HIBCH (3-hydroxyisobutyryl-CoA hydrolase, mitochondrial; EC 3.1.2.4) is a soluble enzyme of the mitochondrial matrix belonging to the enoyl-CoA hydratase/isomerase (crotonase) superfamily. It catalyzes a dedicated step of valine catabolism, the hydrolysis of the thioester 3-hydroxyisobutyryl-CoA (HIBYL-CoA; 3-hydroxy-2-methylpropanoyl-CoA) to 3-hydroxyisobutyrate (3-hydroxy-2-methylpropanoate) plus free coenzyme A. By deacylating this intermediate, HIBCH diverts the valine-degradation pathway to the free-acid branch; this hydrolytic step is thought to protect against accumulation of the upstream reactive intermediate methacrylyl-CoA. The enzyme also shows activity toward related short-chain acyl-CoA thioesters, including isobutyryl-CoA and 3-hydroxypropanoyl-CoA (a propionate-metabolism intermediate). HIBCH is synthesized as a precursor with a cleaved N-terminal mitochondrial targeting presequence, and the mature protein resides in the matrix; it is most abundant in liver, heart and kidney, and is also detected in muscle and brain. Biallelic loss-of-function variants cause 3-hydroxyisobutyryl-CoA hydrolase deficiency (HIBCHD), an autosomal-recessive inborn error of valine metabolism presenting as a Leigh-like progressive infantile neurodegeneration with basal ganglia lesions, lactic acidosis and 3-hydroxyisobutyric aciduria.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003860 3-hydroxyisobutyryl-CoA hydrolase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference of the defining 3-hydroxyisobutyryl-CoA hydrolase activity from the crotonase-superfamily tree. This is the core, experimentally established molecular function of HIBCH. Reason: Correct core molecular function; independently supported by direct experimental evidence on the human enzyme (PMID:8824301) and by the EC 3.1.2.4 / RHEA:20888 catalytic-activity annotation in UniProt. Supporting Evidence: PMID:8824301 beta-Hydroxyisobutyryl-CoA (HIBYL-CoA) hydrolase is responsible for the specific |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) localization to the mitochondrion. HIBCH is a well-established mitochondrial matrix enzyme, so the compartment is correct but less specific than the mitochondrial matrix annotations. Reason: Correct compartment but subsumed by the more precise mitochondrial matrix annotation (GO:0005759), which better captures where this soluble enzyme acts. |
| GO:0006574 L-valine catabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference that HIBCH participates in L-valine catabolism, consistent with its role as the 3-hydroxyisobutyryl-CoA hydrolase of the valine degradation pathway. Reason: A core biological process for HIBCH; the UniProt PATHWAY line assigns it to L-valine degradation and its deficiency is defined as an inborn error of valine metabolism. Also supported by direct genetic (IMP) evidence (PMID:40056416). Supporting Evidence: file:human/HIBCH/HIBCH-uniprot.txt Amino-acid degradation; L-valine degradation. |
| GO:0003860 3-hydroxyisobutyryl-CoA hydrolase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic assignment of the defining EC 3.1.2.4 / RHEA:20888 activity via combined IEA methods (InterPro signature IPR032259, plus EC and RHEA mapping). This is the core molecular function and is independently confirmed by experimental data. Reason: Correct core molecular function; duplicate of the experimentally supported GO:0003860 annotation, backed by the HIBYL-CoA-H-specific InterPro entry and the EC 3.1.2.4 catalytic activity in UniProt. Supporting Evidence: file:human/HIBCH/HIBCH-uniprot.txt EC=3.1.2.4; |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic localization to the mitochondrion from the UniProt Subcellular Location controlled vocabulary (SL-0173). Correct compartment, consistent with experimental evidence, but less specific than the matrix annotation. Reason: Correct but subsumed by the more precise mitochondrial matrix localization (GO:0005759). Supporting Evidence: file:human/HIBCH/HIBCH-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0009083 branched-chain amino acid catabolic process | TAS Reactome:R-HSA-70895 | KEEP AS NON CORE | Summary: Reactome (TAS) placement of HIBCH in branched-chain amino acid catabolism. HIBCH acts specifically in the valine branch (hydrolysis of 3-hydroxyisobutyryl-CoA), so this broader parent term is correct. Reason: Correct but broad; the specific and experimentally supported L-valine catabolic process (GO:0006574) is the more informative core biological process. |
| GO:0006574 L-valine catabolic process | IEA GO_REF:0000041 | ACCEPT | Summary: Electronic assignment of L-valine catabolic process from the UniPathway mapping (UPA00362, L-valine degradation). Correct and consistent with the core biological process. Reason: Correct core biological process; duplicate of the IBA/IMP-supported annotation and of the UniProt PATHWAY assignment to L-valine degradation. Supporting Evidence: file:human/HIBCH/HIBCH-uniprot.txt Amino-acid degradation; L-valine degradation. |
| GO:0003860 3-hydroxyisobutyryl-CoA hydrolase activity | EXP PMID:8824301 Primary structure and tissue-specific expression of human be... | ACCEPT | Summary: Experimental (EXP) annotation of the defining 3-hydroxyisobutyryl-CoA hydrolase activity. The recombinant human enzyme, expressed in E. coli, reproduced the substrate specificity of the purified rat liver enzyme for HIBYL-CoA hydrolysis. Reason: Direct experimental demonstration of the core molecular function of HIBCH. Supporting Evidence: PMID:8824301 beta-Hydroxyisobutyryl-CoA (HIBYL-CoA) hydrolase is responsible for the specific |
