A3 domain region 1803-1818 contributes to the stability of activated factor VIII and includes a binding site for activated factor IX

Factor VIII A3 domain residues 1793-1795 represent a factor IXa-interactive site in the tenase complex

BACKGROUND

Factor (F)VIII functions as a cofactor in the tenase complex responsible for conversion of FX to FXa by FIXa. Earlier studies indicated that one of the FIXa-binding sites is located in residues 1811-1818 (crucially F1816) of the FVIII A3 domain. A putative, three-dimensional structure model of the FVIIIa molecule suggested that residues 1790-1798 form a V-shaped loop, and juxtapose residues 1811-1818 on the extended surface of FVIIIa.

AIM

To examine FIXa molecular interactions in the clustered acidic sites of FVIII including residues 1790-1798.

METHODS AND RESULTS

Specific ELISA's demonstrated that the synthetic peptides, encompassing residues 1790-1798 and 1811-1818, competitively inhibited the binding of FVIII light chain to active-site-blocked Glu-Gly-Arg-FIXa (EGR-FIXa) (IC50; 19.2 and 42.9 μM, respectively), in keeping with a possible role for the 1790-1798 in FIXa interactions. Surface plasmon resonance-based analyses demonstrated that variants of FVIII, in which the clustered acidic residues (E1793/E1794/D1793) or F1816 contained substituted alanine, bound to immobilized biotin labeled-Phe-Pro-Arg-FIXa (bFPR-FIXa) with a 1.5-2.2-fold greater KD compared to wild-type FVIII (WT). Similarly, FXa generation assays indicated that E1793A/E1794A/D1795A and F1816A mutants increased the Km by 1.6-2.8-fold relative to WT. Furthermore, E1793A/E1794A/D1795A/F1816A mutant showed that the Km was increased by 3.4-fold and the Vmax was decreased by 0.75-fold, compared to WT. Molecular dynamics simulation analyses revealed the subtle changes between WT and E1793A/E1794A/D1795A mutant, supportive of the contribution of these residues for FIXa interaction.

CONCLUSION

The 1790-1798 region in the A3 domain, especially clustered acidic residues E1793/E1794/D1795, contains a FIXa-interactive site.

SAXS analysis of the intrinsic tenase complex bound to a lipid nanodisc highlights intermolecular contacts between factors VIIIa/IXa

The intrinsic tenase (Xase) complex, formed by factors (f) VIIIa and fIXa, forms on activated platelet surfaces and catalyzes the activation of factor X to Xa, stimulating thrombin production in the blood coagulation cascade. The structural organization of the membrane-bound Xase complex remains largely unknown, hindering our understanding of the structural underpinnings that guide Xase complex assembly. Here, we aimed to characterize the Xase complex bound to a lipid nanodisc with biolayer interferometry (BLI), Michaelis-Menten kinetics, and small-angle X-ray scattering (SAXS). Using immobilized lipid nanodiscs, we measured binding rates and nanomolar affinities for fVIIIa, fIXa, and the Xase complex. Enzyme kinetic measurements demonstrated the assembly of an active enzyme complex in the presence of lipid nanodiscs. An ab initio molecular envelope of the nanodisc-bound Xase complex allowed us to computationally model fVIIIa and fIXa docked onto a flexible lipid membrane and identify protein-protein interactions. Our results highlight multiple points of contact between fVIIIa and fIXa, including a novel interaction with fIXa at the fVIIIa A1-A3 domain interface. Lastly, we identified hemophilia A/B-related mutations with varying severities at the fVIIIa/fIXa interface that may regulate Xase complex assembly. Together, our results support the use of SAXS as an emergent tool to investigate the membrane-bound Xase complex and illustrate how mutations at the fVIIIa/fIXa dimer interface may disrupt or stabilize the activated enzyme complex.

Unveiling the influence of factor VIII physicochemical properties on hemophilia A phenotype through an in silico methodology

BACKGROUND AND OBJECTIVES

Hemophilia A (HA) is an X-linked blood disorder. It is caused by pathogenic F8 gene variants, among which missense mutations are the most prevalent. The resulting amino acid substitutions may have different impacts on physicochemical properties and, consequently, on protein functionality. Regular prediction tools do not include structural elements and their physiological significance, which hampers our ability to functionally link variants to disease phenotype, opening an ample field for investigation. The present study aims to elucidate how physicochemical changes generated by substitutions in different protein domains relate to HA, and which of these features are more consequential to protein function and its impact on HA phenotype.

