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Structural Modeling Insights into Human VKORC1 Phenotypes.

Czogalla KJ, Watzka M, Oldenburg J - Nutrients (2015)

Bottom Line: Vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1) catalyses the reduction of vitamin K and its 2,3-epoxide essential to sustain γ-carboxylation of vitamin K-dependent proteins.Two different phenotypes are associated with mutations in human VKORC1.Here, we summarize published experimental data and in silico modeling results in order to rationalize the mechanisms of VKA resistance and VKCFD2.

View Article: PubMed Central - PubMed

Affiliation: Institute of Experimental Hematology and Transfusion Medicine, University Clinic Bonn, Bonn 53105, Germany. katrin.czogalla@ukb.uni-bonn.de.

ABSTRACT
Vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1) catalyses the reduction of vitamin K and its 2,3-epoxide essential to sustain γ-carboxylation of vitamin K-dependent proteins. Two different phenotypes are associated with mutations in human VKORC1. The majority of mutations cause resistance to 4-hydroxycoumarin- and indandione-based vitamin K antagonists (VKA) used in the prevention and therapy of thromboembolism. Patients with these mutations require greater doses of VKA for stable anticoagulation than patients without mutations. The second phenotype, a very rare autosomal-recessive bleeding disorder caused by combined deficiency of vitamin K dependent clotting factors type 2 (VKCFD2) arises from a homozygous Arg98Trp mutation. The bleeding phenotype can be corrected by vitamin K administration. Here, we summarize published experimental data and in silico modeling results in order to rationalize the mechanisms of VKA resistance and VKCFD2.

No MeSH data available.


Related in: MedlinePlus

3TM and 4TM topological models for hVKORC1 (modified from Tie et al., 2012 [26]). In both models, conserved cysteines (Cys132 and Cys135 of the active center (CXXC motif), green; loop cysteines Cys43 and Cys51, blue) and Arg98_Arg100 of the di-arginine endoplasmic reticulum (ER) retention motif (red) are labeled with colored circles. Amino acid positions for which mutations were reported to be associated with either vitamin K antagonist (VKA) resistance or combined deficiency of vitamin K dependent clotting factors type 2 (VKCFD2) are marked by filled circles (mutations causing VKA resistance, red; VKCFD2 mutation, yellow). (A) Shows the putative topology for hVKORC1 as a 3 TM membrane-embedded protein with the loop located in the cytoplasm. The N-terminus is located in the ER lumen, whereas the C-terminus is in the cytoplasm. (B) Shows the putative 4TM topology for hVKORC1 with the loop containing the conserved cysteines Cys43 and Cys51 in the ER lumen with both termini located in the cytoplasm.
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nutrients-07-05313-f001: 3TM and 4TM topological models for hVKORC1 (modified from Tie et al., 2012 [26]). In both models, conserved cysteines (Cys132 and Cys135 of the active center (CXXC motif), green; loop cysteines Cys43 and Cys51, blue) and Arg98_Arg100 of the di-arginine endoplasmic reticulum (ER) retention motif (red) are labeled with colored circles. Amino acid positions for which mutations were reported to be associated with either vitamin K antagonist (VKA) resistance or combined deficiency of vitamin K dependent clotting factors type 2 (VKCFD2) are marked by filled circles (mutations causing VKA resistance, red; VKCFD2 mutation, yellow). (A) Shows the putative topology for hVKORC1 as a 3 TM membrane-embedded protein with the loop located in the cytoplasm. The N-terminus is located in the ER lumen, whereas the C-terminus is in the cytoplasm. (B) Shows the putative 4TM topology for hVKORC1 with the loop containing the conserved cysteines Cys43 and Cys51 in the ER lumen with both termini located in the cytoplasm.

