Structural and functional studies of γ-carboxyglutamic acid domains of factor VIIa and activated Protein C: role of magnesium at physiological calcium

Identification of a broad lipid repertoire associated to the endothelial cell protein C receptor (EPCR)

Evidence is mounting that the nature of the lipid bound to the endothelial cell protein C receptor (EPCR) has an impact on its biological roles, as observed in anticoagulation and more recently, in autoimmune disease. Phosphatidylethanolamine and phosphatidylcholine species dominate the EPCR lipid cargo, yet, the extent of diversity in the EPCR-associated lipid repertoire is still unknown and remains to be uncovered. We undertook mass spectrometry analyses to decipher the EPCR lipidome, and identified species not yet described as EPCR ligands, such as phosphatidylinositols and phosphatidylserines. Remarkably, we found further, more structurally divergent lipids classes, represented by ceramides and sphingomyelins, both in less abundant quantities. In support of our mass spectrometry results and previous studies, high-resolution crystal structures of EPCR in three different space groups point to a prevalent diacyl phospholipid moiety in EPCR’s pocket but a mobile and ambiguous lipid polar head group. In sum, these studies indicate that EPCR can associate with varied lipid classes, which might impact its properties in anticoagulation and the onset of autoimmune disease.

Antioxidant/anti-inflammatory effect of Mg2+ in coronavirus disease 2019 (COVID-19)

Severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19), characterised by high levels of inflammation and oxidative stress (OS). Oxidative stress induces oxidative damage to lipids, proteins, and DNA, causing tissue damage. Both inflammation and OS contribute to multi-organ failure in severe cases. Magnesium (Mg²⁺ ) regulates many processes, including antioxidant and anti-inflammatory responses, as well as the proper functioning of other micronutrients such as vitamin D. In addition, Mg²⁺ participates as a second signalling messenger in the activation of T cells. Therefore, Mg²⁺ deficiency can cause immunodeficiency, exaggerated acute inflammatory response, decreased antioxidant response, and OS. Supplementation with Mg²⁺ has an anti-inflammatory response by reducing the levels of nuclear factor kappa B (NF-κB), interleukin (IL) -6, and tumor necrosis factor alpha. Furthermore, Mg²⁺ supplementation improves mitochondrial function and increases the antioxidant glutathione (GSH) content, reducing OS. Therefore, Mg²⁺ supplementation is a potential way to reduce inflammation and OS, strengthening the immune system to manage COVID-19. This narrative review will address Mg²⁺ deficiency associated with a worse disease prognosis, Mg²⁺ supplementation as a potent antioxidant and anti-inflammatory therapy during and after COVID-19 disease, and suggest that randomised controlled trials are indicated.

Factors affecting the quality of therapeutic proteins in recombinant Chinese hamster ovary cell culture

Chinese hamster ovary (CHO) cells are the most widely used mammalian host cells for the commercial production of therapeutic proteins. Fed-batch culture is widely used to produce therapeutic proteins, including monoclonal antibodies, because of its operational simplicity and high product titer. Despite technical advances in the development of culture media and cell cultures, it is still challenging to maintain high productivity in fed-batch cultures while also ensuring good product quality. In this review, factors that affect the quality attributes of therapeutic proteins in recombinant CHO (rCHO) cell culture, such as glycosylation, charge variation, aggregation, and degradation, are summarized and categorized into three groups: culture environments, chemical additives, and host cell proteins accumulated in culture supernatants. Understanding the factors that influence the therapeutic protein quality in rCHO cell culture will facilitate the development of large-scale, high-yield fed-batch culture processes for the production of high-quality therapeutic proteins.

Blood Products, Crystalloids, and Rapid Infusion: An Experimental Study With Magnesium

OBJECTIVES

Calcium and magnesium are concentration-dependent pro- and anticoagulant cofactors, and magnesium behaves similarly to calcium in the presence of citrate. The authors hypothesized that magnesium can cause clot formation (primary objective) when mixed with coagulation factor-containing blood products diluted with different crystalloids in a rapid- infuser reservoir. A secondary objective was the observation of any infuser alarms and stops in the event of clotting.