METHODS

An in silico evaluation of 71 F8 variants found in patients with different HA phenotypes (mild, moderate, severe) was performed to understand protein modifications and functional impact. Homology modeling was used for the structural analysis of physicochemical changes including electrostatic potential, hydrophobicity, solvent-accessible/excluded surface areas, disulfide disruptions, and substitutions indexes. These variants and properties were analyzed by hierarchical clustering analysis (HCA) and principal component analysis (PCA), independently and in combination, to investigate their relative contribution.

RESULTS

About 69% of variants show electrostatic changes, and almost all show hydrophobicity and surface area modifications. HCA combining all physicochemical properties analyzed was better in reflecting the impact of different variants in disease severity, more so than the single feature analysis. On the other hand, PCA led to the identification of prominent properties involved in the clustering results for variants of different domains.

CONCLUSIONS

The methodology developed here enables the assessment of structural features not available in other prediction tools (e.g., surface distribution of electrostatic potential), evaluating what kind of physicochemical changes are involved in FVIII functional disruption. HCA results allow distinguishing substitutions according to their properties, and yielded clusters which were more homogeneous in phenotype. All evaluated properties are involved in determining disease severity. The nature, as well as the position of the variants in the protein, were shown to be relevant for physicochemical changes, demonstrating that all these aspects must be collectively considered to fine-tune an approach to predict HA severity.

Hydrogen-Deuterium Exchange Mass Spectrometry Identifies Activated Factor IX-Induced molecular Changes in Activated Factor VIII

Hydrogen-deuterium exchange mass spectrometry (HDX-MS) was employed to gain insight into the changes in factor VIII (FVIII) that occur upon its activation and assembly with activated factor IX (FIXa) on phospholipid membranes. HDX-MS analysis of thrombin-activated FVIII (FVIIIa) revealed a marked increase in deuterium incorporation of amino acid residues along the A1-A2 and A2-A3 interface. Rapid dissociation of the A2 domain from FVIIIa can explain this observation. In the presence of FIXa, enhanced deuterium incorporation at the interface of FVIIIa was similar to that of FVIII. This is compatible with the previous finding that FIXa contributes to A2 domain retention in FVIIIa. A2 domain region Leu631-Tyr637, which is not part of the interface between the A domains, also showed a marked increase in deuterium incorporation in FVIIIa compared with FVIII. Deuterium uptake of this region was decreased in the presence of FIXa beyond that observed in FVIII. This implies that FIXa alters the conformation or directly interacts with this region in FVIIIa. Replacement of Val634 in FVIII by alanine using site-directed mutagenesis almost completely impaired the ability of the activated cofactor to enhance the activity of FIXa. Surface plasmon resonance analysis revealed that the rates of A2 domain dissociation from FVIIIa and FVIIIa-Val634Ala were indistinguishable. HDX-MS analysis showed, however, that FIXa was unable to retain the A2 domain in FVIIIa-Val634Ala. The combined results of this study suggest that the local structure of Leu631-Tyr637 is altered by FIXa and that this region contributes to the cofactor function of FVIII.

Unique surface-exposed hydrophobic residues in the C1 domain of factor VIII contribute to cofactor function and von Willebrand factor binding

BACKGROUND

The identity of the amino acid regions of factor VIII (FVIII) that contribute to factor IXa (FIXa) and von Willebrand factor (VWF) binding has not been fully resolved. Previously, we observed that replacing the FVIII C1 domain for the one of factor V (FV) markedly reduces VWF binding and cofactor function. Compared to the FV C1 domain, this implies that the FVIII C1 domain comprises unique surface-exposed elements involved in VWF and FIXa interaction.

OBJECTIVE

The aim of this study is to identify residues in the FVIII C1 domain that contribute to VWF and FIXa binding.

METHODS

Structures and primary sequences of FVIII and FV were compared to identify surface-exposed residues unique to the FVIII C1 domain. The identified residues were replaced with alanine residues to identify their role in FIXa and VWF interaction. This role was assessed employing surface plasmon resonance analysis studies and enzyme kinetic assays.

RESULTS

Five surface-exposed hydrophobic residues unique to the FVIII C1 domain, ie, F2035, F2068, F2127, V2130, I2139 were identified. Functional analysis indicated that residues F2068, V2130, and especially F2127 contribute to VWF and/or FIXa interaction. Substitution into alanine of the also surface-exposed V2125, which is spatially next to F2127, affected only VWF binding.

