysfunctional endogenous FIX impairs prophylaxis in a mouse hemophilia B model.

Factor IX (FIX) binds to collagen IV (Col4) in the subendothelial basement membrane. In hemophilia B, this FIX-Col4 interaction reduces the plasma recovery of infused FIX and plays a role in hemostasis. Studies examining the recovery of infused BeneFix (FIXWT) in null (cross-reactive material negative, CRM-) hemophilia B mice suggest the concentration of Col4 readily available for binding FIX is ∼405 nM with a 95% confidence interval of 374 to 436 nM. Thus, the vascular cache of FIX bound to Col4 is several-fold the FIX level measured in plasma. In a mouse model of prophylactic therapy (testing hemostasis by saphenous vein bleeding 7 days after infusion of 150 IU/kg FIX), FIXWT and the increased half-life FIXs Alprolix (FIXFC) and Idelvion (FIXAlb) produce comparable hemostatic results in CRM- mice. In bleeding CRM- hemophilia B mice, the times to first clot at a saphenous vein injury site after the infusions of the FIX agents are significantly different, at FIXWT < FIXFC < FIXAlb Dysfunctional forms of FIX, however, circulate in the majority of patients with hemophilia B (CRM+). In the mouse prophylactic therapy model, none of the FIX products improves hemostasis in CRM+ mice expressing a dysfunctional FIX, FIXR333Q, that nevertheless competes with infused FIX for Col4 binding and potentially other processes involving FIX. The results in this mouse model of CRM+ hemophilia B demonstrate that the endogenous expression of a dysfunctional FIX can deleteriously affect the hemostatic response to prophylactic therapy.

Evaluation of oral anticoagulants with vitamin K epoxide reductase in its native milieu.

Warfarin, acenocoumarol, phenprocoumon, and fluindione are commonly prescribed oral anticoagulants for the prevention and treatment of thromboembolic disorders. These anticoagulants function by impairing the biosynthesis of active vitamin K-dependent coagulation factors through the inhibition of vitamin K epoxide reductase (VKOR). Genetic variations in VKOR have been closely associated with the resistant phenotype of oral anticoagulation therapy. However, the relative efficacy of these anticoagulants, their mechanisms of action, and their resistance variations among naturally occurring VKOR mutations remain elusive. Here, we explored these questions using our recently established cell-based VKOR activity assay with the endogenous VKOR function ablated. Our results show that the efficacy of these anticoagulants on VKOR inactivation, from most to least, is: acenocoumarol > phenprocoumon > warfarin > fluindione. This is consistent with their effective clinical dosages for stable anticoagulation control. Cell-based functional studies of how each of the 27 naturally occurring VKOR mutations responds to these 4 oral anticoagulants indicate that phenprocoumon has the largest resistance variation (up to 199-fold), whereas the resistance of acenocoumarol varies the least (<14-fold). Cell-based kinetics studies show that fluindione appears to be a competitive inhibitor of VKOR, whereas warfarin is likely to be a mixed-type inhibitor of VKOR. The anticoagulation effect of these oral anticoagulants can be reversed by the administration of a high dose of vitamin K, apparently due to the existence of a different enzyme that can directly reduce vitamin K. These findings provide new insights into the selection of oral anticoagulants, their effective dosage management, and their mechanisms of anticoagulation.

Warfarin and vitamin K epoxide reductase: a molecular accounting for observed inhibition.

Vitamin K epoxide reductase (VKOR), an endoplasmic reticulum membrane protein, is the key enzyme for vitamin K-dependent carboxylation, a posttranslational modification that is essential for the biological functions of coagulation factors. VKOR is the target of the most widely prescribed oral anticoagulant, warfarin. However, the topological structure of VKOR and the mechanism of warfarin's inhibition of VKOR remain elusive. Additionally, it is not clear why warfarin-resistant VKOR mutations identified in patients significantly decrease warfarin's binding affinity, but have only a minor effect on vitamin K binding. Here, we used immunofluorescence confocal imaging of VKOR in live mammalian cells and PEGylation of VKOR's endogenous cytoplasmic-accessible cysteines in intact microsomes to probe the membrane topology of human VKOR. Our results show that the disputed loop sequence between the first and second transmembrane (TM) domain of VKOR is located in the cytoplasm, supporting a 3-TM topological structure of human VKOR. Using molecular dynamics (MD) simulations, a T-shaped stacking interaction between warfarin and tyrosine residue 139, within the proposed TY139A warfarin-binding motif, was observed. Furthermore, a reversible dynamic warfarin-binding pocket opening and conformational changes were observed when warfarin binds to VKOR. Several residues (Y25, A26, and Y139) were found essential for warfarin binding to VKOR by MD simulations, and these were confirmed by the functional study of VKOR and its mutants in their native milieu using a cell-based assay. Our findings provide new insights into the dynamics of the binding of warfarin to VKOR, as well as into warfarin's mechanism of anticoagulation.