DESIGN

An experimental in vitro study with blood products, crystalloids, magnesium, and calcium in a rapid infuser with a reservoir using a closed-loop system.

SETTING

Anesthesia research laboratory at an urban academic tertiary medical center PARTICIPANTS: Not applicable.

INTERVENTIONS

Exposure of fresh frozen plasma (FFP) and packed red blood cells alone (control) or in combination with either normal saline (NS), lactated Ringer's solution (LR), or Plasma-Lyte A (PL) to increasing concentrations of magnesium sulphate (MgSO₄) up to 1 g. After each incremental MgSO₄ change, the authors applied a specific pump-flow sequence in a closed-loop system with a rapid-infuser reservoir, and if no clot was observed, the authors incrementally added calcium chloride (CaCl₂) up to 1 g.

MEASUREMENTS AND MAIN RESULTS

Observation of macroscopic clot and time to event, as well as occurrence and type of any pump alarms or stops. LR experiments resulted in clot observation in the reservoir by a dedicated observer after MgSO₄ 275 ± 206 mg (95% confidence interval [CI], 9-541). Adding MgSO₄ 1 g in the NS, PL, or the control experiments did not result in clot observation. Only when CaCl₂ 166.7 ± 51.64 mg (95% CI, 112.0-22.01) was added to the combination of blood products alone or mixed with NS and PL, clotting occurred. The mean FFP volume was 281 ± 48.6 mL (range, 204-340 mL) and was not different between groups (p = 0.44). Pump alarms and stops were inconsistent.

CONCLUSIONS

The addition of magnesium to a combination of LR with coagulation factor- containing blood products consistently resulted in a visible blood clot in the rapid-infuser reservoir in the authors' experimental setup. In addition to MgSO₄ 1 g in the control, NS, and PL experiments, CaCl₂ is needed before a clot can be observed.

Structure of human factor VIIa-soluble tissue factor with calcium, magnesium and rubidium

Coagulation factor VIIa (FVIIa) consists of a γ-carboxyglutamic acid (GLA) domain, two epidermal growth factor-like (EGF) domains and a protease domain. FVIIa binds three Mg2+ ions and four Ca2+ ions in the GLA domain, one Ca2+ ion in the EGF1 domain and one Ca2+ ion in the protease domain. Further, FVIIa contains an Na+ site in the protease domain. Since Na+ and water share the same number of electrons, Na+ sites in proteins are difficult to distinguish from waters in X-ray structures. Here, to verify the Na+ site in FVIIa, the structure of the FVIIa-soluble tissue factor (TF) complex was solved at 1.8 Å resolution containing Mg2+, Ca2+ and Rb+ ions. In this structure, Rb+ replaced two Ca2+ sites in the GLA domain and occupied three non-metal sites in the protease domain. However, Rb+ was not detected at the expected Na+ site. In kinetic experiments, Na+ increased the amidolytic activity of FVIIa towards the synthetic substrate S-2288 (H-D-Ile-Pro-Arg-p-nitroanilide) by ∼20-fold; however, in the presence of Ca2+, Na+ had a negligible effect. Ca2+ increased the hydrolytic activity of FVIIa towards S-2288 by ∼60-fold in the absence of Na+ and by ∼82-fold in the presence of Na+. In molecular-dynamics simulations, Na+ stabilized the two Na+-binding loops (the 184-loop and 220-loop) and the TF-binding region spanning residues 163-180. Ca2+ stabilized the Ca2+-binding loop (the 70-loop) and Na+-binding loops but not the TF-binding region. Na+ and Ca2+ together stabilized both the Na+-binding and Ca2+-binding loops and the TF-binding region. Previously, Rb+ has been used to define the Na+ site in thrombin; however, it was unsuccessful in detecting the Na+ site in FVIIa. A conceivable explanation for this observation is provided.