CONCLUSION

The surface-exposed hydrophobic residues in C1 domain contribute to cofactor function and VWF binding. These findings provide novel information on the fundamental role of the C1 domain in FVIII life cycle.

Mild hemophilia A patient with novel Pro1809Leu mutation develops an anti-C2 antibody inhibiting allogeneic but not autologous factor VIII activity

BACKGROUND

In mild hemophilia A (MHA) patients, the risk of inhibitor development is generally low, but some factor VIII (FVIII) gene missense mutations are associated with a higher inhibitor incidence.

OBJECTIVE

To investigate the mechanism(s) of inhibitor development in MHA.

METHODS AND RESULTS

A patient, HA78, with MHA with a novel P1809L missense mutation in the A3 domain, exhibited significant residual FVIII activity (

FVIII

C ~10 IU dL(-1) ), despite the development of an inhibitor (5.6 BU mL(-1) ). Purified HA78-IgG significantly depressed

FVIII

C from normal plasma but not from patient's plasma without inhibitor, indicating that this IgG inhibited allogeneic but not autologous FVIII. The HA78-IgG blocked thrombin and FXa-catalyzed FVIII cleavage but had little effect on FVIII binding to von Willebrand factor and phospholipid. The IgG recognized a C2 epitope close or overlapping the previously described anti-C2 ESH8 epitope. Similarly, a recombinant FVIII-P1809L mutant was little inactivated by HA78-IgG. This mutant demonstrated ~3-fold lower binding affinities to von Willebrand factor and phospholipid compared with wild-type, while reactions with thrombin or FXa were not impaired. Reaction of FVIII-P1809L with the alternative anti-C2 ESH4 showed only an ~20% inhibition compared with wild-type FVIII but was similar to wild-type after incubation with ESH8. A surface plasmon resonance-based assay demonstrated that anti-C2 ESH4 bound to FVIII-P1809L with ~10(2) -fold lower affinity compared with ESH8.

CONCLUSION

These results indicated that the P1809L mutation in A3 induced the conformational change in the FVIII molecule that hampered antigenic determinant(s) located in the C2 domain and might result in the inhibitor development.

Dimeric Organization of Blood Coagulation Factor VIII bound to Lipid Nanotubes

Membrane-bound Factor VIII (FVIII) has a critical function in blood coagulation as the pro-cofactor to the serine-protease Factor IXa (FIXa) in the FVIIIa-FIXa complex assembled on the activated platelet membrane. Defects or deficiency of FVIII cause Hemophilia A, a mild to severe bleeding disorder. Despite existing crystal structures for FVIII, its membrane-bound organization has not been resolved. Here we present the dimeric FVIII membrane-bound structure when bound to lipid nanotubes, as determined by cryo-electron microscopy. By combining the structural information obtained from helical reconstruction and single particle subtomogram averaging at intermediate resolution (15-20 Å), we show unambiguously that FVIII forms dimers on lipid nanotubes. We also demonstrate that the organization of the FVIII membrane-bound domains is consistently different from the crystal structure in solution. The presented results are a critical step towards understanding the mechanism of the FVIIIa-FIXa complex assembly on the activated platelet surface in the propagation phase of blood coagulation.

Structural insights into the interaction of blood coagulation co-factor VIIIa with factor IXa: a computational protein-protein docking and molecular dynamics refinement study

Coagulation factor X (FX) zymogen activation by factor IXa (FIXa) enzyme plays a critical role in the middle-phase of coagulation cascade. The activation process is catalytically inert and requires FIXa binding and complex formation with co-factor VIIIa (FVIIIa). In order to understand the structural details of the FVIIIa:FIXa complex, we employed knowledge-driven protein-protein docking and aqueous-phase MD refinement methods to develop a stable structural complex between FVIIIa and FIXa. The model shows that all four domains of FIXa wrap across FVIIIa that spans the co-factor binding surface of A2, A3 and C1 domains. The region surrounding the 558-helix of the A2-domain of FVIIIa is predicted to be the key interaction site with the helical segments of Lys293-Lys301 and Asp332-Arg338 residues of the serine-protease domain of FIXa. The hydrophobic helical stack between the GLA and EGF1 domains of FIXa is predicted to be primary interacting region with the A3-C2 domain interface of FVIIIa.