Phosphatidylserine binding alters the conformation and specifically enhances the cofactor activity of bovine factor Va

Phosphatidylserine Regulation of Coagulation Proteins Factor IXa and Factor VIIIa

Blood coagulation is an intricate process, and it requires precise control of the activities of pro- and anticoagulant factors and sensitive signaling systems to monitor and respond to blood vessel insults. These requirements are fulfilled by phosphatidylserine, a relatively miniscule-sized lipid molecule amid the myriad of large coagulation proteins. This review limelight the role of platelet membrane phosphatidylserine (PS) in regulating a key enzymatic reaction of blood coagulation; conversion of factor X to factor Xa by the enzyme factor IXa and its cofactor factor VIIIa. PS is normally located on the inner leaflet of the resting platelet membrane but appears on the outer leaflet surface of the membrane surface after an injury happens. Human platelet activation leads to exposure of buried PS molecules on the surface of the platelet-derived membranes and the exposed PS binds to discrete and specific sites on factors IXa and VIIIa. PS binding to these sites allosterically regulates both factors IXa and VIIIa. The exposure of PS and its binding to factors IXa/VIIIa is a vital step during clotting. Insufficient exposure or a defective binding of PS to these clotting proteins is responsible for various hematologic diseases which are discussed in this review.

Blood coagulation factor Va's key interactive residues and regions for prothrombinase assembly and prothrombin binding

Blood coagulation factor Va serves an indispensable role in hemostasis as cofactor for the serine protease factor Xa. In the presence of an anionic phospholipid membrane and calcium ions, factors Va and Xa assemble into the prothrombinase complex. Following formation of the ternary complex with the macromolecular zymogen substrate prothrombin, the latter is rapidly converted into thrombin, the key regulatory enzyme of coagulation. Over the years, multiple binding sites have been identified in factor Va that play a role in the interaction of the cofactor with factor Xa, prothrombin, or the anionic phospholipid membrane surface. In this review, an overview of the currently available information on these interactive sites in factor Va is provided, and data from biochemical approaches and 3D structural protein complex models are discussed. The structural models have been generated in recent years and provide novel insights into the molecular requirements for assembly of both the prothrombinase and the ternary prothrombinase-prothrombin complexes. Integrated knowledge of functionally important regions in factor Va will allow for a better understanding of factor Va cofactor activity.

Phosphatidylserine and Phosphatidylethanolamine Bind to Protein Z Cooperatively and with Equal Affinity

Protein Z (PZ) is an anticoagulant that binds with high affinity to Protein Z-dependent protease inhibitor (ZPI) and accelerates the rate of ZPI-mediated inhibition of factor Xa (fXa) by more than 1000-fold in the presence of Ca²⁺ and phospholipids. PZ promotion of the ZPI-fXa interaction results from the anchoring of the Gla domain of PZ onto phospholipid surfaces and positioning the bound ZPI in close proximity to the Gla-anchored fXa, forming a ternary complex of PZ/ZPI/fXa. Although interaction of PZ with phospholipid membrane appears to be absolutely crucial for its cofactor activity, little is known about the binding of different phospholipids to PZ. The present study was conceived to understand the interaction of different phospholipids with PZ. Experiments with both soluble lipids and model membranes revealed that PZ binds to phosphatidylserine (PS) and phosphatidylethanolamine (PE) with equal affinity (K_d~48 μM); further, PS and PE bound to PZ synergistically. Equilibrium dialysis experiments revealed two lipid-binding sites for both PS and PE. PZ binds with weaker affinity to other phospholipids, e.g., phosphatidic acid, phosphatidylglycerol, phosphatidylcholine and binding of these lipids is not synergistic with respect to PS. Both PS and PE -containing membranes supported the formation of a fXa-PZ complex. PZ protection of fXa from antithrombin inhibition were also shown to be comparable in presence of both PS: PC and PE: PC membranes. These findings are particularly important and intriguing since they suggest a special affinity of PZ, in vivo, towards activated platelets, the primary membrane involved in blood coagulation process.