Mentions: The CXXC motif is located in the last TM and is oriented towards the ER lumen in both 3TM and 4TM models for hVKORC1 (Figure 1). All published reports show complete loss of VKOR activity if one of the cysteines in the CXXC motif is mutated (Cys132 and Cys135, Table 1) [3,19,30,31]. These data clearly demonstrate that the CXXC motif is the active center essential for substrate reduction. The functional role of the conserved cysteines in the large loop, Cys43 and Cys51, is not completely clear. Due to their localisation Cys43 and Cys51 are also called “loop cysteines”. Of the two topology models, studies proposing the 4TM model claim that the loop cysteines are essential for in vivo VKOR activity [27,33], whereas studies supporting the 3TM model argue that they are not necessary [3,26,34]. The main difference between both models is the orientation of the N-terminus, the first TM, and the large loop. In the 3TM model, the loop is located in the cytoplasm, whereas in the 4TM model, the loop is in the ER lumen (Figure 1). In the 4TM model, the loop cysteines are required for electron transfer from redox partners located in the ER lumen to reduce the CXXC motif. This postulated electron transfer pathway is necessary for catalytic VKOR activity and is also present in bacterial VKOR homologues. In synVKOR, the disulfide bridge formed between the loop cysteines is reduced by a periplasmic Trx-like redox partner naturally fused to the VKOR core protein. In vertebrates, this natural fused redox partner is missing. Therefore, Trx-like domain containing proteins are thought to reduce the loop cysteines in vertebrates. Candidate partner oxidoreductases include protein disulfide isomerases (PDIs) that are either separate globular proteins in the ER lumen or type-I ER membrane-anchored proteins with redox-active cysteines facing the ER lumen [27,35]. Thus, electron transfer mediated by the loop cysteines is feasible for the 4TM model only, as most PDIs are resident in the ER lumen. However, for the 3TM model, PDIs would not be able to reduce the loop cysteines if the loop is located in the cytoplasm. Here, the two loop cysteines could not be essential for VKOR activity, and electron transfer would presumably take place through direct reduction of the CXXC motif. Thus, location of the loop cysteines in the ER lumen might not be a strict requirement for hVKORC1 function if the 3TM model were to be correct.


Structural Modeling Insights into Human VKORC1 Phenotypes.

Czogalla KJ, Watzka M, Oldenburg J - Nutrients (2015)

3TM and 4TM topological models for hVKORC1 (modified from Tie et al., 2012 [26]). In both models, conserved cysteines (Cys132 and Cys135 of the active center (CXXC motif), green; loop cysteines Cys43 and Cys51, blue) and Arg98_Arg100 of the di-arginine endoplasmic reticulum (ER) retention motif (red) are labeled with colored circles. Amino acid positions for which mutations were reported to be associated with either vitamin K antagonist (VKA) resistance or combined deficiency of vitamin K dependent clotting factors type 2 (VKCFD2) are marked by filled circles (mutations causing VKA resistance, red; VKCFD2 mutation, yellow). (A) Shows the putative topology for hVKORC1 as a 3 TM membrane-embedded protein with the loop located in the cytoplasm. The N-terminus is located in the ER lumen, whereas the C-terminus is in the cytoplasm. (B) Shows the putative 4TM topology for hVKORC1 with the loop containing the conserved cysteines Cys43 and Cys51 in the ER lumen with both termini located in the cytoplasm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4555152&req=5

