PDHX (component X / E3-binding protein, E3BP) is a structural, non-catalytic subunit of the mitochondrial pyruvate dehydrogenase complex (PDC), which catalyses the oxidative decarboxylation of pyruvate to acetyl-CoA linking glycolysis to the TCA cycle. PDHX has a segmented multidomain architecture analogous to the E2 component (DLAT): an N-terminal lipoyl domain (lipoylated at Lys97), a peripheral subunit-binding / E3-binding domain, and a C-terminal E2-like inner-core (I') domain that packs into the dodecahedral E2 (DLAT) core. Its established function is to bind and anchor the E3 dihydrolipoyl dehydrogenase (DLD) to the E2 core, positioning E3 to reoxidise the reduced lipoyl arms; this tethering is essential for a functional PDC. Unlike E2, the catalytic-site histidine of the inner-core acyltransferase fold is replaced by serine in human E3BP, so PDHX does not itself catalyse acetyl-CoA formation. PDHX resides in the mitochondrial matrix. Its lipoyl-Lys97 is a substrate of the SIRT4 lipoamidase, providing a regulatory input to PDC activity. Loss-of-function variants cause pyruvate dehydrogenase E3-binding protein deficiency (a form of PDC deficiency with congenital lactic acidosis, hypotonia and psychomotor retardation).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: PDHX is a mitochondrial matrix protein and subunit of the mitochondrial PDC. Phylogenetic (IBA) placement in the mitochondrion is correct, though less specific than mitochondrial matrix. Reason: UniProt records subcellular location "Mitochondrion matrix" and a cleavable mitochondrial transit peptide (residues 1-53). The term is correct; the more specific GO:0005759 (mitochondrial matrix) annotation is also present. Supporting Evidence: file:human/PDHX/PDHX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion matrix. |
| GO:0045254 pyruvate dehydrogenase complex | IBA GO_REF:0000033 | ACCEPT | Summary: PDHX is a bona fide subunit of the pyruvate dehydrogenase complex, constituting part of the E2/E3BP inner core. Correct and core. Reason: Direct experimental and structural evidence establishes PDHX as part of the PDC inner core; the IBA phylogenetic annotation is consistent with this. Supporting Evidence: PMID:19240034 a noncatalytic component, E3-binding protein (E3BP), which specifically tethers E3 dimers to the PDC. |
| GO:0004742 dihydrolipoyllysine-residue acetyltransferase activity | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: This is a homology/family-based electronic annotation propagated from the E2 (acetyltransferase) fold that PDHX shares. However, human E3BP is not a functional acetyltransferase: the catalytic-site histidine of the inner-core acyltransferase domain is replaced by a serine, so CoA acetylation by this protein is unlikely. Reason: PDHX retains the E2-like inner-core fold (hence the ARBA family mapping) but lacks the catalytic histidine required for transacetylase activity; it is a structural/E3-anchoring subunit, not a catalytic acyltransferase. The annotation reflects fold similarity, not true molecular function. Supporting Evidence: PMID:9242632 The putative catalytic site histidine residue present in the inner core domains of all dihydrolipoamide acyltransferases is replaced by a serine residue in human E3BP; thus, catalysis of coenzyme A acetylation by this protein is unlikely. PMID:19240034 a noncatalytic component, E3-binding protein (E3BP), which specifically tethers E3 dimers to the PDC. |
| GO:0005759 mitochondrial matrix | IEA GO_REF:0000044 | ACCEPT | Summary: Correct and the most specific localization for PDHX; matches the UniProt subcellular location. Reason: UniProt SubCell mapping (SL-0170) to mitochondrial matrix is consistent with the curated UniProt location and with PDHX being an assembled subunit of the matrix-resident PDC. Supporting Evidence: file:human/PDHX/PDHX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion matrix. |
| GO:0006086 pyruvate decarboxylation to acetyl-CoA | IEA GO_REF:0000120 | ACCEPT | Summary: PDHX is a required structural subunit of the PDC, which performs the oxidative decarboxylation of pyruvate to acetyl-CoA. As part_of the complex it is legitimately "involved_in" this process even though it is not itself catalytic. Reason: Anchoring E3 to the E2 core is essential for a functional PDC; loss of PDHX causes PDC deficiency. The BP annotation captures the pathway role of the subunit and is corroborated by the ComplexPortal IDA (PMID:24534072) and the IC annotation (PMID:9242632). Supporting Evidence: PMID:9242632 Protein X, recently renamed dihydrolipoamide dehydrogenase-binding protein (E3BP), is required for anchoring dihydrolipoamide dehydrogenase (E3) to the dihydrolipoamide transacetylase (E2) core of the pyruvate dehydrogenase complexes of eukaryotes. |