The Role of Serum Calcium Level in Intracerebral Hemorrhage Hematoma Expansion: Is There Any?

Spontaneous intracerebral hemorrhage (ICH) is a devastating form of stroke, with a high rate of mortality and morbidity. Even with the best current medical or surgical interventions, outcomes remain poor. The location and initial hematoma volume are strong predictors of mortality. Hematoma expansion (HE) is a further marker of poor prognosis that may be at least partly preventable. Several risk factors for HE have been identified, including baseline ICH volume, anticoagulation, and computed tomography angiography spot signs. Recent studies have shown the correlation of serum calcium (Ca⁺⁺) levels on admission with HE. Low serum Ca⁺⁺ level has been associated with larger hematoma volume at the time of presentation, HE, and worse outcome. Although the causal and mechanistic links between low serum Ca⁺⁺ level and HE are not well understood, several mechanisms have been proposed including coagulopathy, platelet dysfunction, and higher blood pressure (BP) in the context of low serum Ca⁺⁺ level. However, low serum Ca⁺⁺ level might be only a biomarker of the adaptive response due to acute inflammatory response following acute ICH. The purpose of the current review is to discuss the evidence regarding the possible role of low serum Ca⁺⁺ level on HE in acute ICH.

Reduced Plasma Magnesium Levels in Type-1 Diabetes Associate with Prothrombotic Changes in Fibrin Clotting and Fibrinolysis

Individuals with type-1 diabetes mellitus (T1DM) have a higher risk of thrombosis and low plasma magnesium concentrations. As magnesium is a known regulator of fibrin network formation, we investigated potential associations between fibrin clot properties and plasma magnesium concentrations in 45 individuals with T1DM and 47 age- and sex-matched controls without diabetes. Fibrin clot characteristics were assessed using a validated turbidimetric assay and associations with plasma magnesium concentration were examined. Plasma concentrations of fibrinogen, plasminogen activator inhibitor-1 (PAI-1), and lipids were measured and fibrin fiber diameters assessed using scanning electron microscopy. Fibrin clot maximum absorbance was unchanged in subjects with T1DM compared with controls, while lysis time was prolonged (p = 0.0273). No differences in fibrin fiber diameters or in lipid profile were observed between T1DM and controls. PAI-1 concentration was lower in the T1DM group compared with the controls (p = 0.0232) and positively correlated with lysis time (p = 0.0023). Plasma magnesium concentration was lower in the T1DM group compared with controls (p < 0.0001). Magnesium concentration negatively correlated with clot maximum absorbance (p = 0.0215) and lysis time (p = 0.0464). A turbidimetric fibrin clot lysis assay performed in a purified system that included PAI-1 and 0 to 3.2 mM Mg2+ showed a shortening of lysis time with increasing Mg2+ concentrations (p = 0.0004). Our findings reveal that plasma magnesium concentration is associated with changes in fibrin clot and lysis parameters.

Coagulatory Defects in Type-1 and Type-2 Diabetes

Diabetes (both type-1 and type-2) affects millions of individuals worldwide. A major cause of death for individuals with diabetes is cardiovascular diseases, in part since both types of diabetes lead to physiological changes that affect haemostasis. Those changes include altered concentrations of coagulatory proteins, hyper-activation of platelets, changes in metal ion homeostasis, alterations in lipid metabolism (leading to lipotoxicity in the heart and atherosclerosis), the presence of pro-coagulatory microparticles and endothelial dysfunction. In this review, we explore the different mechanisms by which diabetes leads to an increased risk of developing coagulatory disorders and how this differs between type-1 and type-2 diabetes.

Hematoma Expansion Predictors: Laboratory and Radiological Risk Factors in Patients with Acute Intracerebral Hemorrhage: A Prospective Observational Study

BACKGROUND

Intracerebral hemorrhage (ICH) is considered a devastating neurologic emergency and carried a higher morbidity and mortality rates. Early hematoma expansion (HE) is considered one of the poor prognostic factors after ICH. Consequently, determination of the possible risk factors for HE could be effective in early detection of high-risk patients and hence directing management course aiming to improving ICH outcome.