Soluble phosphatidylserine binds to two sites on human factor IXa in a Ca2+ dependent fashion to specifically regulate structure and activity

Clinical studies have demonstrated a correlation between elevated levels of FIX and the risk of coronary heart disease, while reduced plasma FIX causes hemophilia B. FIXa interacts with FVIIIa in the presence of Ca²⁺ and phosphatidylserine (PS)-containing membranes to form a factor X-activating complex (Xase) that is key to propagation of the initiated blood coagulation process in human. We test the hypothesis that PS in these membranes up-regulates the catalytic activity of this essential enzyme. We used a soluble form of phosphatidylserine, 1, 2-dicaproyl-sn-glycero-3-phospho-L-serine (C6PS), as a tool to do so. C6PS and PS in membranes are reported to regulate the homologous FXa nearly identically. FIXa binds a molecule of C6PS at each of with two sites with such different affinities (∼100-fold) that these appear to be independent. A high affinity C6PS binding site (K_d∼1.4 µM) regulates structure, whereas a low-affinity binding site (K_d∼140 µM) regulates activity. Equilibrium dialysis experiments were analyzed globally with four other data sets (proteolytic and amidolytic activities, intrinsic fluorescence, ellipticity) to unequivocally demonstrate stoichiometries of one for both sites. Michaelis-Menten parameters for FIXa proteolytic activity were the same in the presence of C6PS or PS/PC membranes. We conclude that the PS molecule and not a membrane surface is the key regulator of both factors Xa and IXa. Despite some minor differences in the details of regulation of factors Xa and IXa, the similarities we found suggest that lipid regulation of these two proteases may be similar, a hypothesis that we continue to test.

Phosphatidylserine-induced factor Xa dimerization and binding to factor Va are competing processes in solution

A soluble, short chain phosphatidylserine, 1,2-dicaproyl-sn-glycero-3-phospho-l-serine (C6PS), binds to discrete sites on FXa, FVa, and prothrombin to alter their conformations, to promote FXa dimerization (K(d) ~ 14 nM), and to enhance both the catalytic activity of FXa and the cofactor activity of FVa. In the presence of calcium, C6PS binds to two sites on FXa, one in the epidermal growth factor-like (EGF) domain and one in the catalytic domain; the latter interaction is sensitive to Na(+) binding and probably represents a protein recognition site. Here we ask whether dimerization of FXa and its binding to FVa in the presence of C6PS are competitive processes. We monitored FXa activity at 5, 20, and 50 nM FXa while titrating with FVa in the presence of 400 μM C6PS and 3 or 5 mM Ca(2+) to show that the apparent K(d) of FVa-FXa interaction increased with an increase in FXa concentration at 5 mM Ca(2+), but the K(d) was only slightly affected at 3 mM Ca(2+). A mixture of 50 nM FXa and 50 nM FVa in the presence of 400 μM C6PS yielded both Xa homodimers and Xa·Va heterodimers, but no FXa dimers bound to FVa. A mutant FXa (R165A) that has reduced prothrombinase activity showed both weakened dimerization (K(d) ~ 147 nM) and weakened FVa binding (apparent K(d) values of 58, 92, and 128 nM for 5, 20, and 50 nM R165A FXa, respectively). Native gel electrophoresis showed that the GLA-EGF(NC) fragment of FXa (lacking the catalytic domain) neither dimerized nor formed a complex with FVa in the presence of 400 μM C6PS and 5 mM Ca(2+). Our results demonstrate that the dimerization site and FVa-binding site are both located in the catalytic domain of FXa and that these sites are linked thermodynamically.

Modulation of prothrombinase assembly and activity by phosphatidylethanolamine

Constituents of platelet membranes regulate the activity of the prothrombinase complex. We demonstrate that membranes containing phosphatidylcholine and phosphatidylethanolamine (PE) bind factor Va with high affinity (K(d) = ∼10 nm) in the absence of phosphatidylserine (PS). These membranes support formation of a 60-70% functional prothrombinase complex at saturating factor Va concentrations. Although reduced interfacial packing does contribute to factor Va binding in the absence of PS, it does not correlate with the enhanced activity of the Xa-Va complex assembled on PE-containing membranes. Instead, specific protein-PE interactions appear to contribute to the effects of PE. In support of this, soluble C6PE binds to recombinant factor Va(2) (K(d) = ∼6.5 μm) and to factor Xa (K(d) = ∼91 μm). C6PE and C6PS binding sites of factor Xa are specific, distinct, and linked, because binding of one lipid enhances the binding and activity effects of the other. C6PE triggers assembly (K(d)(app) = ∼40 nm) of a partially active prothrombinase complex between factor Xa and factor Va(2), compared with K(d)(app) for C6PS ∼2 nm. These findings provide new insights into the possible synergistic roles of platelet PE and PS in regulating thrombin formation, particularly when exposed membrane PS may be limiting.

