F2

Prothrombin (F2) is the zymogen precursor of thrombin and is essential for both developmental and postnatal hemostasis, as homozygous knockout in mice causes mid-gestation lethality from failed yolk-sac vascular integrity and neonatal death from hemorrhage (PMID:11171556, PMID:9714898). It is activated by the prothrombinase complex (fXa/fVa on a phospholipid membrane), and a cryo-EM structure of the prothrombin-prothrombinase complex shows that fVa's A2 domain closes like a lid on the fXa protease to fix active-site orientation while a YDYQNRL segment sequesters R271 against D697 and directs R320 toward the fXa active site, establishing the meizothrombin activation pathway (PMID:35427420). Activation specificity is governed predominantly by exosite binding between substrate and enzyme rather than recognition of the scissile bond, and thrombin reversibly interconverts between zymogen-like and proteinase-like states depending on bound ligands (PMID:23809130). Prothrombin itself populates two conformations: the closed form favors R320 cleavage (meizothrombin) and the open form favors R271 cleavage (prethrombin-2), with R296 linking the open-closed equilibrium to cleavage-site exposure and removal of the Gla domain, kringles, or linkers stabilizing the open form (PMID:30992526); proper interdomain folding is required to stabilize the physiologically predominant closed conformation (PMID:42173288). Thrombosis-associated mutations potentiate coagulation by conferring antithrombin resistance and/or impairing the protein C pathway, including Phe382 substitutions that disrupt thrombomodulin binding and reduce protein C activation (PMID:38914130). Beyond its hemostatic role, circulating prothrombin/thrombin drives podocyte injury in glomerular disease (PMID:40152945), and hepatic prothrombin secretion is suppressed by ApoM acting through Arid5B (PMID:40719152).

1. 1988 Medium

   Resolving how the prothrombin gene is organized established its evolutionary relationship to other vitamin-K-dependent coagulation factors and the modular origin of its domains.

   Evidence Partial DNA sequencing of the cloned bovine prothrombin gene with intron-exon junction analysis

   PMID:3379642

   Open questions at the time

   • Does not address protein function or activation mechanism
   • Bovine gene; human regulatory architecture not directly characterized here
2. 1992 Medium

   Tested whether abnormal PIVKA-II prothrombin in hepatocellular carcinoma arises from coding mutations, ruling out Gla-domain and carboxylase-recognition-site lesions as the cause.

   Evidence PCR and direct sequencing of prothrombin exons I/II and cDNA from HCC cell lines and tumors

   PMID:1309675

   Open questions at the time

   • Negative result; mechanism of PIVKA-II production left unresolved
   • Limited to two exons and a small sample set
3. 1997 Medium

   Identified a membrane-associated prothrombin activator induced by tissue injury, indicating prothrombin can be activated locally outside classical plasma coagulation during inflammation and regeneration.

   Evidence Enzymatic activity assays on fractions from normal and chemically injured mouse tissues with phospholipid-requirement testing

   PMID:9276460

   Open questions at the time

   • Molecular identity of MAPA not defined
   • Physiological role in inflammation/regeneration inferred, not demonstrated
4. 2001 High

   Established that prothrombin is non-redundantly required in vivo by showing its loss causes embryonic and neonatal lethality, linking it to both developmental vascular integrity and postnatal hemostasis.

   Evidence Gene targeting (knockout) in mice with embryonic and neonatal phenotypic analysis

   PMID:11171556 PMID:9714898

   Open questions at the time

   • Does not separate developmental coagulation from possible non-hemostatic roles
   • Molecular cause of yolk-sac defect not dissected at the cellular level
5. 2013 High

   Reframed prothrombin activation specificity as exosite-driven rather than scissile-bond-driven and established that thrombin is an allosteric, ligand-responsive proteinase.

   Evidence Synthesis of biochemical/biophysical reconstitution and exosite mutagenesis studies

   PMID:23809130

   Open questions at the time

   • Structural basis of exosite engagement not yet visualized at this stage
   • Does not define how cofactors direct cleavage-site choice
6. 2019 Medium

   Demonstrated that prothrombin populates open and closed conformations that dictate which scissile bond (R320 vs R271) is cleaved, identifying R296 as the allosteric link.

   Evidence Limited chymotryptic proteolysis with domain-deletion variants of recombinant prothrombin

   PMID:30992526

   Open questions at the time

   • Single lab; equilibrium populations under physiological conditions not quantified
   • Coupling to cofactor binding inferred indirectly
7. 2022 High

   Provided the structural mechanism by which fVa orients fXa and positions prothrombin to enforce the meizothrombin pathway, explaining cofactor-directed cleavage-site selection.

   Evidence 4.1-Å cryo-EM of the fVa-fXa-prothrombin complex on nanodiscs

   PMID:35427420

   Open questions at the time

   • Captures one assembly state; dynamics of pathway switching not resolved
   • Does not address how mutations remodel the interface
8. 2023 Medium

   Connected upstream coagulation factor availability to prothrombin activation kinetics, showing FV supply and thrombin feedback control R271 cleavage in plasma.

   Evidence FRET assay monitoring R271 cleavage in factor-depleted (FV, FVIII, FIX, FXI) plasmas

   PMID:36931601

   Open questions at the time

   • Focuses on R271; meizothrombin (R320) pathway contribution not tracked
   • Single-lab assay system
9. 2023 Medium

   Assessed how a sodium/antithrombin-binding-site mutation (Belgrade) alters clot properties, showing effects on clot architecture without changing net fibrinolytic capacity.

   Evidence Recombinant mutant prothrombin in reconstituted plasma assays with clot turbidity and microscopy

   PMID:37918971

   Open questions at the time

   • Clinical thrombotic phenotype not established
   • Single-lab functional characterization
10. 2024 Medium

    Defined the molecular routes by which thrombosis-associated prothrombin mutations potentiate coagulation, separating antithrombin resistance from protein C pathway impairment.

    Evidence Recombinant mutant expression in HEK293T with thrombin generation and antithrombin/protein C functional assays

    PMID:38914130

    Open questions at the time

    • Heterozygous physiological context approximated in vitro
    • Structural basis of thrombomodulin-binding loss not shown
11. 2025 Medium

    Extended prothrombin biology beyond hemostasis by showing circulating prothrombin/thrombin drives podocyte injury, and identified ApoM-Arid5B as a hepatic regulator of prothrombin secretion.

    Evidence ASO knockdown and infusion in a rat glomerular disease model; ApoM/Arid5B gain- and loss-of-function in HepG2 and mice

    PMID:40152945 PMID:40719152

    Open questions at the time

    • Direct podocyte receptor/effector mechanism not defined
    • ApoM-Arid5B regulatory link characterized in single labs
12. 2026 Medium

    Showed that the physiologically predominant closed conformation depends on hierarchical domain folding and the formation of stabilizing interdomain contacts.

    Evidence Chemical denaturation with single-molecule and ensemble spectroscopy on recombinant prothrombin

    PMID:42173288

    Open questions at the time

    • Single study, not yet replicated
    • Folding pathway in cellular context not validated

• How conformational equilibrium, cofactor engagement, and disease mutations are integrated to control the choice between meizothrombin and prethrombin-2 pathways in vivo remains incompletely defined.
• No in vivo measurement linking conformational state to pathway choice
• Identity and regulation of tissue prothrombin activators undefined
• Mechanism of non-hemostatic prothrombin signaling in target tissues unresolved