Cryo-EM structure of the prothrombin-prothrombinase complex

Elucidating the complex membrane binding of a protein with multiple anchoring domains using extHMMM

Membrane binding is a crucial mechanism for many proteins, but understanding the specific interactions between proteins and membranes remains a challenging endeavor. Coagulation factor Va (FVa) is a large protein whose membrane interactions are complicated due to the presence of multiple anchoring domains that individually can bind to lipid membranes. Using molecular dynamics simulations, we investigate the membrane binding of FVa and identify the key mechanisms that govern its interaction with membranes. Our results reveal that FVa can either adopt an upright or a tilted molecular orientation upon membrane binding. We further find that the domain organization of FVa deviates (sometimes significantly) from its crystallographic reference structure, and that the molecular orientation of the protein matches with domain reorganization to align the C2 domain toward its favored membrane-normal orientation. We identify specific amino acid residues that exhibit contact preference with phosphatidylserine lipids over phosphatidylcholine lipids, and we observe that mostly electrostatic effects contribute to this preference. The observed lipid-binding process and characteristics, specific to FVa or common among other membrane proteins, in concert with domain reorganization and molecular tilt, elucidate the complex membrane binding dynamics of FVa and provide important insights into the molecular mechanisms of protein-membrane interactions. An updated version of the HMMM model, termed extHMMM, is successfully employed for efficiently observing membrane bindings of systems containing the whole FVa molecule.

Structural basis for inhibition of coagulation factor VIII reveals a shared antigenic hotspot on the C1 domain

BACKGROUND

Hemophilia A arises from dysfunctional or deficient coagulation factor (F)VIII and leads to inefficient fibrin clot formation and uncontrolled bleeding events. The development of antibody inhibitors is a clinical complication in hemophilia A patients receiving FVIII replacement therapy. LE2E9 is an anti-C1 domain inhibitor previously isolated from a mild/moderate hemophilia A patient and disrupts FVIII interactions with von Willebrand factor and FIXa, though the intermolecular contacts that underpin LE2E9-mediated FVIII neutralization are undefined.

OBJECTIVES

To determine the structure of the complex between FVIII and LE2E9 and characterize its mechanism of inhibition.

METHODS

FVIII was bound to the antigen binding fragment (Fab) of NB2E9, a recombinant construct of LE2E9, and its structure was determined by cryogenic electron microscopy.

RESULTS

This report communicates the 3.46 Å structure of FVIII bound to NB2E9, with its epitope comprising FVIII residues S2040 to Y2043, K2065 to W2070, and R2150 to H2155. Structural analysis reveals that the LE2E9 epitope overlaps with portions of the epitope for 2A9, a murine-derived inhibitor, suggesting that these residues represent a shared antigenic region on the C1 domain between FVIII-/- mice and hemophilia A patients. Furthermore, the FVIII:NB2E9 structure elucidates the orientation of the LE2E9 glycan, illustrating how the glycan sterically blocks interactions between the FVIII C1 domain and the von Willebrand factor D' domain. A putative model of the FVIIIa:FIXa complex suggests potential clashing between the NB2E9 glycan and FIXa light chain.

CONCLUSION

These results describe an antigenic "hotspot" on the FVIII C1 domain and provide a structural basis for engineering FVIII replacement therapeutics with reduced antigenicity.

Nanodiscs for the study of membrane proteins

Nanodiscs represent a versatile tool for studies of membrane proteins and protein-membrane interactions under native-like conditions. Multiple variations of the Nanodisc platform, as well as new experimental methods, have been recently developed to understand various aspects of structure, dynamics and functional properties of systems involved in signaling, transport, blood coagulation and many other critically important processes. In this mini-review, we focus on some of these exciting recent developments that utilize the Nanodisc platform.