Vitamin K epoxide reductase and its paralogous enzyme have different structures and functions.

Vitamin K epoxide reductase (VKOR) is an essential enzyme for vitamin K-dependent carboxylation, while the physiological function of its paralogous enzyme VKOR-like (VKORL) is yet unknown. Although these two enzymes share approximately 50% protein sequence homology, the membrane topology of VKOR is still in debate. Here, we explored the differences in the membrane topology and disulfide-linked oligomerization of these two enzymes. Results from mutating the critical amino acid residues in the disputed transmembrane (TM) regions revealed that the second TM domain in the proposed 4-TM model of VKOR does not function as an authentic TM helix; supporting VKOR is a 3-TM protein, which is different from VKORL. Additionally, altering the loop sequence between the two conserved cysteine residues of VKORL affects its activity, supporting the notion that the conserved loop cysteines of VKORL are involved in its active site regeneration. However, a similar mutation in VKOR does not affect its enzymatic activity. Finally, our results show that although both VKOR and VKORL form disulfide-linked oligomers, the cysteine residues involved in the oligomerization appear to be different. Overall, the structural and functional differences between VKOR and VKORL shown here indicate that VKORL might have a different physiological function other than recycling vitamin K.

Extravascular FIX and coagulation.

This review summarizes the evidence that collagen IV binding is physiologically important, and that the extravascular compartment of FIX is composed of type IV collagen. As we have previously demonstrated, 7 days post-infusion, FIXWT (BeneFIX) is able to control bleeding as well as the same dosage of Alprolix in hemophilia B mice, tested using the saphenous vein bleeding model (Alprolix is a chimeric FIX molecule joined at its C terminus to a Fc domain). Furthermore, we have shown that in hemophilia B mice, doses of BeneFIX or Alprolix (up to a dose of 150 IU/kg) have increased bleeding-control effectiveness in proportion to the dose up to a certain limit: higher doses are no more effective than the 150 IU/kg dose. These studies suggest that in hemophilia B mice, tested using the saphenous vein bleeding model, three things are true: first, extravascular FIX is at least as important for coagulation as is circulating FIX; second, measuring circulating levels of FIX may not be the best criterion for designing new "longer lasting" FIX molecules; and third, trough levels are less diagnostic for FIX therapy than they are for FVIII therapy.

Prophylactic efficacy of BeneFIX vs Alprolix in hemophilia B mice.

FIX binds tightly to collagen IV. Furthermore, a FIX mutant, FIXK5R, which binds better than wild-type FIX to collagen IV, provides better hemostasis than wild-type FIX, long after both are undetectable in the plasma. There is also credible evidence of extravascular FIX. Here, we use the saphenous vein bleeding model to compare the efficacy of recombinant FIXFc (Alprolix) and wild-type FIX (BeneFIX) in hemophilia B mice 7 days postinfusion. Although the terminal half-life of Alprolix is significantly longer than that of BeneFIX, at equal doses Alprolix is not better at controlling bleeding 7 days postinfusion, presumably because of the extravascular FIX. Both BeneFIX and Alprolix exhibit a linear response in clotting efficacy up to 150 IU/kg, where they appear to saturate an extravascular compartment, because there is no additional prophylactic benefit from higher doses. A robust pool of extravascular FIX is clearly observed surrounding blood vessels, localized to the same region as collagen IV, in 2 representative human tissues: liver and skeletal muscle. We see no increased risk for thrombosis at 250 IU/kg FIX at 6 hours postinfusion. In summary, 7 days postinfusion into hemophilia B mice, BeneFIX and Alprolix are hemostatically indistinguishable despite the latter's increased half-life. We predict that doses of FIX ∼3 times higher than the currently recommended 40 to 50 IU/kg will, because of FIX's large extravascular compartment, efficiently prolong prophylactic hemostasis without thrombotic risk.