METHODS

One-hundred and thirty-six spontaneous ICH patients were included and prospectively evaluated for the presence of HE. Demographic, laboratory, and certain radiological factors were studied and compared between those with HE and those without, the in-hospital mortality rates were assessed as well.

RESULTS

HE was observed in 30% of the studied cohort, those who developed HE had more neurologic impairment (Glasgow coma scale, median 9; National Institute of Health Stroke Scale, median 34), and higher in-hospital mortality rate (53.6%) than those without HE. HE was related to the presence of higher red blood cell distribution width (RDW), reduced total cholesterol, low-density lipoprotein-C (LDL-C), and Ca levels. Among the radiological factors, hematoma density (heterogeneous), and shape (irregular) are highly related to the occurrence of HE. The computed tomography angiography (CTA) spot sign among patients with ICH was associated with HE development.

CONCLUSIONS

Abnormal RDW; low cholesterol, LDL, and Ca level; heterogeneous density, irregular shape hemorrhage, and presence of CTA spot sign were associated with the development of HE in the setting of spontaneous ICH.

Sodium-site in serine protease domain of human coagulation factor IXa: evidence from the crystal structure and molecular dynamics simulations study

Essentials Consensus sequence and biochemical data suggest a Na+ -site in the factor (F) IXa protease domain. X-ray structure of the FIXa EGF2/protease domain at 1.37 Å reveals a Na+ -site not observed earlier. Molecular dynamics simulations data support that Na+  ± Ca2+ promote FIXa protease domain stability. Sulfate ions found in the protease domain mimic heparin sulfate binding mode in FIXa. SUMMARY: Background Activated coagulation factor IX (FIXa) consists of a γ-carboxyglutamic acid domain, two epidermal growth factor-like (EGF) domains, and a C-terminal protease domain. Consensus sequence and biochemical data support the existence of a Na+ -site in the FIXa protease domain. However, soaking experiments or crystals grown in high concentration of ammonium sulfate did not reveal a Na+ -site in wild-type or mutant FIXa EGF2/protease domain structure. Objective Determine the structure of the FIXa EGF2/protease domain in the presence of Na+ ; perform molecular dynamics (MD) simulations to explore the role of Na+ in stabilizing FIXa structure. Methods Crystallography, MD simulations, and modeling heparin binding to FIXa. Results Crystal structure at 1.37-Å resolution revealed that Na+ is coordinated to carbonyl groups of residues 184A, 185, 221A, and 224 in the FIXa protease domain. The Na+ -site in FIXa is similar to that of FXa and is linked to the Asp189 S1-site. In MD simulations, Na+ reduced fluctuations in residues 217-225 (Na+ -loop) and 70-80 (Ca2+ -loop), whereas Ca2+ reduced fluctuations only in residues of the Ca2+ -loop. Ca2+ and Na+ together reduced fluctuations in residues of the Ca2+ -loop and Na+ -loop (residues 70-80, 183-194, and 217-225). Moreover, we observed four sulfate ions that make salt bridges with FIXa protease domain Arg/Lys residues, which have been implicated in heparin binding. Based upon locations of the sulfate ions, we modeled heparin binding to FIXa, which is similar to the heparin binding in thrombin. Conclusions The FIXa Na+ -site in association with Ca2+ contributes to stabilization of the FIXa protease domain. The heparin binding mode in FIXa is similar to that in thrombin.

