Importance of Mg2+ in the Ca2+-Dependent Folding of the gamma-Carboxyglutamic Acid Domains of Vitamin K-Dependent clotting and anticlotting Proteins
[PROC_HUMAN] Defects in PROC are the cause of thrombophilia due to protein C deficiency, autosomal dominant (THPH3) [MIM:176860]. A hemostatic disorder characterized by impaired regulation of blood coagulation and a tendency to recurrent venous thrombosis. However, many adults with heterozygous disease may be asymptomatic. Individuals with decreased amounts of protein C are classically referred to as having type I protein C deficiency and those with normal amounts of a functionally defective protein as having type II deficiency.              Defects in PROC are the cause of thrombophilia due to protein C deficiency, autosomal recessive (THPH4) [MIM:612304]. A hemostatic disorder characterized by impaired regulation of blood coagulation and a tendency to recurrent venous thrombosis. It results in a thrombotic condition that can manifest as a severe neonatal disorder or as a milder disorder with late-onset thrombophilia. The severe form leads to neonatal death through massive neonatal venous thrombosis. Often associated with ecchymotic skin lesions which can turn necrotic called purpura fulminans, this disorder is very rare.
[EPCR_HUMAN] Binds activated protein C. Enhances protein C activation by the thrombin-thrombomodulin complex; plays a role in the protein C pathway controlling blood coagulation. [PROC_HUMAN] Protein C is a vitamin K-dependent serine protease that regulates blood coagulation by inactivating factors Va and VIIIa in the presence of calcium ions and phospholipids.
Publication Abstract from PubMed
Crystal structures of factor (F) VIIa/soluble tissue factor (TF), obtained under high Mg2+ (50mM Mg2+/5mM Ca2+), have three of seven Ca2+ sites in the gamma-carboxyglutamic acid (Gla) domain replaced by Mg2+ at positions 1, 4, and 7. We now report structures under low Mg2+ (2.5mM Mg2+/5mM Ca2+) as well as under high Ca2+ (5mM Mg2+/45mM Ca2+). Under low Mg2+, four Ca2+ and three Mg2+ occupy the same positions as in high-Mg2+ structures. Conversely, under low Mg2+, reexamination of the structure of Gla domain of activated Protein C (APC) complexed with soluble endothelial Protein C receptor (sEPCR) has position 4 occupied by Ca2+ and positions 1 and 7 by Mg2+. Nonetheless, in direct binding experiments, Mg2+ replaced three Ca2+ sites in the unliganded Protein C or APC. Further, the high-Ca2+ condition was necessary to replace Mg4 in the FVIIa/soluble TF structure. In biological studies, Mg2+ enhanced phospholipid binding to FVIIa and APC at physiological Ca2+. Additionally, Mg2+ potentiated phospholipid-dependent activations of FIX and FX by FVIIa/TF and inactivation of activated factor V by APC. Since APC and FVIIa bind to sEPCR involving similar interactions, we conclude that under the low-Mg2+ condition, sEPCR binding to APC-Gla (or FVIIa-Gla) replaces Mg4 by Ca4 with an attendant conformational change in the Gla domain omega-loop. Moreover, since phospholipid and sEPCR bind to FVIIa or APC via the omega-loop, we predict that phospholipid binding also induces the functional Ca4 conformation in this loop. Cumulatively, the data illustrate that Mg2+ and Ca2+ act in concert to promote coagulation and anticoagulation.
Structural and Functional Studies of gamma-Carboxyglutamic Acid Domains of Factor VIIa and Activated Protein C: Role of Magnesium at Physiological Calcium.,Vadivel K, Agah S, Messer AS, Cascio D, Bajaj MS, Krishnaswamy S, Esmon CT, Padmanabhan K, Bajaj SP J Mol Biol. 2013 Feb 20. pii: S0022-2836(13)00104-6. doi:, 10.1016/j.jmb.2013.02.017. PMID:23454357
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