Characterization of vitamin K-dependent carboxylase mutations that cause bleeding and nonbleeding disorders.

Vitamin K-dependent coagulation factors deficiency is a bleeding disorder mainly associated with mutations in γ-glutamyl carboxylase (GGCX) that often has fatal outcomes. Some patients with nonbleeding syndromes linked to GGCX mutations, however, show no coagulation abnormalities. The correlation between GGCX genotypes and their clinical phenotypes has been previously unknown. Here we report the identification and characterization of novel GGCX mutations in a patient with both severe cerebral bleeding disorder and comorbid Keutel syndrome, a nonbleeding malady caused by functional defects of matrix γ-carboxyglutamate protein (MGP). To characterize GGCX mutants in a cellular milieu, we established a cell-based assay by stably expressing 2 reporter proteins (a chimeric coagulation factor and MGP) in HEK293 cells. The endogenous GGCX gene in these cells was knocked out by CRISPR-Cas9-mediated genome editing. Our results show that, compared with wild-type GGCX, the patient's GGCX D153G mutant significantly decreased coagulation factor carboxylation and abolished MGP carboxylation at the physiological concentration of vitamin K. Higher vitamin K concentrations can restore up to 60% of coagulation factor carboxylation but do not ameliorate MGP carboxylation. These results are consistent with the clinical results obtained from the patient treated with vitamin K, suggesting that the D153G alteration in GGCX is the causative mutation for both the bleeding and nonbleeding disorders in our patient. These findings provide the first evidence of a GGCX mutation resulting in 2 distinct clinical phenotypes; the established cell-based assay provides a powerful tool for studying the clinical consequences of naturally occurring GGCX mutations in vivo.

Employing a gain-of-function factor IX variant R338L to advance the efficacy and safety of hemophilia B human gene therapy: preclinical evaluation supporting an ongoing adeno-associated virus clinical trial.

Vector capsid dose-dependent inflammation of transduced liver has limited the ability of adeno-associated virus (AAV) factor IX (FIX) gene therapy vectors to reliably convert severe to mild hemophilia B in human clinical trials. These trials also identified the need to understand AAV neutralizing antibodies and empty AAV capsids regarding their impact on clinical success. To address these safety concerns, we have used a scalable manufacturing process to produce GMP-grade AAV8 expressing the FIXR338L gain-of-function variant with minimal (<10%) empty capsid and have performed comprehensive dose-response, biodistribution, and safety evaluations in clinically relevant hemophilia models. The scAAV8.FIXR338L vector produced greater than 6-fold increased FIX specific activity compared with wild-type FIX and demonstrated linear dose responses from doses that produced 2-500% FIX activity, associated with dose-dependent hemostasis in a tail transection bleeding challenge. More importantly, using a bleeding model that closely mimics the clinical morbidity of hemophilic arthropathy, mice that received the scAAV8.FIXR338L vector developed minimal histopathological findings of synovitis after hemarthrosis, when compared with mice that received identical doses of wild-type FIX vector. Hemostatically normal mice (n=20) and hemophilic mice (n=88) developed no FIX antibodies after peripheral intravenous vector delivery. No CD8(+) T cell liver infiltrates were observed, despite the marked tropism of scAAV8.FIXR338L for the liver in a comprehensive biodistribution evaluation (n=60 animals). With respect to the role of empty capsids, we demonstrated that in vivo FIXR338L expression was not influenced by the presence of empty AAV particles, either in the presence or absence of various titers of AAV8-neutralizing antibodies. Necropsy of FIX(-/-) mice 8-10 months after vector delivery revealed no microvascular or macrovascular thrombosis in mice expressing FIXR338L (plasma FIX activity, 100-500%). These preclinical studies demonstrate a safety:efficacy profile supporting an ongoing phase 1/2 human clinical trial of the scAAV8.FIXR338L vector (designated BAX335).

Conserved loop cysteines of vitamin K epoxide reductase complex subunit 1-like 1 (VKORC1L1) are involved in its active site regeneration.