| GO:0003860 3-hydroxyisobutyryl-CoA hydrolase activity | TAS Reactome:R-HSA-9916727 | ACCEPT | Summary: Reactome (TAS) annotation of 3-hydroxyisobutyryl-CoA hydrolase activity in the context of the HIBCH-deficiency disease pathway (pathogenic variants that lose the ability to synthesize beta-hydroxyisobutyrate). Reason: Correct core molecular function applied in the disease context; consistent with the experimental EXP/IDA evidence. |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | KEEP AS NON CORE | Summary: Immunofluorescence-based (IDA, HPA) mitochondrial localization. Correct compartment but less precise than the matrix annotation. Reason: Subsumed by the more specific mitochondrial matrix localization (GO:0005759). |
| GO:0005759 mitochondrial matrix | IDA PMID:40056416 Ectopic protein lysine methacrylation contributes to defects... | ACCEPT | Summary: Direct experimental (IDA) localization of HIBCH to the mitochondrial matrix, the precise compartment for this soluble matrix enzyme. Reason: Best-supported and most specific subcellular localization; this is the core location of HIBCH. Supporting Evidence: PMID:40056416 results in abnormal mitochondrial morphology and respiratory defects. |
| GO:0006574 L-valine catabolic process | IMP PMID:40056416 Ectopic protein lysine methacrylation contributes to defects... | ACCEPT | Summary: Direct experimental (IMP) evidence that HIBCH acts in valine catabolism: its loss (like loss of the upstream enzyme ECHS1) elevates protein lysine methacrylation, reflecting build-up of the reactive valine-pathway intermediate methacrylyl-CoA whose downstream product HIBCH normally processes. Reason: A core biological process of HIBCH, supported by genetic loss-of-function evidence and consistent with HIBCH deficiency being an inborn error of valine metabolism. Supporting Evidence: PMID:40056416 methacrylation (Kmea) is observed in both HIBCH- and ECHS1-deficient cells and |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-9916727 | ACCEPT | Summary: Reactome (TAS) mitochondrial matrix localization within the HIBCH-deficiency disease pathway. Reason: Correct, specific localization; consistent with the IDA evidence. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | KEEP AS NON CORE | Summary: High-throughput (HTP) detection of HIBCH in a high-confidence human mitochondrial proteome (MitoCoP). Confirms mitochondrial localization at lower specificity than the matrix term. Reason: Supports mitochondrial localization but is subsumed by the more specific mitochondrial matrix annotation. |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-70881 | ACCEPT | Summary: Reactome (TAS) mitochondrial matrix localization for the HIBCH-catalyzed reaction (beta-hydroxyisobutyryl-CoA + H2O => beta-hydroxyisobutyrate + CoA). Reason: Correct, specific localization; consistent with the experimental IDA evidence. |
| GO:0003860 3-hydroxyisobutyryl-CoA hydrolase activity | IDA PMID:8824301 Primary structure and tissue-specific expression of human be... | ACCEPT | Summary: Direct experimental (IDA) demonstration of 3-hydroxyisobutyryl-CoA hydrolase activity using purified/recombinant enzyme, the strongest evidence for the gene's core molecular function. Reason: Definitive experimental support for the core 3-hydroxyisobutyryl-CoA hydrolase activity of HIBCH. Supporting Evidence: PMID:8824301 beta-Hydroxyisobutyryl-CoA (HIBYL-CoA) hydrolase is responsible for the specific |
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Download this section (compressed HTML)Q: Beyond 3-hydroxyisobutyryl-CoA, how physiologically significant are HIBCH's activities toward isobutyryl-CoA and 3-hydroxypropanoyl-CoA, and do they represent distinct biological roles (e.g. in a minor pathway of propionate metabolism) worth capturing?
Q: Is the neurotoxicity of HIBCH deficiency driven primarily by accumulation of methacrylyl-CoA / 3-hydroxyisobutyryl-CoA and consequent ectopic protein lysine methacrylation, or by depletion of the free-acid product 3-hydroxyisobutyrate?
Experiment: Measure in vitro hydrolase kinetics (KM/kcat) of purified recombinant human HIBCH against 3-hydroxyisobutyryl-CoA, isobutyryl-CoA and 3-hydroxypropanoyl-CoA to define its true substrate preference and rank its contribution to valine versus propionate metabolism.
Hypothesis: HIBCH is a valine-pathway-dedicated hydrolase with highest catalytic efficiency for 3-hydroxyisobutyryl-CoA, explaining why its deficiency presents as an inborn error of valine metabolism.
Experiment: In HIBCH-null cells and patient fibroblasts, perform metabolomic tracing of valine carbon combined with quantitative proteomics of lysine methacrylation to determine which accumulating thioester most strongly correlates with mitochondrial dysfunction.
Hypothesis: Accumulation of reactive valine-pathway acyl-CoA thioesters and ensuing ectopic lysine methacrylation, rather than loss of 3-hydroxyisobutyrate production, is the principal driver of the HIBCH-deficiency phenotype.
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