Phosphatidylcholine in the groove of endothelial cell protein C receptor (EPCR) regulates EPCR conformation and protein C recognition

Endothelial cell protein C receptor (EPCR), the cellular receptor for protein C (PC), facilitates PC activation through the thrombin/thrombomodulin complex and regulates thrombin generation. Under pathophysiological conditions like sepsis, the interactions between EPCR and PC become impaired. Previous studies have demonstrated that the EPCR contains a phospholipid in the antigen-binding groove that is responsible for the structural stability of the EPCR and for PC recognition. However, an understanding at the atomic level during ligand recognition is not fully developed. Molecular dynamics simulations along with potential of mean force (PMF) calculations were carried out in order to provide molecular insight into the dynamics and free energies of EPCR-PC in the absence/presence of phospholipid, namely lysophosphatidylcholine (lysoPCh) and phosphatidylcholine (PCh) in the antigen-binding groove of the EPCR. Our data reveal that the presence of lipid maintains the optimal conformation of the EPCR for PC binding. PMF data further suggest that the PCh system is the most stable in comparison with the other systems (lysoPCh and no lipid). With regards to the two hydrophobic tails of PCh, one lipid tail regulates EPCR conformation while the other promotes ligand recognition by interacting with the keel residue (Phe-4) of PC. Due to the lack of one hydrophobic tail for the lysoPCh system, the EPCR conformation is retained but the affinity of the EPCR towards the ligand (PC) is reduced. Our studies for the first time explore the possible mode of ligand recognition by the EPCR via the involvement of phosphatidylcholine within its hydrophobic groove. The present work provides insight into PCh-dependent ligand recognition and hence regulation of the protein C/EPCR complex formation.

Platelet-Activation Mechanisms and Vascular Remodeling

This overview article for the Comprehensive Physiology collection is focused on detailing platelets, how platelets respond to various stimuli, how platelets interact with their external biochemical environment, and the role of platelets in physiological and pathological processes. Specifically, we will discuss the four major functions of platelets: activation, adhesion, aggregation, and inflammation. We will extend this discussion to include various mechanisms that can induce these functional changes and a discussion of some of the salient receptors that are responsible for platelets interacting with their external environment. We will finish with a discussion of how platelets interact with their vascular environment, with a special focus on interactions with the extracellular matrix and endothelial cells, and finally how platelets can aid and possibly initiate the progression of various vascular diseases. Throughout this overview, we will highlight both the historical investigations into the role of platelets in health and disease as well as some of the more current work. Overall, the authors aim for the readers to gain an appreciation for the complexity of platelet functions and the multifaceted role of platelets in the vascular system. © 2017 American Physiological Society. Compr Physiol 8:1117-1156, 2018.

Lipid specificity of the membrane binding domain of coagulation factor X

Essentials Membrane-binding GLA domains of coagulation factors are essential for proper clot formation. Factor X (FX) is specific to phosphatidylserine (PS) lipids through unknown atomic-level interactions. Molecular dynamics simulations were used to develop the first membrane-bound model of FX-GLA. PS binding modes of FX-GLA were described, and potential PS-specific binding sites identified.

SUMMARY

Background Factor X (FX) binds to cell membranes in a highly phospholipid-dependent manner and, in complex with tissue factor and factor VIIa (FVIIa), initiates the clotting cascade. Experimental information concerning the membrane-bound structure of FX with atomic resolution has remained elusive because of the fluid nature of cellular membranes. FX is known to bind preferentially to phosphatidylserine (PS). Objectives To develop the first membrane-bound model of the FX-GLA domain to PS at atomic level, and to identify PS-specific binding sites of the FX-GLA domain. Methods Molecular dynamics (MD) simulations were performed to develop an atomic-level model for the FX-GLA domain bound to PS bilayers. We utilized a membrane representation with enhanced lipid mobility, termed the highly mobile membrane mimetic (HMMM), permitting spontaneous membrane binding and insertion by FX-GLA in multiple 100-ns simulations. In 14 independent simulations, FX-GLA bound spontaneously to the membrane. The resulting membrane-bound models were converted from HMMM to conventional membrane and simulated for an additional 100 ns. Results The final membrane-bound FX-GLA model allowed for detailed characterization of the orientation, insertion depth and lipid interactions of the domain, providing insight into the molecular basis of its PS specificity. All binding simulations converged to the same configuration despite differing initial orientations. Conclusions Analysis of interactions between residues in FX-GLA and lipid-charged groups allowed for potential PS-specific binding sites to be identified. This new structural and dynamic information provides an additional step towards a full understanding of the role of atomic-level lipid-protein interactions in regulating the critical and complex clotting cascade.