Differential activation and inhibition of human platelet thrombin receptors by structurally distinct α-, β- and γ-thrombin

The development of drugs to neutralize the action of thrombin has to date focused on the alpha form of the protease. It is generally agreed that inactive prothrombin is proteolytically converted to active alpha-thrombin which may be further hydrolyzed to beta- and gamma-thrombin. While all three forms of the enzyme retain catalytic activities, only alpha-thrombin is presumed to be physiologically important. The beta- and gamma-thrombin are presumed to be degradation products of no physiological significance. Our demonstration that beta- and gamma-thrombin selectively activate PAR-4 in this and a previous report (J. Biol. Chem. 276, 21173-21183, 2001) necessitates a reevaluation of how we view their physiological roles and how we approach the pharmacological regulation of their actions. Beta-thrombin, like gamma-thrombin, at nM levels selectively activates PAR-4. This was demonstrated by full retention of aggregatory activity with platelets whose PAR-1 and GP Ib receptors were inactivated. Furthermore, the beta-thrombin response was abrogated by desensitizing platelets with suboptimal levels of the thrombin receptor activating peptide for PAR-4 (TRAP-4). For beta-thrombin and gamma-thrombin to have a physiological role, it is necessary to show they can be generated under physiological conditions. We demonstrate, for the first time, that alpha-thrombin is hydrolyzed in less than 1 min by activated factor X at physiological pH, in vitro. This implies that alpha-thrombin may be rapidly converted to beta-thrombin and/or gamma-thrombin in vivo in the proper microenvironment. The differential activation of the three platelet thrombin receptors by alpha-, beta- and gamma-thrombin implies selective structural variations between these thrombin species. Structural differences are likely to account for the marked differential responses observed with the antithrombotic, hirudin, which inhibits alpha-thrombin , is a slightly weaker inhibitor of beta-thrombin and a very weak inhibitor of gamma-thrombin -induced platelet aggregations. The converse order of inhibition is observed with the physiological protease inhibitor, alpha(1)-antitrypsin. Finally, a non-traditional inhibitor, histone-1, selectively inhibits only beta- and gamma-thrombin , primarily at the receptor level of PAR-4 rather than on the thrombin molecule. Trypsin, like beta- and gamma-thrombin , activates PAR-4 and is also inactive with TRAP-4 desensitized platelets. Therefore, it was reasoned that trypsin would be more structurally similar to gamma-thrombin than to alpha-thrombin. The analysis of the crystalline structures of alpha-, gamma-thrombin and trypsin from the databases confirm that this is the case. These findings should help to elucidate structure-function relationships of the different thrombins and may aid in the development of new anti-thrombotic drugs.

Functional and structural characterization of factor Xa dimer in solution

Previous studies showed that binding of water-soluble phosphatidylserine (C6PS) to bovine factor Xa (FXa) leads to Ca2+-dependent dimerization in solution. We report the effects of Ca2+, C6PS, and dimerization on the activity and structure of human and bovine FXa. Both human and bovine dimers are 10(6)- to 10(7)-fold less active toward prothrombin than the monomer, with the decrease being attributed mainly to a substantial decrease in k(cat). Dimerization appears not to block the active site, since amidolytic activity toward a synthetic substrate is largely unaffected. Circular dichroism reveals a substantial change in tertiary or quaternary structure with a concomitant decrease in alpha-helix upon dimerization. Mass spectrometry identifies a lysine (K(270)) in the catalytic domain that appears to be buried at the dimer interface and is part of a synthetic peptide sequence reported to interfere with factor Va (FVa) binding. C6PS binding exposes K(351) (part of a reported FVa binding region), K(242) (adjacent to the catalytic triad), and K(420) (part of a substrate exosite). We interpret our results to mean that C6PS-induced dimerization produces substantial conformational changes or domain rearrangements such that structural data on PS-activated FXa is required to understand the structure of the FXa dimer or the FXa-FVa complex.