nutrients-07-05313-f001: 3TM and 4TM topological models for hVKORC1 (modified from Tie et al., 2012 [26]). In both models, conserved cysteines (Cys132 and Cys135 of the active center (CXXC motif), green; loop cysteines Cys43 and Cys51, blue) and Arg98_Arg100 of the di-arginine endoplasmic reticulum (ER) retention motif (red) are labeled with colored circles. Amino acid positions for which mutations were reported to be associated with either vitamin K antagonist (VKA) resistance or combined deficiency of vitamin K dependent clotting factors type 2 (VKCFD2) are marked by filled circles (mutations causing VKA resistance, red; VKCFD2 mutation, yellow). (A) Shows the putative topology for hVKORC1 as a 3 TM membrane-embedded protein with the loop located in the cytoplasm. The N-terminus is located in the ER lumen, whereas the C-terminus is in the cytoplasm. (B) Shows the putative 4TM topology for hVKORC1 with the loop containing the conserved cysteines Cys43 and Cys51 in the ER lumen with both termini located in the cytoplasm.
Mentions: The CXXC motif is located in the last TM and is oriented towards the ER lumen in both 3TM and 4TM models for hVKORC1 (Figure 1). All published reports show complete loss of VKOR activity if one of the cysteines in the CXXC motif is mutated (Cys132 and Cys135, Table 1) [3,19,30,31]. These data clearly demonstrate that the CXXC motif is the active center essential for substrate reduction. The functional role of the conserved cysteines in the large loop, Cys43 and Cys51, is not completely clear. Due to their localisation Cys43 and Cys51 are also called “loop cysteines”. Of the two topology models, studies proposing the 4TM model claim that the loop cysteines are essential for in vivo VKOR activity [27,33], whereas studies supporting the 3TM model argue that they are not necessary [3,26,34]. The main difference between both models is the orientation of the N-terminus, the first TM, and the large loop. In the 3TM model, the loop is located in the cytoplasm, whereas in the 4TM model, the loop is in the ER lumen (Figure 1). In the 4TM model, the loop cysteines are required for electron transfer from redox partners located in the ER lumen to reduce the CXXC motif. This postulated electron transfer pathway is necessary for catalytic VKOR activity and is also present in bacterial VKOR homologues. In synVKOR, the disulfide bridge formed between the loop cysteines is reduced by a periplasmic Trx-like redox partner naturally fused to the VKOR core protein. In vertebrates, this natural fused redox partner is missing. Therefore, Trx-like domain containing proteins are thought to reduce the loop cysteines in vertebrates. Candidate partner oxidoreductases include protein disulfide isomerases (PDIs) that are either separate globular proteins in the ER lumen or type-I ER membrane-anchored proteins with redox-active cysteines facing the ER lumen [27,35]. Thus, electron transfer mediated by the loop cysteines is feasible for the 4TM model only, as most PDIs are resident in the ER lumen. However, for the 3TM model, PDIs would not be able to reduce the loop cysteines if the loop is located in the cytoplasm. Here, the two loop cysteines could not be essential for VKOR activity, and electron transfer would presumably take place through direct reduction of the CXXC motif. Thus, location of the loop cysteines in the ER lumen might not be a strict requirement for hVKORC1 function if the 3TM model were to be correct.

Bottom Line: Vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1) catalyses the reduction of vitamin K and its 2,3-epoxide essential to sustain γ-carboxylation of vitamin K-dependent proteins.Two different phenotypes are associated with mutations in human VKORC1.Here, we summarize published experimental data and in silico modeling results in order to rationalize the mechanisms of VKA resistance and VKCFD2.

View Article: PubMed Central - PubMed

Affiliation: Institute of Experimental Hematology and Transfusion Medicine, University Clinic Bonn, Bonn 53105, Germany. katrin.czogalla@ukb.uni-bonn.de.

ABSTRACT
Vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1) catalyses the reduction of vitamin K and its 2,3-epoxide essential to sustain γ-carboxylation of vitamin K-dependent proteins. Two different phenotypes are associated with mutations in human VKORC1. The majority of mutations cause resistance to 4-hydroxycoumarin- and indandione-based vitamin K antagonists (VKA) used in the prevention and therapy of thromboembolism. Patients with these mutations require greater doses of VKA for stable anticoagulation than patients without mutations. The second phenotype, a very rare autosomal-recessive bleeding disorder caused by combined deficiency of vitamin K dependent clotting factors type 2 (VKCFD2) arises from a homozygous Arg98Trp mutation. The bleeding phenotype can be corrected by vitamin K administration. Here, we summarize published experimental data and in silico modeling results in order to rationalize the mechanisms of VKA resistance and VKCFD2.

No MeSH data available.


Related in: MedlinePlus