| GO:0016746 acyltransferase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Broad acyltransferase MF mapped from InterPro domains of the shared E2-like fold. As with the more specific GO:0004742, PDHX does not catalyse acyl transfer (catalytic His->Ser), so this is a fold-based over-annotation. Reason: The InterPro signatures (2-oxoacid_DH acyltransferase, E2/Pdx1) reflect the E2-like inner-core fold that PDHX possesses structurally; the catalytic residue is absent and no acyltransferase activity has been demonstrated. Supporting Evidence: PMID:9242632 The putative catalytic site histidine residue present in the inner core domains of all dihydrolipoamide acyltransferases is replaced by a serine residue in human E3BP; thus, catalysis of coenzyme A acetylation by this protein is unlikely. |
| GO:0045254 pyruvate dehydrogenase complex | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (IEA) part_of PDC annotation, duplicating the well-supported experimental/IBA annotations. Correct and core. Reason: Consistent with all other evidence that PDHX is a subunit of the PDC inner core. Supporting Evidence: PMID:16263718 The dihydrolipoamide dehydrogenase-binding protein (E3BP) and the dihydrolipoamide acetyltransferase (E2) component enzyme form the structural core of the human pyruvate dehydrogenase complex |
| GO:0005515 protein binding | IPI PMID:16263718 How dihydrolipoamide dehydrogenase-binding protein binds dih... | MARK AS OVER ANNOTATED | Summary: IPI interaction with DLD (E3, P09622) from the crystal structure of the E3BP E3-binding domain in complex with E3. This is the key biological interaction of PDHX, but the bare "protein binding" term is uninformative; the specific E3-anchoring/adaptor role is captured in core_functions. Reason: Per curation guidelines, bare GO:0005515 "protein binding" adds no functional information. The underlying DLD interaction is real and central, but is better represented by the E3-anchoring adaptor function (GO:0030674) in core_functions rather than by this generic term. Supporting Evidence: PMID:16263718 How dihydrolipoamide dehydrogenase-binding protein binds dihydrolipoamide dehydrogenase in the human pyruvate dehydrogenase complex. |
| GO:0005515 protein binding | IPI PMID:16442803 Structural insight into interactions between dihydrolipoamid... | MARK AS OVER ANNOTATED | Summary: IPI interaction with DLD (E3, P09622) from a second crystal structure of the E3-binding domain in complex with E3, including hot-spot mutagenesis. Real but bare/uninformative. Reason: Meaningful DLD interaction but recorded as generic protein binding; the specific anchoring/adaptor function is captured in core_functions. Supporting Evidence: PMID:16442803 utilizes the specific dihydrolipoamide dehydrogenase (E3) binding protein (E3BP) to tether the essential E3 component to the 60-meric core of the complex |
| GO:0005515 protein binding | IPI PMID:21516116 Next-generation sequencing to generate interactome datasets. | MARK AS OVER ANNOTATED | Summary: Interaction with AGTRAP (Q6RW13) from a high-throughput next-generation sequencing interactome (Y2H) screen. No established biological role of PDHX involves AGTRAP; a bare, high-throughput protein-binding hit. Reason: Generic protein-binding annotation from a proteome-scale screen with no functional context for PDHX; uninformative for the gene's function. Supporting Evidence: PMID:21516116 Next-generation sequencing to generate interactome datasets. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | MARK AS OVER ANNOTATED | Summary: Interaction with AGTRAP (Q6RW13) from a proteome-scale binary interactome map. High-throughput, bare protein-binding hit with no functional context. Reason: Uninformative generic protein-binding from a large-scale interactome screen. Supporting Evidence: PMID:25416956 A proteome-scale map of the human interactome network. |