The PT1-Ca2+ Gla domain binds to a membrane through two dipalmitoylphosphatidylserines. A computational study

Binding of vitamin K-dependent proteins to cell membranes containing phosphatidylserine (PS) via gamma-carboxyglutamic acid (Gla) domains is one of the essential steps in the blood coagulation pathway. During activation of the coagulation cascade, prothrombin is converted to thrombin by prothrombinase, a complex consisting of serine protease FXa and cofactor FVa, anchored to anionic phospholipids on the surface of activated platelets in the presence of calcium ions. To investigate the binding of the Gla domain of prothrombin fragment 1 (PT1) to anionic lipids in the presence of Ca2+, we have conducted MD simulations of the protein with one and two dipalmitoylphosphatidylserines (DPPS) in a dipalmitoylphosphatidylcholine (DPPC) bilayer membrane. The results show a well-defined phosphatidylserine binding site, which agrees generally with crystallographic studies [Huang, M., et al. (2003) Nat. Struct. Biol. 10, 751-756]. However, in the presence of the lipid membrane, some of the interactions observed in the crystal structure adjust during the simulations possibly because in our system the PT1-Ca2+ complex is embedded in a DPPC lipid membrane. Our simulations confirm the existence of a second phospholipid headgroup binding site on the opposite face of the PT1-Ca2+ complex as suggested by MacDonald et al. [(1997) Biochemistry 36, 5120-5127]. The serine headgroup in the second site binds through a Gla domain-bound calcium ion Ca1, Gla30, and Lys11. On the basis of free energy simulations, we estimate the energy of binding of the PT1-Ca2+ complex to a single DPPS to be around -11.5 kcal/mol. The estimated free energy of binding of a DPPS lipid to the second binding site is around -8.8 kcal/mol and is in part caused by the nature of the second site and in part by entropic effects.

A phosphatidylserine binding site in factor Va C1 domain regulates both assembly and activity of the prothrombinase complex

Tightly associated factor V(a) (FVa) and factor X(a) (FXa) serve as the essential prothrombin-activating complex that assembles on phosphatidylserine (PS)-containing platelet membranes during blood coagulation. We have previously shown that (1) a soluble form of PS (C6PS) triggers assembly of a fully active FVa-FXa complex in solution and (2) that 2 molecules of C6PS bind to FVa light chain with one occupying a site in the C2 domain. We expressed human factor V(a) (rFVa) with mutations in either the C1 domain (Y1956,L1957)A, the C2 domain (W2063,W2064)A, or both C domains (Y1956,L1957,W2063,W2064)A. Mutations in the C1 and C1-C2 domains of rFVa reduced the rate of activation of prothrombin to thrombin by FXa in the presence of 400 muM C6PS by 14 000- to 15 000-fold relative to either wild-type or C2 mutant factor rFVa. The K(d')s of FXa binding with rFVa (wild-type, C2 mutant, C1 mutant, and C1-C2 mutant) were 3, 4, 564, and 624 nM, respectively. Equilibrium dialysis experiments detected binding of 4, 3, and 2 molecules of C6PS to wild-type rFVa, C1-mutated, and C1,C2-mutated rFVa, respectively. Because FVa heavy chain binds 2 molecules of C6PS, we conclude that both C2 and C1 domains bind one C6PS, with binding to the C1 domain regulating prothrombinase complex assembly.

Protein structure and oligomerization are important for the formation of export-competent HIV-1 Rev-RRE complexes

The translation of the unspliced and partially spliced viral mRNAs that encode the late, structural proteins of HIV-1 depends on the viral-protein Rev. Oligomeric binding of Rev to the Rev response element (RRE) in these mRNAs promotes their export from the nucleus and thus controls their expression. Here, we compared the effects of hydrophobic to hydrophilic mutations within the oligomerization domain of Rev using assays for oligomeric RNA binding, protein structure, and export from the nucleus. Oligomeric RNA binding alone does not correlate well with RNA transport activity in the subset of mutants. However, protein structure as judged by CD spectroscopy does correlate well with Rev function. The oligomeric assembly of Rev-L18T is impaired but exhibits minor defects in structure and retains a basal level of activity in vivo. The prevalence of L18T in infected individuals suggests a positive selection mechanism for L18T modulation of Rev activity that may delay the onset of AIDS.