Cryo-EM structure and functional basis of prothrombin recognition by a type I antiprothrombin antiphospholipid antibody

ABSTRACT

Antiprothrombin antibodies are found in antiphospholipid patients, but how they interact with prothrombin remains elusive. Prothrombin adopts closed and open forms. We recently discovered type I and type II antibodies and proposed that type I recognizes the open form. In this study, we report the discovery and structural and functional characterization in human plasma of a type I antibody, POmAb (prothrombin open monoclonal antibody). Using surface plasmon resonance and single-molecule spectroscopy, we show that POmAb interacts with kringle-1 of prothrombin, shifting the equilibrium toward the open form. Using single-particle cryogenic electron microscopy (cryo-EM), we establish that the epitope targeted by POmAb is in kringle-1, comprising an extended binding interface centered at residues R90-Y93. The 3.2-Å cryo-EM structure of the complex reveals that the epitope overlaps with the position occupied by the protease domain of prothrombin in the closed state, explaining the exclusive binding of POmAb to the open form. In human plasma, POmAb prolongs phospholipid-initiated and diluted Russell's viper venom clotting time, which could be partly rescued by excess phospholipids, indicating POmAb is an anticoagulant but exerts a weak lupus anticoagulant effect. These studies reveal the structural basis of prothrombin recognition by a type I antiphospholipid antibody and uncover an exciting new strategy to achieve anticoagulation in human plasma.

Membrane binding and lipid-protein interaction of the C2 domain from coagulation factor V

Anchoring of coagulation factors to anionic regions of the membrane involves the C2 domain as a key player. The rate of enzymatic reactions of the coagulation factors is increased by several orders of magnitude upon membrane binding. However, the precise mechanisms behind the rate acceleration remain unclear, primarily because of a lack of understanding of the conformational dynamics of the C2-containing factors and corresponding complexes. We elucidate the membrane-bound form of the C2 domain from human coagulation factor V (FV-C2) by characterizing its membrane binding the specific lipid-protein interactions. Employing all-atom molecular dynamics simulations and leveraging the highly mobile membrane-mimetic (HMMM) model, we observed spontaneous binding of FV-C2 to a phosphatidylserine (PS)-containing membrane within 2-25 ns across twelve independent simulations. FV-C2 interacted with the membrane through three loops (spikes 1-3), achieving a converged, stable orientation. Multiple HMMM trajectories of the spontaneous membrane binding provided extensive sampling and ample data to examine the membrane-induced effects on the conformational dynamics of C2 as well as specific lipid-protein interactions. Despite existing crystal structures representing presumed "open" and "closed" states of FV-C2, our results revealed a continuous distribution of structures between these states, with the most populated structures differing from both "open" and "closed" states observed in crystal environments. Lastly, we characterized a putative PS-specific binding site formed by K23, Q48, and S78 located in the groove enclosed by spikes 1-3 (PS-specificity pocket), suggesting a different orientation of a bound headgroup moiety compared to previous proposals based upon analysis of static crystal structures.

A simple method to resolve rate constants when the binding mechanism obeys induced fit or conformational selection

Many interactions involving a ligand and its molecular target are studied by rapid kinetics using a stopped-flow apparatus. Information obtained from these studies is often limited to a single, saturable relaxation that is insufficient to resolve all independent rate constants even for a two-step mechanism of binding obeying induced fit (IF) or conformational selection (CS). We introduce a simple method of general applicability where this limitation is overcome. The method accurately reproduces the rate constants for ligand binding to the serine protease thrombin determined independently from the analysis of multiple relaxations. Application to the inactive zymogen precursor of thrombin, prethrombin-2, resolves all rate constants for a binding mechanism of IF or CS from a single, saturable relaxation. Comparison with thrombin shows that the prethrombin-2 to thrombin conversion enhances ligand binding to the active site not by improving accessibility through the value of kon but by reducing the rate of dissociation koff. The conclusion holds regardless of whether binding is interpreted in terms of IF or CS and has general relevance for the mechanism of zymogen activation of serine proteases. The method also provides a simple test of the validity of IF and CS and indicates when more complex mechanisms of binding should be considered.

Humanization and functional characterization of enhanced coagulation factor IX variants identified through ancestral sequence reconstruction

BACKGROUND

Laboratory resurrection of ancient coagulation factor (F) IX variants generated through ancestral sequence reconstruction led to the discovery of a FIX variant, designated An96, which possesses enhanced specific activity independent of and additive to that provided by human p.Arg384Lys, referred to as FIX-Padua.

OBJECTIVES

The goal of the current study was to identify the amino acid substitution(s) responsible for the enhanced activity of An96 and create a humanized An96 FIX transgene for gene therapy application.

METHODS

Reductionist screening approaches, including domain swapping and scanning residue substitution, were used and guided by one-stage FIX activity assays. In vitro characterization of top candidates included recombinant high-purity preparation, specific activity determination, and enzyme kinetic analysis. Final candidates were packaged into adeno-associated viral (AAV) vectors and delivered to hemophilia B mice.