Exogenous Magnesium Chloride Reduces the Activated Partial Thromboplastin Times of Lupus Anticoagulant-Positive Patients

The activated partial thromboplastin time (APTT) assay is a basic hemostatic assay based on the time it takes for clots to form in plasma samples after the addition of calcium chloride. It is used to screen for various coagulation disorders. Several previous reports have suggested that magnesium (Mg) might contribute to coagulation reactions by binding to specific coagulation proteins. We investigated the effects of Mg on the APTT. In healthy controls, the APTT was significantly prolonged in proportion to the increase in the concentration of magnesium chloride in the range from 2.1 to 16.7 mmol/L. Among eight samples from patients with various disorders that exhibited prolonged APTT, two samples demonstrated shorter APTT when Mg was added, both of which were from patients that were positive for lupus anticoagulant. When we examined 206 clinical APTT samples, we found that Mg shortened the APTT of two samples. These two samples were also from lupus anticoagulant-positive patients (p-value: <0.003). Our findings regarding the unique effects of exogenous Mg on the APTT of lupus anticoagulant-positive patients might shed light on the role of Mg in APTT assays and lead to the development of a novel screening method for lupus anticoagulant.

In vitro induction and proteomics characterisation of a uranyl-protein interaction network in bovine serum

Uranyl ions (UO2(2+)) were shown to interact with a number of foetal serum proteins, leading to the formation of a complex that could be isolated by ultracentrifugation. The molecular weight of the complex was estimated based on size-exclusion chromatography as 650 000 Da. Online ICP AES detection indicated that UO2(2+) in the complex co-eluted with minor amounts of calcium and phosphorous, but not with magnesium. A 1D gel electrophoresis of the U-complex produced more than 10 bands of similar intensity compared with only 2-3 intense bands corresponding to the main serum proteins in the control serum, indicative of the specific interaction of UO2(2+) with minor proteins. A proteomics approach allowed for the identification of 74 proteins in the complex. Analysis of the protein-protein interaction network in the UO2(2+) complex identified 32 proteins responsible for protein-protein complex formation and 34 with demonstrated ion-binding function, suggesting that UO2(2+) stimulates the formation of protein functional networks rather than using a particular molecule as its target.

Structure-Function Relationship of the Interaction between Tissue Factor and Factor VIIa

Interactions between tissue factor and factor VIIa are the primary initiators of coagulation in hemostasis and certain thrombotic diseases. Tissue factor, an integral membrane protein expressed extensively outside of the vasculature, is the regulatory protein cofactor for coagulation factor VIIa. Factor VIIa, a trypsin-like serine protease homologous with other blood coagulation proteases, is weakly active when free in solution and must bind its membrane-bound cofactor for physiologically relevant activity. Tissue factor allosterically activates factor VIIa by several mechanisms such as active site positioning, spatial stabilization, and direct interactions with the substrate. Protein-membrane interactions between tissue factor, factor VIIa, and substrates all play critical roles in modulating the activity of this enzyme complex. Additionally, divalent cations such as Ca(2+) and Mg(2+) are critical for correct protein folding, as well as protein-membrane and protein-protein interactions. The contributions of these factors toward tissue factor-factor VIIa activity are discussed in this review.

Tissue Factor Residues That Modulate Magnesium-Dependent Rate Enhancements of the Tissue Factor/Factor VIIa Complex