| GO:0005515 protein binding | IPI PMID:25525879 Sirtuin 4 is a lipoamidase regulating pyruvate dehydrogenase... | MARK AS OVER ANNOTATED | Summary: IPI interaction with SIRT4 (Q9Y6E7). SIRT4 is a lipoamidase that removes the lipoyl modification from PDHX (Lys97) and DLAT to regulate PDC activity, so this is a biologically meaningful interaction, but bare "protein binding" is uninformative. Reason: The SIRT4 interaction is functionally relevant (regulatory delipoylation), but as recorded it is a generic protein-binding term; the regulatory relationship is better captured as PTM/regulation, not as a core MF. Supporting Evidence: PMID:25525879 SIRT4 removed lipoamide from DLAT and PDHX peptides |
| GO:0005515 protein binding | IPI PMID:28514442 Architecture of the human interactome defines protein commun... | MARK AS OVER ANNOTATED | Summary: Interaction with DLD (P09622) captured in a large-scale AP-MS interactome (BioPlex). Consistent with the biological E3(DLD) interaction, but a bare, uninformative protein-binding term. Reason: High-throughput DLD interaction; real but generic. Anchoring function is in core_functions. Supporting Evidence: PMID:28514442 Architecture of the human interactome defines protein communities and disease networks. |
| GO:0005515 protein binding | IPI PMID:31515488 Extensive disruption of protein interactions by genetic vari... | MARK AS OVER ANNOTATED | Summary: Interaction with AGTRAP (Q6RW13) from a variant/interaction disruption study. High-throughput, bare protein-binding hit with no functional role for PDHX. Reason: Generic protein-binding from a large-scale interaction screen; uninformative. Supporting Evidence: PMID:31515488 Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: Interactions with DLD (P09622) and CIDEB (Q9UHD4) from a reference binary interactome map (HuRI). The DLD hit is consistent with biology; CIDEB has no established role with PDHX. Bare protein-binding term. Reason: Mixed high-throughput hits recorded as generic protein binding; the DLD interaction is real (anchoring, in core_functions) and CIDEB is an uncharacterized screen hit. Supporting Evidence: PMID:32296183 A reference map of the human binary protein interactome. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MARK AS OVER ANNOTATED | Summary: Interaction with DLD (P09622) from a cell-specific AP-MS interactome (BioPlex 3.0). Consistent with the biological E3(DLD) interaction, but bare. Reason: High-throughput DLD interaction; real but recorded as generic protein binding. Supporting Evidence: PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling of the human interactome. |
| GO:0005739 mitochondrion | NAS PMID:24534072 Component co-expression and purification of recombinant huma... | ACCEPT | Summary: Non-traceable statement of mitochondrial localization (ComplexPortal), consistent with the well-established matrix localization. Correct but less specific than mitochondrial matrix. Reason: PDHX is a matrix-resident PDC subunit; the mitochondrion annotation is correct. Supporting Evidence: file:human/PDHX/PDHX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion matrix. |
| GO:0006086 pyruvate decarboxylation to acetyl-CoA | IDA PMID:24534072 Component co-expression and purification of recombinant huma... | ACCEPT | Summary: ComplexPortal IDA: reconstituted recombinant human PDC (including E3BP) was shown to be functional in producing acetyl-CoA. PDHX participates as a required structural subunit. Correct as involved_in. Reason: A well-defined recombinant human PDC containing PDHX was assembled and shown catalytically competent, supporting the subunit's involvement in pyruvate decarboxylation to acetyl-CoA (as part of the complex, not as a catalyst). Supporting Evidence: PMID:24534072 The mammalian pyruvate dehydrogenase complex (PDC) is a multi-component mitochondrial enzyme that plays a key role in the conversion of pyruvate to acetyl-CoA connecting glycolysis to the citric acid cycle. |
| GO:0045254 pyruvate dehydrogenase complex | IPI PMID:19240034 Subunit and catalytic component stoichiometries of an in vit... | ACCEPT | Summary: Experimental (IPI) part_of PDC from stoichiometry/reconstitution work establishing PDHX as ~20 copies (40/20 model) within the E2/E3BP core. Correct and core. Reason: Direct biophysical characterization places E3BP within the PDC core and shows it tethers E3 to the complex. Supporting Evidence: PMID:19240034 The human PDC is organized around a 60-meric dodecahedral core comprising the C-terminal domains of E2p and a noncatalytic component, E3-binding protein (E3BP), which specifically tethers E3 dimers to the PDC. |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | ACCEPT | Summary: Immunofluorescence (HPA) localization to mitochondrion. Consistent with the established matrix localization. Reason: Direct imaging evidence of mitochondrial localization; correct though less specific than mitochondrial matrix. Supporting Evidence: file:human/PDHX/PDHX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion matrix. |