RESULTS

Five of 42 total amino acid substitutions in An96 appear sufficient to retain the enhanced activity of An96 in an otherwise human FIX variant. Additional substitution of the Padua variant further increased the specific activity 5-fold. This candidate, designated ET9, demonstrated 51-fold greater specific activity than hFIX. AAV2/8-ET9 treated hemophilia B mice produced plasma FIX activities equivalent to those observed previously for AAV2/8-An96-Padua, which were 10-fold higher than AAV2/8-hFIX-Padua.

CONCLUSION

Starting from computationally inferred ancient FIX sequences, novel amino acid substitutions conferring activity enhancement were identified and translated into an AAV-FIX gene therapy cassette demonstrating high potency. This ancestral sequence reconstruction discovery and sequence mapping refinement approach represents a promising platform for broader protein drug and gene therapy candidate optimization.

Structural architecture of the acidic region of the B domain of coagulation factor V

BACKGROUND

Coagulation factor (F)V features an A1-A2-B-A3-C1-C2 domain organization and functions as the inactive precursor of FVa, a component of the prothrombinase complex required for rapid thrombin generation in the penultimate step of the coagulation cascade. An intramolecular interaction within the large B domain (residues 710-1545) involves the basic region (BR, residues 963-1008) and acidic region (AR, residues 1493-1537) and locks FV in its inactive state. However, structural information on this important regulatory interaction or on the separate architecture of the AR and BR remains elusive due to conformational disorder of the B domain.

OBJECTIVES

To reveal the structure of the BR-AR interaction or of its separate components.

METHODS

The structure of FV is solved by cryogenic electron microscopy.

RESULTS

A new 3.05 Å resolution cryogenic electron microscopy structure of FV confirms the overall organization of the A and C domains but resolves the segment 1507 to 1545 within a largely disordered B domain. The segment contains most of the AR and is organized as recently reported in FV short, a spliced variant of FV with a significantly shorter and less disordered B domain.

CONCLUSION

The similar architecture of the AR in FV and FV short provides structural context for physiologically important interactions of this region with the BR in FV and with the basic C-terminal end of tissue factor pathway inhibitor α in FV short.

The Prothrombin-Prothrombinase Interaction

The hemostatic response to vascular injury entails a sequence of proteolytic events where several inactive zymogens of the trypsin family are converted to active proteases. The cascade starts with exposure of tissue factor from the damaged endothelium and culminates with conversion of prothrombin to thrombin in a reaction catalyzed by the prothrombinase complex composed of the enzyme factor Xa, cofactor Va, Ca2+, and phospholipids. This cofactor-dependent activation is paradigmatic of analogous reactions of the blood coagulation and complement cascades, which makes elucidation of its molecular mechanism of broad significance to the large class of trypsin-like zymogens to which prothrombin belongs. Because of its relevance as the most important reaction in the physiological response to vascular injury, as well as the main trigger of pathological thrombotic complications, the mechanism of prothrombin activation has been studied extensively. However, a molecular interpretation of this mechanism has become available only recently from important developments in structural biology. Here we review current knowledge on the prothrombin-prothrombinase interaction and outline future directions for the study of this key reaction of the coagulation cascade.

Factor V variants in bleeding and thrombosis

A state-of-the-art lecture titled "Factor V variants in bleeding and thrombosis" was presented at the International Society on Thrombosis and Haemostasis (ISTH) congress in 2023. Blood coagulation is a finely regulated cascade of enzymatic reactions culminating in thrombin formation and fibrin deposition at the site of injury. Factor V (FV) plays a central role in this process, as its activated form is an essential procoagulant cofactor in prothrombin activation. However, other molecular forms of FV act as anticoagulant cofactors of activated protein C and tissue factor pathway inhibitor α, respectively, thereby contributing to the regulation of coagulation. This dual procoagulant and anticoagulant character makes FV a central regulator of the hemostatic balance, and quantitative and qualitative alterations of FV may be associated with an increased risk of bleeding or venous thrombosis. Here, we review the procoagulant and anticoagulant functions of FV and the manifold mechanisms by which F5 gene mutations may affect the balance between these opposite functions and thereby predispose individuals to bleeding or venous thrombosis. In particular, we discuss our current understanding of the 3 main pathological conditions related to FV, namely FV deficiency, activated protein C resistance, and the overexpression of FV-short, a minor splicing isoform of FV with tissue factor pathway inhibitor α-dependent anticoagulant properties and an emerging role as a key regulator of the initiation of coagulation. Finally, we summarize relevant new data on this topic presented during the 2023 ISTH Congress.