Vitamin K epoxide reductase complex subunit 1 (VKORC1) reduces vitamin K epoxide in the vitamin K cycle for post-translational modification of proteins that are involved in a variety of biological functions. However, the physiological function of VKORC1-like 1 (VKORC1L1), a paralogous enzyme sharing about 50% protein identity with VKORC1, is unknown. Here we determined the structural and functional differences of these two enzymes using fluorescence protease protection (FPP) assay and an in vivo cell-based activity assay. We show that in vivo VKORC1L1 reduces vitamin K epoxide to support vitamin K-dependent carboxylation as efficiently as does VKORC1. However, FPP assays show that unlike VKORC1, VKORC1L1 is a four-transmembrane domain protein with both its termini located in the cytoplasm. Moreover, the conserved loop cysteines, which are not required for VKORC1 activity, are essential for VKORC1L1's active site regeneration. Results from domain exchanges between VKORC1L1 and VKORC1 suggest that it is VKORC1L1's overall structure that uniquely allows for active site regeneration by the conserved loop cysteines. Intermediate disulfide trapping results confirmed an intra-molecular electron transfer pathway for VKORC1L1's active site reduction. Our results allow us to propose a concerted action of the four conserved cysteines of VKORC1L1 for active site regeneration; the second loop cysteine, Cys-58, attacks the active site disulfide, forming an intermediate disulfide with Cys-139; the first loop cysteine, Cys-50, attacks the intermediate disulfide resulting in active site reduction. The different membrane topologies and reaction mechanisms between VKORC1L1 and VKORC1 suggest that these two proteins might have different physiological functions.

A conformational investigation of propeptide binding to the integral membrane protein γ-glutamyl carboxylase using nanodisc hydrogen exchange mass spectrometry.

Gamma (γ)-glutamyl carboxylase (GGCX) is an integral membrane protein responsible for the post-translational catalytic conversion of select glutamic acid (Glu) residues to γ-carboxy glutamic acid (Gla) in vitamin K-dependent (VKD) proteins. Understanding the mechanism of carboxylation and the role of GGCX in the vitamin K cycle is of biological interest in the development of therapeutics for blood coagulation disorders. Historically, biophysical investigations and structural characterizations of GGCX have been limited due to complexities involving the availability of an appropriate model membrane system. In previous work, a hydrogen exchange mass spectrometry (HX MS) platform was developed to study the structural configuration of GGCX in a near-native nanodisc phospholipid environment. Here we have applied the nanodisc-HX MS approach to characterize specific domains of GGCX that exhibit structural rearrangements upon binding the high-affinity consensus propeptide (pCon; AVFLSREQANQVLQRRRR). pCon binding was shown to be specific for monomeric GGCX-nanodiscs and promoted enhanced structural stability to the nanodisc-integrated complex while maintaining catalytic activity in the presence of carboxylation co-substrates. Noteworthy modifications in HX of GGCX were prominently observed in GGCX peptides 491-507 and 395-401 upon pCon association, consistent with regions previously identified as sites for propeptide and glutamate binding. Several additional protein regions exhibited minor gains in solvent protection upon propeptide incorporation, providing evidence for a structural reorientation of the GGCX complex in association with VKD carboxylation. The results herein demonstrate that nanodisc-HX MS can be utilized to study molecular interactions of membrane-bound enzymes in the absence of a complete three-dimensional structure and to map dynamic rearrangements induced upon ligand binding.

FVIIa as used pharmacologically is not TF dependent in hemophilia B mice.

Activated factor VII is approved for treating hemophilia patients with autoantibodies to their factor IX or FVIII; however, its mechanism of action remains controversial. Some studies suggest that FVIIa requires tissue factor (TF) for function and that the reason for the high dose requirement is that it must compete with endogenous FVII for tissue factor. Others suggest that FVIIa binds platelets where it activates FX directly; the high concentration required would result from FVIIa's weak affinity for phospholipids. We address this question by infusing a chimera of mouse FIX (Gla and EGF1) with FVIIa (EGF2 and catalytic domain) into hemophilia B mice. This mutant has no TF-dependent activity because it cannot functionally bind TF at physiologically relevant concentrations. In vivo, this mutant is as effective as mouse FVIIa in controlling bleeding in hemophilia B mice. Our results suggest that the hemostatic effect of pharmacologic doses of FVIIa is TF independent.