Inhibition of Ribonuclease A by polyphenols present in green tea

We report the effect of the natural polyphenolic compounds from green tea on the catalytic activity of Ribonuclease A (RNase A). The compounds behave as noncompetitive inhibitors of the protein with inhibition constants ranging from 80-1300 microM. The dissociation constants range from 50-150 microM for the RNase A-polyphenol complexes as determined by ultraviolet (UV) and circular dichroism (CD) studies. We have also investigated the changes in the secondary structure of RNase A on complex formation by CD and Fourier transformed infrared (FTIR) spectroscopy. The presence of the gallate moiety has been shown to be important for the inhibition of enzymatic activity. Docking studies for these compounds indicate that the preferred site of binding is the region encompassing residues 34-39 with possible hydrogen bonding with Lys 7 and Arg 10. Finally we have also looked at changes in the accessible surface area of the interacting residues on complex formation for an insight into the residues involved in the interaction.

O-phospho-L-serine, multi-functional excipient for B domain deleted recombinant factor VIII

Factor VIII (FVIII) is an important cofactor in the blood coagulation cascade. A deficiency or dysfunction of FVIII causes hemophilia A, a life-threatening bleeding disorder. FVIII circulates in plasma as a heterodimer comprising 6 domains (heavy chain, A1-A2-B and light chain, A3-C1-C2). Replacement therapy using FVIII is the leading therapy in the management of hemophilia A. However, approximately 15% to 30% of patients develop inhibitory antibodies that neutralize the activity of the protein. Neutralizing antibodies to epitopes in the lipid binding region of FVIII are commonly identified in patients' plasma. In this report, we investigated the effect of O-phospho-L-serine (OPLS), which binds to the lipid binding region, on the immunogenicity of B domain deleted recombinant factor VIII (BDDrFVIII). Sandwich enzyme-linked immunosorbent assay (ELISA) studies showed that OPLS specifically bind to the lipid binding region, localized in the C2 domain of the coagulation factor. Size exclusion chromatography and fluorescence anisotropy studies showed that OPLS interfered with the aggregation of BDDrFVIII. Immunogenicity of free- vs BDDrFVIII-OPLS complex was evaluated in a murine model of hemophilia A. Animals administered subcutaneous (sc) injections of BDDrFVIII-OPLS had lower neutralizing titers compared with animals treated with BDDrFVIII alone. Based on these studies, we hypothesize that specific molecular interactions between OPLS and BDDrFVIII may improve the stability and reduce the immunogenicity of BDDrFVIII formulations.

Association free energy of dipalmitoylphosphatidylserines in a mixed dipalmitoylphosphatidylcholine membrane

Blood coagulation is strongly dependent on the binding of vitamin K-dependent proteins to cell membranes containing phosphatidylserine (PS) via gamma-carboxyglutamic acid (Gla) domains. The process depends on calcium, which can induce nonideal behavior in membranes through domain formation. Such domain separation mediated by Ca(2+) ions or proteins can have an important contribution to the thermodynamics of the interaction between charged peripheral proteins and oppositely charged membranes. To characterize the properties of lipid-lipid interactions, molecular dynamics, and free energy simulations in a mixed bilayer membrane containing dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylserine were carried out. The free energy of association between dipalmitoylphosphatidylserines in the environment of dipalmitoylphosphatidylcholines has been calculated by using a novel approach to the dual topology technique of the PS-PC hybrid. Two different methods, free energy perturbation and thermodynamic integration, were used to calculate the free energy difference. In thermodynamic integration runs three schemes were applied to evaluate the integral at the limits of lambda --> 0 or lambda --> 1. Our studies show that the association of two PSs in the environment of PCs is repulsive in the absence of Ca(2+) and becomes favorable in their presence. We also show that the mixed component membrane should exhibit nonideal behavior that will lead to PS clustering.

Interaction of dicaproyl phosphatidylserine with recombinant factor VIII and its impact on immunogenicity