Advances of Blood Coagulation Factor XIII in Bone Healing

The biologic process of bone healing is complicated, involving a variety of cells, cytokines, and growth factors. As a result of bone damage, the activation of a clotting cascade leads to hematoma with a high osteogenic potential in the initial stages of healing. A major factor involved in this course of events is clotting factor XIII (FXIII), which can regulate bone defect repair in different ways during various stages of healing. Autografts and allografts often have defects in clinical practice, making the development of advanced materials that support bone regeneration a critical requirement. Few studies, however, have examined the promotion of bone healing by FXIII in combination with biomaterials, in particular, its effect on blood coagulation and osteogenesis. Therefore, we mainly summarized the role of FXIII in promoting bone regeneration by regulating the extracellular matrix and type I collagen, bone-related cells, angiogenesis, and platelets, and described the research progress of FXIII = related biomaterials on osteogenesis. This review provides a reference for investigators to explore the mechanism by which FXIII promotes bone healing and the combination of FXIII with biomaterials to achieve targeted bone tissue repair.

Lactadherin's Multistate Binding Predicts Stable Membrane-Bound Conformations of Factors V and VIII's C Domains

Protein binding to negatively charged lipids is essential for maintaining numerous vital cellular processes where its dysfunction can lead to various diseases. One such protein that plays a crucial role in this process is lactadherin, which competes with coagulation factors for membrane binding sites to regulate blood clotting. Despite identifying key binding regions of these proteins through structural and biochemical studies, models incorporating membrane dynamics are still lacking. In this study, we report on the multimodal binding of lactadherin and use it to gain insight into the binding mechanisms of its C domain homologs, factor V and factor VIII. Molecular dynamics simulations enhanced with the highly mobile mimetic model enabled the determination of lactadherin's multimodal binding on membranes that revealed critical interacting residues consistent with prior NMR and mutagenesis data. The binding occurred primarily via two dynamic structural ensembles: an inserted state and an unreported, highly conserved side-lying state driven by a cationic patch. We utilized these findings to analyze the membrane binding domains of coagulation factors V and VIII and identified their preferred membrane-bound conformations. Specifically, factor V's C domains maintained an inserted state, while factor VIII preferred a tilted, side-lying state that permitted antibody binding. Insight into lactadherin's atomistically resolved membrane interactions from a multistate perspective can guide new therapeutic opportunities in treating diseases related to blood coagulation.

Lipid nanodiscs as a template for high-resolution cryo-EM structures of peripheral membrane proteins

Peripheral membrane proteins are ubiquitous throughout cell biology and are required for a variety of cellular processes such as signal transduction, membrane trafficking, and autophagy. Transient binding to the membrane has a profound impact on protein function, serving to induce conformational changes and alter biochemical and biophysical parameters by increasing the local concentration of factors and restricting diffusion to two dimensions. Despite the centrality of the membrane in serving as a template for cell biology, there are few reported high-resolution structures of peripheral membrane proteins bound to the membrane. We analyzed the utility of lipid nanodiscs to serve as a template for cryo-EM analysis of peripheral membrane proteins. We tested a variety of nanodiscs and we report a 3.3 Å structure of the AP2 clathrin adaptor complex bound to a 17-nm nanodisc, with sufficient resolution to visualize a bound lipid head group. Our data demonstrate that lipid nanodiscs are amenable to high-resolution structure determination of peripheral membrane proteins and provide a framework for extending this analysis to other systems.

New insight into the traditional model of the coagulation cascade and its regulation: illustrated review of a three-dimensional view

The coagulation process relies on an intricate network of three-dimensional structural interactions and subtle biological regulations. In the present review, we illustrate the state of the art of the structural biology of the coagulation cascade by surveying the Protein Data Bank and the EBI AlphaFold databases. Investigations performed in the last decade have provided structural information on essentially all players involved in the process. Indeed, the initial characterization of specific and rather canonical domains has been progressively extended to complicated multidomain proteins. Recently, the application of cryogenic electron microscopy techniques has unraveled the structural features of highly complex coagulation factors, which has led to enhanced understanding. This review initially focuses on the structure of the individual factors as a function of their involvement in intrinsic, extrinsic, and common pathways. A specific emphasis is given to what is known or unknown on the structural basis of each step of the cascade. Available data providing clues on the structural recognition of the factors involved in the functional partnerships of the pathways are illustrated. Recent structures of important complexes formed by these proteins with regulators are described, focusing on the drugs used as anticoagulants and on their reversal agents. Finally, we highlight the different roles that innovative biomolecules such as aptamers may have in the regulation of the cascade.