The blood coagulation cascade is initiated when the cell-surface complex of factor VIIa (FVIIa, a trypsin-like serine protease) and tissue factor (TF, an integral membrane protein) proteolytically activates factor X (FX). Both FVIIa and FX bind to membranes via their γ-carboxyglutamate-rich domains (GLA domains). GLA domains contain seven to nine bound Ca(2+) ions that are critical for their folding and function, and most biochemical studies of blood clotting have employed supraphysiologic Ca(2+) concentrations to ensure saturation of these domains with bound Ca(2+). Recently, it has become clear that, at plasma concentrations of metal ions, Mg(2+) actually occupies two or three of the divalent metal ion-binding sites in GLA domains, and that these bound Mg(2+) ions are required for full function of these clotting proteins. In this study, we investigated how Mg(2+) influences FVIIa enzymatic activity. We found that the presence of TF was required for Mg(2+) to enhance the rate of FX activation by FVIIa, and we used alanine-scanning mutagenesis to identify TF residues important for mediating this response to Mg(2+). Several TF mutations, including those at residues G164, K166, and Y185, blunted the ability of Mg(2+) to enhance the activity of the TF/FVIIa complex. Our results suggest that these TF residues interact with the GLA domain of FX in a Mg(2+)-dependent manner (although effects of Mg(2+) on the FVIIa GLA domain cannot be ruled out). Notably, these TF residues are located within or immediately adjacent to the putative substrate-binding exosite of TF.

Quantifying vitamin K-dependent holoprotein compaction caused by differential γ-carboxylation using high-pressure size exclusion chromatography

This study uses high-pressure size exclusion chromatography (HPSEC) to quantify divalent metal ion (X(2+))-induced compaction found in vitamin K-dependent (VKD) proteins. Multiple X(2+) binding sites formed by the presence of up to 12 γ-carboxyglutamic acid (Gla) residues are present in plasma-derived FIX (pd-FIX) and recombinant FIX (r-FIX). Analytical ultracentrifugation (AUC) was used to calibrate the Stokes radius (R) measured by HPSEC. A compaction of pd-FIX caused by the filling of Ca(2+) and Mg(2+) binding sites resulted in a 5 to 6% decrease in radius of hydration as observed by HPSEC. The filling of Ca(2+) sites resulted in greater compaction than for Mg(2+) alone where this effect was additive or greater when both ions were present at physiological levels. Less X(2+)-induced compaction was observed in r-FIX with lower Gla content populations, which enabled the separation of biologically active r-FIX species from inactive ones by HPSEC. HPSEC was sensitive to R changes of approximately 0.01nm that enabled the detection of FIX compaction that was likely cooperative in nature between lower avidity X(2+) sites of the Gla domain and higher avidity X(2+) sites of the epidermal growth factor 1 (EGF1)-like domain.

Ca2+ switches the effect of PS-containing membranes on Factor Xa from activating to inhibiting: implications for initiation of blood coagulation

Calcium (Ca2+) plays a pivotal role in cellular and organismal physiology. The Ca2+ ion has an intermediate protein-binding affinity and thus it can serve as an on/off switch in the regulation of different biochemical processes. The serum level of ionized Ca2+ is regulated with normal ionized Ca2+ being in the range 1.10-1.3 mM. Hypocalcaemia (free Ca2+<1.1 mM) in critically ill patients is commonly accompanied by haemostatic abnormalities, ranging from isolated thrombocytopenia to complex defects such as disseminated intravascular coagulation, commonly thought to be due to insufficient functioning of anticoagulation pathways. A small amount of fXa (Factor Xa) produced by Factor VIIa and exposed tissue factor is key to initiating blood coagulation by producing enough thrombin to induce the later stages of coagulation. fXa must bind to PS (phosphatidylserine)-containing membranes to produce thrombin at a physiologically significant rate. In the present study, we show that overall fXa activity on PS-containing membranes is sharply regulated by a 'Ca2+ switch' centred at 1.16 mM, below which fXa is active and above which fXa forms inactive dimers on PS-exposing membranes. Our data lead to a mathematical model that predicts the variation of fXa activity as a function of both Ca2+ and membrane concentrations. Because the critical Ca2+ concentration is at the lower end of the normal plasma ionized Ca2+ concentration range, we propose a new regulatory mechanism by which local Ca2+ concentration switches fXa from an intrinsically active form to a form requiring its cofactor [fVa (Factor Va)] to achieve significant activity.