Replacement therapy with exogenous recombinant factor VIII (rFVIII) to control bleeding episodes results in the development of inhibitory antibodies in 15% to 30% of hemophilia A patients. The inhibitory antibodies are mainly directed against specific and universal immunodominant epitopes located in the C2 domain. Previously we have shown that complexation of O-phospho-L-serine (phosphatidylserine head group) with the phospholipid binding region of the C2 domain can lead to an overall reduction in the immunogenicity of rFVIII. Here, we have investigated the hypothesis that dicaproyl phosphatidylserine, a short-chain water-soluble phospholipid, can reduce the immunogenicity of rFVIII. Circular dichroism and fluorescence spectroscopy studies suggest that dicaproyl phosphatidylserine interacts with rFVIII, causing subtle changes in the tertiary and secondary structure of the protein. Sandwich enzyme-linked immunosorbent assay studies indicate that dicaproyl phosphatidylserine probably interacts with the phospholipid binding region of the C2 domain. The immunogenicity of FVIII-dicaproyl phosphatidylserine complexes prepared at concentrations above and below the critical micellar concentrations of the lipid were evaluated in hemophilia A mice. Our results suggest that micellar dicaproyl phosphatidylserine may be useful to reduce the immunogenicity of rFVIII preparations.

Asymmetric Distribution of Phospholipids in Biomembranes

In eukaryotic cells, the biological membrane is characterized by a non-uniform distribution of membrane lipids, vertically as well as laterally. The paradigm for the vertical non-random distribution is the plasma membrane, where phosphatidylcholine (PC), sphingomyelin (SM), and glycosphingolipids are primarily located on the exoplasmic leaflet, while aminophospholipids, including phosphatidylserine (PS) and phosphatidylethanolamine (PE), are generally enriched in the cytoplasmic leaflet. Other minor phospholipids, such as phosphatidic acid and phosphatidylinositol (PI), are also enriched on the cytoplasmic face. Such asymmetrical distribution is related to each lipid regulating various biological events through interaction with other molecules. The clarification of the regulatory mechanism of the distribution and movement of membrane lipids is crucial to understanding the physiological roles of lipids. Here we focus on PS, which has been reported to be involved in apoptosis, blood coagulation and other biological phenomena, and summarize the present understanding of the dynamics of this phospholipid, including biosynthesis, metabolism, transport, and transbilayer movement. We also refer to diseases that have been reported to be related to phospholipid asymmetry.

Reduction of circulating annexin A5 levels and resistance to annexin A5 anticoagulant activity in women with recurrent spontaneous pregnancy losses

OBJECTIVE

We investigated whether levels of annexin A5, evidence for resistance to annexin A5 activity, and levels anti-annexin A5 antibodies might be altered in women with a history of recurrent spontaneous pregnancy losses.

STUDY DESIGN

These annexin A5 parameters were assayed in 70 nonpregnant women with a history of > or = 3 recurrent spontaneous pregnancy losses (cases) and 50 women without adverse pregnancy history (control subjects).

RESULTS

Cases had significantly lower plasma annexin A5 levels than control subjects (median, 4.7 ng/mL [range, 0.3-40.4 ng/mL] vs 6.7 ng/mL [range, 0.7-56.0]; P = .01), significantly reduced anticoagulant ratios (188% [range, 119%-279%] vs 238% [range, 159%-286%]; P < .0001), and reduced binding of annexin A5 to phospholipid (6.3 ng/aliquot phospholipid [range, 1.5-16.4 ng/aliquot phospholipid] vs 9.7 ng/aliquot phospholipid (range, 3.5-17.0 ng/aliquot phospholipid]; P = .0002). There were no significant differences in anti-annexin A5 antibody levels.

CONCLUSION

Reduction of annexin A5 and interference with its anticoagulant and binding activities are associated significantly with a history of recurrent spontaneous pregnancy losses. These data support the concept of a significant role for annexin A5 in the maintenance of pregnancy.

Lower inhibitor development in hemophilia A mice following administration of recombinant factor VIII-O-phospho-L-serine complex

Factor VIII is a multidomain protein composed of A1, A2, B, A3, C1, and C2 domains. Deficiency or dysfunction of factor VIII causes hemophilia A, a bleeding disorder. Administration of exogenous recombinant factor VIII as a replacement leads to development of inhibitory antibodies against factor VIII in 15-30% of hemophilia A patients. Hence, less immunogenic preparations of factor VIII are highly desirable. Inhibitory antibodies against factor VIII are mainly directed against immunodominant epitopes in C2, A3, and A2 domains. Further, several universal epitopes for CD4+ T-cells have been identified within the C2 domain. The C2 domain is also known to interact specifically with phosphatidylserine-rich lipid vesicles. Here, we have investigated the hypothesis that complexation of O-phospho-l-serine, the head group of phosphatidylserine, with the C2 domain can reduce the overall immunogenicity of factor VIII. The biophysical (circular dichroism and fluorescence) and biochemical studies (ELISA and size exclusion chromatography) showed that O-phospho-l-serine binds to the phospholipid-binding region in the C2 domain, and this interaction causes subtle changes in the tertiary structure of the protein. O-Phospho-l-serine also prevented aggregation of the protein under thermal stress. The immunogenicity of the factor VIII-O-phospho-l-serine complex was evaluated in hemophilia A mice. The total and inhibitory antibody titers were lower for factor VIII-O-phospho-l-serine complex compared with factor VIII alone. Moreover, factor VIII administered as a complex with O-phospho-l-serine retained in vivo activity in hemophilia A mice. Our results suggest that factor VIII-O-phospho-l-serine complex may be beneficial to increase the physical stability and reduce immunogenicity of recombinant factor VIII preparations.