Monitoring prothrombin activation in plasma through loss of Förster resonance energy transfer

BACKGROUND

Current assays that monitor thrombin generation in plasma rely on fluorogenic substrates to follow the kinetics of zymogen activation, which may be complicated by substrate cleavage from other proteases. In addition, these assays depend on activation following cleavage at the prothrombin R320 site and fail to report the cleavage at the alternative R271 site, leading to the shedding of the auxiliary Gla and kringle domains of prothrombin.

OBJECTIVES

To develop a plasma assay that directly monitors prothrombin activation independent of fluorogenic substrate hydrolysis.

METHODS

Cleavage at the R271 site of prothrombin is monitored through loss of Förster resonance energy transfer in plasma coagulated along the extrinsic or intrinsic pathway.

RESULTS

The availability of factor (F)V in plasma strongly influences the rate of prothrombin activation. The rate of thrombin formation is equally perturbed in FV or prothrombin-depleted plasma, implicating that the thrombin-catalyzed feedback reactions that amplify the coagulation response play an important role in generating sufficient amounts of FVa required for the assembly of prothrombinase. Congenital deficiencies in FVIII and FIX significantly slow down cleavage at R271 in plasma coagulated along the extrinsic and intrinsic pathways. Prothrombin activation in FXI-deficient plasma is only perturbed when coagulation is triggered along the intrinsic pathway.

CONCLUSION

The Förster resonance energy transfer assay enables direct monitoring of prothrombin activation through cleavage at R271 without the need for fluorogenic substrates. The assay is sensitive enough to assess how deficiencies in coagulation factors affect thrombin formation.

Cryo-EM structure of coagulation factor V short

Coagulation factor V (fV) is the precursor of activated fV (fVa), an essential component of the prothrombinase complex required for the rapid activation of prothrombin in the penultimate step of the coagulation cascade. In addition, fV regulates the tissue factor pathway inhibitor α (TFPIα) and protein C pathways that inhibit the coagulation response. A recent cryogenic electron microscopy (cryo-EM) structure of fV has revealed the architecture of its A1-A2-B-A3-C1-C2 assembly but left the mechanism that keeps fV in its inactive state unresolved because of an intrinsic disorder in the B domain. A splice variant of fV, fV short, carries a large deletion of the B domain that produces constitutive fVa-like activity and unmasks epitopes for the binding of TFPIα. The cryo-EM structure of fV short was solved at 3.2 Å resolution and revealed the arrangement of the entire A1-A2-B-A3-C1-C2 assembly. The shorter B domain stretches across the entire width of the protein, making contacts with the A1, A2, and A3 domains but suspended over the C1 and C2 domains. In the portion distal to the splice site, several hydrophobic clusters and acidic residues provide a potential binding site for the basic C-terminal end of TFPIα. In fV, these epitopes may bind intramolecularly to the basic region of the B domain. The cryo-EM structure reported in this study advances our understanding of the mechanism that keeps fV in its inactive state, provides new targets for mutagenesis and facilitates future structural analysis of fV short in complex with TFPIα, protein S, and fXa.

A bispecific inhibitor of complement and coagulation blocks activation in complementopathy models via a novel mechanism