| GO:0004742 dihydrolipoyllysine-residue acetyltransferase activity | IDA PMID:9242632 Dihydrolipoamide dehydrogenase-binding protein of the human ... | MARK AS OVER ANNOTATED | Summary: Legacy IDA (MGI) assigning transacetylase activity, but the same primary paper explicitly concludes that CoA acetylation by human E3BP is unlikely because the catalytic-site histidine is replaced by serine. This MF should not be treated as a genuine catalytic function of PDHX. Reason: The cited paper reconstituted a functional PDC using E3BP but attributes the acetyltransferase catalysis to E2; it states the E3BP inner-core domain lacks the catalytic histidine and is unlikely to catalyse acetylation. PDHX is a structural (E3-anchoring) subunit, so this catalytic MF is an over-annotation of the fold rather than a demonstrated activity. Supporting Evidence: PMID:9242632 The putative catalytic site histidine residue present in the inner core domains of all dihydrolipoamide acyltransferases is replaced by a serine residue in human E3BP; thus, catalysis of coenzyme A acetylation by this protein is unlikely. |
| GO:0006086 pyruvate decarboxylation to acetyl-CoA | IC PMID:9242632 Dihydrolipoamide dehydrogenase-binding protein of the human ... | ACCEPT | Summary: Curator inference (IC) that PDHX participates in pyruvate decarboxylation to acetyl-CoA, based on its role in reconstituting a functional PDC. Consistent with the subunit's essential contribution to the pathway. Reason: PDHX is required for a functional PDC; its involvement in the pyruvate->acetyl-CoA conversion is a reasonable curator inference and is supported by reconstitution of the complex. Supporting Evidence: PMID:9242632 Coexpression of cDNAs for E3BP and E2 resulted in the formation of an E2.E3BP subcomplex that spontaneously reconstituted the pyruvate dehydrogenase complex in the presence of native E3 and recombinant pyruvate decarboxylase (E1). |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: High-throughput mitochondrial proteome localization. Consistent with the established matrix localization of PDHX. Reason: Corroborates mitochondrial localization; less specific than mitochondrial matrix but correct. Supporting Evidence: PMID:34800366 Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context. |
| GO:0045254 pyruvate dehydrogenase complex | IDA PMID:9242632 Dihydrolipoamide dehydrogenase-binding protein of the human ... | ACCEPT | Summary: Direct experimental (IDA) part_of PDC from reconstitution of the E2.E3BP subcomplex and functional PDC. Correct and core. Reason: PDHX (E3BP) was shown to assemble into the PDC core with E2 and reconstitute a functional complex, directly demonstrating it is part of the PDC. Supporting Evidence: PMID:9242632 Coexpression of cDNAs for E3BP and E2 resulted in the formation of an E2.E3BP subcomplex that spontaneously reconstituted the pyruvate dehydrogenase complex in the presence of native E3 and recombinant pyruvate decarboxylase (E1). |
| GO:0045254 pyruvate dehydrogenase complex | IDA PMID:25525879 Sirtuin 4 is a lipoamidase regulating pyruvate dehydrogenase... | ACCEPT | Summary: IDA part_of PDC (UniProt), consistent with PDHX being a structural subunit of the PDH complex characterized in the SIRT4 study. Correct and core. Reason: The study describes PDHX as a structural subunit of the PDH complex and immuno-isolates it as part of the complex. Supporting Evidence: PMID:25525879 a structural subunit (PDH-binding component X, PDHX) |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-203946 | ACCEPT | Summary: Reactome TAS localization to mitochondrial matrix (PDH regulation reaction context). Correct and matches the UniProt subcellular location. Reason: Consistent with the curated matrix localization of PDHX as a PDC subunit. Supporting Evidence: file:human/PDHX/PDHX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion matrix. |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-204169 | ACCEPT | Summary: Reactome TAS localization to mitochondrial matrix. Correct. Reason: Consistent with the curated matrix localization of PDHX. Supporting Evidence: file:human/PDHX/PDHX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion matrix. |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-9861616 | ACCEPT | Summary: Reactome TAS localization to mitochondrial matrix (DLD dehydrogenation reaction context). Correct. Reason: Consistent with the curated matrix localization of PDHX. Supporting Evidence: file:human/PDHX/PDHX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion matrix. |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-9861667 | ACCEPT | Summary: Reactome TAS localization to mitochondrial matrix (DLAT acetyl transfer reaction context). Correct. Reason: Consistent with the curated matrix localization of PDHX. Supporting Evidence: file:human/PDHX/PDHX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion matrix. |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-9861734 | ACCEPT | Summary: Reactome TAS localization to mitochondrial matrix (E1 decarboxylation reaction context). Correct. Reason: Consistent with the curated matrix localization of PDHX. Supporting Evidence: file:human/PDHX/PDHX-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion matrix. |
| GO:0030674 protein-macromolecule adaptor activity | IDA PMID:16263718 How dihydrolipoamide dehydrogenase-binding protein binds dih... | NEW | Summary: Proposed molecular function capturing PDHX's established E3-anchoring role: PDHX acts as an adaptor that tethers the E3 (DLD) dimer to the E2 (DLAT) core of the PDC. Supported by two crystal structures of the E3BP E3-binding domain bound to E3 and by mutagenesis of the interface hot-spot residues. Reason: The bare GO:0005515 protein-binding IPIs and the mis-attributed catalytic MF terms fail to capture what PDHX actually does molecularly. Its function is a protein-macromolecule adaptor/anchoring activity tethering E3 to the E2 core. Supporting Evidence: PMID:16263718 The dihydrolipoamide dehydrogenase-binding protein (E3BP) and the dihydrolipoamide acetyltransferase (E2) component enzyme form the structural core of the human pyruvate dehydrogenase complex PMID:9242632 Protein X, recently renamed dihydrolipoamide dehydrogenase-binding protein (E3BP), is required for anchoring dihydrolipoamide dehydrogenase (E3) to the dihydrolipoamide transacetylase (E2) core of the pyruvate dehydrogenase complexes of eukaryotes. |
| GO:0005198 structural molecule activity | IDA PMID:16263718 How dihydrolipoamide dehydrogenase-binding protein binds dih... | NEW | Summary: Proposed molecular function reflecting PDHX's role as a structural constituent of the PDC inner core: together with E2 (DLAT) it forms the dodecahedral scaffold that binds the peripheral E1 and E3 catalytic components. PDHX itself is non-catalytic. Reason: Captures the structural/scaffolding contribution of PDHX to the PDC core, complementing the adaptor (E3-anchoring) function and replacing the fold-based catalytic over-annotations. Supporting Evidence: PMID:16263718 The dihydrolipoamide dehydrogenase-binding protein (E3BP) and the dihydrolipoamide acetyltransferase (E2) component enzyme form the structural core of the human pyruvate dehydrogenase complex |
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Download this section (compressed HTML)Q: Beyond its structural E3-anchoring role, does the lipoyl domain of PDHX (lipoylated at Lys97) participate directly in the reductive acetylation / lipoyl-arm shuttling cycle of the PDC, or is it functionally redundant with the DLAT lipoyl domains?
Q: What is the physiological significance of SIRT4-mediated delipoylation of PDHX Lys97 relative to delipoylation of DLAT for regulating PDC flux?
Experiment: Reconstitute human PDC with lipoyl-null (K97) PDHX to test whether the PDHX lipoyl group is catalytically required or purely structural.
Hypothesis: The PDHX lipoyl group is dispensable for PDC catalysis, consistent with a purely structural/anchoring role.
Experiment: Assay purified human E3BP/PDHX inner-core domain for any residual transacetylase activity to confirm the loss of acyltransferase function implied by the His->Ser substitution.
Hypothesis: The PDHX inner-core domain has no detectable dihydrolipoyllysine-residue acetyltransferase activity because it lacks the catalytic histidine.
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