Apolipoprotein E isoprotein-specific interactions with tissue plasminogen activator

Apolipoprotein E (Apo E) is an important genetic risk factor for multiple neurological, vascular and cardiovascular diseases. Previously, we reported Apo E isoprotein-specific modulation of tissue plasminogen activator (tPA) using an in vitro blood-clotting assay. Here, we studied the conformational changes of Apo E2, E3 and E4 in the presence of tPA and vice versa using circular dichroism (CD) and dual polarization interferometry (DPI). We report isoprotein and state-specific intermolecular interactions between the Apo E isoforms and tPA. Apo E2 interaction with immobilized tPA leads to significant conformational changes which are not observed with Apo E3 or E4. Additionally, tPA induces changes in helicity of lipidated Apo E2 whereas no detectable changes were observed in Apo E3 or E4. The Tukey's test for interaction indicated a significant (P < 0.001) interaction between tPA and Apo E2 in the lipidated environment. These results may be important regarding the mechanism by which Apo E has isoprotein-specific effects on many biological processes and diseases involving blood clotting, proteolysis and perfusion.

Phospholipid Flip-Flop and Phospholipid Scramblase 1 (PLSCR1) Co-localize to Uropod Rafts in Formylated Met-Leu-Phe-stimulated Neutrophils

Movement of phosphatidylserine (PS) to the plasma membrane outer leaflet is a nearly universal marker of apoptosis and occurs during activation of many cells. Neutrophils stimulated with the chemotactic peptide formylated Met-Leu-Phe (fMLP) demonstrated transient PS exposure. Stimulated outward movement of PS was accompanied by enhanced inward movement of several phosphorylcholine lipid probes and was associated with enhanced FM 1-43 staining indicative of phospholipid packing changes. Unlike apoptosis, inward movement of exogenously added fluorescent PS did not decline, and DNA was not cleaved during fMLP stimulation. Movement of phospholipids occurred within minutes following stimulation, was independent of endocytosis/pinocytosis, and was consistent with bidirectional, transbilayer phospholipid flip-flop. While the role of phospholipid scramblase 1 (PLSCR1) is controversial in flip-flop, we sought evidence for its role in enhanced phospholipid movements during fMLP stimulation. Using antibodies to the carboxyl-terminal domain of PLSCR1, its presence in the plasma membranes of non-permeabilized neutrophils was confirmed by flow cytometry. Additionally subcellular fractionation demonstrated that PLSCR1 was also located in secretory vesicles and tertiary and secondary granules. Activation of neutrophils with fMLP, however, did not significantly alter surface labeling suggesting that stimulated phospholipid flip-flop does not require additional mobilization of PLSCR1 to the plasma membrane. As expected for palmitoylated proteins, PLSCR1 was enriched in detergent-insoluble membranes and co-localized with raft markers at the neutrophil uropod after stimulation. Of note, PS exposure, phospholipid uptake, and FM 1-43 staining also localized to the uropod following stimulation demonstrating that both PLSCR1 and phospholipid flip-flop characterize this specialized domain of polarized neutrophils.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Exposure of platelet membrane phosphatidylserine regulates blood coagulation