Inhibitors of complement and coagulation are present in the saliva of a variety of blood-feeding arthropods that transmit parasitic and viral pathogens. Here, we describe the structure and mechanism of action of the sand fly salivary protein lufaxin, which inhibits the formation of the central alternative C3 convertase (C3bBb) and inhibits coagulation factor Xa (fXa). Surface plasmon resonance experiments show that lufaxin stabilizes the binding of serine protease factor B (FB) to C3b but does not detectably bind either C3b or FB alone. The crystal structure of the inhibitor reveals a novel all β-sheet fold containing 2 domains. A structure of the lufaxin-C3bB complex obtained via cryo-electron microscopy (EM) shows that lufaxin binds via its N-terminal domain at an interface containing elements of both C3b and FB. By occupying this spot, the inhibitor locks FB into a closed conformation in which proteolytic activation of FB by FD cannot occur. C3bB-bound lufaxin binds fXa at a separate site in its C-terminal domain. In the cryo-EM structure of a C3bB-lufaxin-fXa complex, the inhibitor binds to both targets simultaneously, and lufaxin inhibits fXa through substrate-like binding of a C-terminal peptide at the active site as well as other interactions in this region. Lufaxin inhibits complement activation in ex vivo models of atypical hemolytic uremic syndrome (aHUS) and paroxysmal nocturnal hemoglobinuria (PNH) as well as thrombin generation in plasma, providing a rationale for the development of a bispecific inhibitor to treat complement-related diseases in which thrombosis is a prominent manifestation.

Natural anticoagulant discovery, the gift that keeps on giving: finding FV-Short

The complex reactions of blood coagulation are balanced by several natural anticoagulants resulting in tuned hemostasis. During several decades, the knowledge base of the natural anticoagulants has greatly increased and we have also learned about antiinflammatory and cytoprotective activities expressed by antithrombin and activated protein C (APC). Some coagulation proteins have also been found to function as anticoagulants; e.g., thrombin when bound to thrombomodulin activates protein C. Another example is factor V (FV), which in addition to being a procofactor to FVa has emerged as an anticoagulant. The discovery of APC resistance, caused by FVLeiden, as a thrombosis risk factor resulted in the identification of FV as an APC cofactor working in synergy with protein S in the regulation of FVIIIa in the Xase complex. More recently, a natural anticoagulant FV splice isoform (FV-Short) was discovered when investigating the East Texas bleeding disorder. In FV-Short, the truncated B domain exposes a high-affinity binding site for tissue factor pathway inhibitor alpha (TFPIα), and together with protein S a high-affinity trimolecular complex is generated. The FXa-inhibitory activity of TFPIα is synergistically stimulated by FV-Short and protein S. The circulating FV-Short/protein S/TFPIα complex concentration is normally low (≈0.2 nM) but provides an anticoagulant threshold. In the East Texas bleeding, the concentration of the complex, and thus the threshold, is increased 10-fold, which results in bleeding manifestations. The anticoagulant properties of FV were discovered during investigations of individual patients and follow the great tradition of bed-to-bench and bench-to-bed research in the coagulation field.

Cryo-EM structures of coagulation factors

A State of the Art lecture titled "Cryo-EM structures of coagulation factors" was presented at the ISTH Congress in 2022. Cryogenic electron microscopy (cryo-EM) is a revolutionary technique capable of solving the structure of high molecular weight proteins and their complexes, unlike nuclear magnetic resonance (NMR), and under conditions not biased by crystal contacts, unlike X-ray crystallography. These features are particularly relevant to the analysis of coagulation factors that are too big for NMR and often recalcitrant to X-ray investigation. Using cryo-EM, we have solved the structures of coagulation factors V and Va, prothrombinase on nanodiscs, and the prothrombin-prothrombinase complex. These structures have advanced basic knowledge in the field of thrombosis and hemostasis, especially on the function of factor V and the molecular mechanism for prothrombin activation, and set the stage for exciting new lines of investigation. Finally, we summarize relevant new data on this topic presented during the 2022 ISTH Congress.

Structural insights into blood coagulation factor VIII: Procoagulant complexes, membrane binding, and antibody inhibition

Advances in structural studies of blood coagulation factor VIII (FVIII) have provided unique insight into FVIII biochemistry. Atomic detail models of the B domain-deleted FVIII structure alone and in complex with its circulatory partner, von Willebrand factor (VWF), provide a structure-based rationale for hemophilia A-associated mutations which impair FVIII stability and increase FVIII clearance rates. In this review, we discuss the findings from these studies and their implications toward the design of a recombinant FVIII with improved circulatory half-life. Additionally, we highlight recent structural studies of FVIII bound to inhibitory antibodies that have refined our understanding of FVIII binding to activated platelet membranes and formation of the intrinsic tenase complex. The combination of bioengineering and structural efforts to understand FVIII biochemistry will improve therapeutics for treating hemophilia A, either through FVIII replacement therapeutics, immune tolerance induction, or gene therapy approaches.