This article addresses the role of platelet membrane phosphatidylserine (PS) in regulating the production of thrombin, the central regulatory molecule of blood coagulation. PS is normally located on the cytoplasmic face of the resting platelet membrane but appears on the plasma-oriented surface of discrete membrane vesicles that derive from activated platelets. Thrombin, the central molecule of coagulation, is produced from prothrombin by a complex ("prothrombinase") between factor Xa and its protein cofactor (factor V(a)) that forms on platelet-derived membranes. This complex enhances the rate of activation of prothrombin to thrombin by roughly 150,000 fold relative to factor X(a) in solution. It is widely accepted that the negatively charged surface of PS-containing platelet-derived membranes is at least partly responsible for this rate enhancement, although there is not universal agreement on mechanism by which this occurs. Our efforts have led to an alternative view, namely that PS molecules bind to discrete regulatory sites on both factors X(a) and V(a) and allosterically alter their proteolytic and cofactor activities. In this view, exposure of PS on the surface of activated platelet vesicles is a key regulatory event in blood coagulation, and PS serves as a second messenger in this regulatory process. This article reviews our knowledge of the prothrombinase reaction and summarizes recent evidence leading to this alternative viewpoint. This viewpoint suggests a key role for PS both in normal hemostasis and in thrombotic disease.

Effects of water soluble phosphotidylserine on bovine factor Xa: functional and structural changes plus dimerization

Previous work has shown that two molecules of a soluble form of phosphatidylserine, C6PS, bind to human and bovine factor X(a). Activity measurements along with the fluorescence of active-site-labeled human factor X(a) showed that two linked sites specifically regulate the active site conformation and proteolytic activity of the human enzyme. These results imply, but cannot demonstrate, a C6PS-induced factor X(a) conformational change. The purpose of this paper is to extend these observations to bovine factor X(a) and to demonstrate that they do reflect conformational changes. We report that the fluorescence of active-site-labeled bovine factor X(a) also varied with C6PS concentration in a sigmoidal manner, whereas amidolytic activity of unlabeled enzyme varied in a simple hyperbolic fashion, also as seen for human factor X(a). C6PS induced a 70-fold increase in bovine factor X(a)'s autolytic activity, consistent with the 60-fold increase in proteolytic activity reported for human factor X(a). In addition, circular dichroism spectroscopy clearly demonstrated that C6PS binding to bovine factor X(a) induces secondary structural changes. In addition, C6PS binding to the tighter of the two sites triggered structural changes that lead to Ca(2+)-dependent dimer formation, as demonstrated by changes in intrinsic fluorescence and quantitative native gel electrophoresis. Dimerization produced further change in secondary structure, either inter- or intramolecularly. These results, along with results presented previously, support a model in which C6PS binds in a roughly sequential fashion to two linked sites whose occupancy in both human and bovine factor X(a) elicits different structural and functional responses.

Soluble phosphatidylserine triggers assembly in solution of a prothrombin-activating complex in the absence of a membrane surface

Factor X(a) (FX(a)) binding to factor V(a) (FV(a)) on platelet-derived membranes containing surface-exposed phosphatidylserine (PS) forms the "prothrombinase complex" that is essential for efficient thrombin generation during blood coagulation. There are two naturally occurring isoforms of FV(a), FV(a1) and FV(a2). These two isoforms differ by a 3-kDa polysaccharide chain (at Asn(2181) in human FV(a1) (Kim, S. W., Ortel, T. L., Quinn-Allen, M. A., Yoo, L., Worfolk, L., Zhai, X., Lentz, B. R., and Kane, W. H. (1999) Biochemistry 38, 11448-11454)) and have different coagulant activities. We examined the interaction of the two bovine isoforms with active site-labeled FX(a), finding no significant difference. A soluble form of PS (C6PS) bound to FV(a1) and FV(a2) with comparable affinities (K(d) = 11-12 microm) and changes in FV(a) intrinsic fluorescence. At concentrations well below its critical micelle concentration, C6PS binding to bovine FV(a2) enhanced its affinity for FX(a) in solution by nearly 3 orders of magnitude (K(d)(eff) = 40-2 nm over a C6PS range of 30-400 microm) but had no effect on the affinity of FV(a1) for FX(a) (K(d) = 1 microm). This results in a soluble complex between FX(a) and FV(a2), whose expected molecular weight was confirmed by calibrated native gel electrophoresis. This complex behaved as a normal Michaelis-Menten enzyme in its ability to produce thrombin from meizothrombin (apparent k(cat)/K(m) congruent with 10(9) m(-1) s(-1)). The ability of soluble PS to trigger formation of a soluble prothrombinase complex suggests that exposure of PS molecules during platelet activation is likely the key event responsible for the assembly of an active membrane-bound complex.