ALDH5A1

UniProt ID: P51649
Organism: Homo sapiens
Review Status: INITIALIZED
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Gene Description

Succinate-semialdehyde dehydrogenase, mitochondrial (SSADH; ALDH5A1) is a mitochondrial NAD+-dependent oxidoreductase (EC 1.2.1.24) that catalyzes the final step of gamma-aminobutyric acid (GABA) degradation, the GABA shunt. It irreversibly oxidizes succinate semialdehyde to succinate (using NAD+ and water, producing NADH), thereby routing the carbon skeleton of the inhibitory neurotransmitter GABA into the tricarboxylic acid (TCA) cycle and central energy metabolism. The protein is synthesized as a precursor with an N-terminal transit peptide, imported into the mitochondrion, and assembles into a homotetramer that resides in the mitochondrial matrix; its activity is redox-regulated through a reversible disulfide bond on a dynamic catalytic loop. It is a member of the aldehyde dehydrogenase superfamily and is expressed broadly, including in brain, liver, kidney and skeletal muscle. Loss-of-function variants cause succinic semialdehyde dehydrogenase deficiency (SSADH deficiency; 4-hydroxybutyric / gamma-hydroxybutyric aciduria), an autosomal recessive disorder in which GABA and 4-hydroxybutyrate (GHB) accumulate, producing developmental delay, hypotonia, intellectual disability, ataxia, seizures and behavioural disturbances.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0004777 succinate-semialdehyde dehydrogenase (NAD+) activity
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetic (IBA) assignment of the enzyme's core NAD+-dependent succinate-semialdehyde dehydrogenase activity, consistent with the family and with direct experimental evidence for the human protein.
Reason: This is the well-established core molecular function of SSADH, catalyzing the NAD+-dependent oxidation of succinate semialdehyde to succinate (EC 1.2.1.24). The IBA is corroborated by multiple experimental annotations for the human enzyme (PMID:12208142, PMID:14635103, PMID:19300440, PMID:16199352, PMID:9683595) and the UniProt catalytic activity statement.
Supporting Evidence:
PMID:19300440
Succinic semialdehyde dehydrogenase (SSADH) is involved in the final degradation step of the inhibitory neurotransmitter gamma-aminobutyric acid by converting succinic semialdehyde to succinic acid in the mitochondrial matrix
file:human/ALDH5A1/ALDH5A1-uniprot.txt
Reaction=succinate semialdehyde + NAD(+) + H2O = succinate + NADH + 2
GO:0005739 mitochondrion
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetic assignment of mitochondrial localization, consistent with the N-terminal mitochondrial transit peptide and multiple experimental localization datasets.
Reason: SSADH is a nuclear-encoded mitochondrial matrix enzyme with an N-terminal transit peptide; mitochondrial localization is well supported. The more precise location (mitochondrial matrix, GO:0005759) is captured by a separate annotation.
Supporting Evidence:
PMID:19300440
converting succinic semialdehyde to succinic acid in the mitochondrial matrix
file:human/ALDH5A1/ALDH5A1-uniprot.txt
SUBCELLULAR LOCATION: Mitochondrion
GO:0009450 GABA catabolic process
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetic assignment of the GABA catabolic process, the pathway in which SSADH performs the terminal step.
Reason: SSADH catalyzes the last committed step of GABA degradation (succinate semialdehyde to succinate). This is a core biological process for the gene, independently supported by IDA (PMID:9683595) and the UniProt/literature pathway descriptions.
Supporting Evidence:
PMID:12208142
represents the last enzyme in the GABA catabolism and irreversibly oxidizes SSA to succinate
GO:0004777 succinate-semialdehyde dehydrogenase (NAD+) activity
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated electronic assignment (ARBA/RHEA/EC mapping) of the core NAD+-dependent SSADH activity, matching EC 1.2.1.24 and RHEA:13217.
Reason: The electronic mapping (EC:1.2.1.24, RHEA:13217) reproduces the correct core molecular function and agrees with the experimental annotations.
Supporting Evidence:
file:human/ALDH5A1/ALDH5A1-uniprot.txt
Reaction=succinate semialdehyde + NAD(+) + H2O = succinate + NADH + 2
GO:0005739 mitochondrion
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated electronic assignment of mitochondrial localization (via mouse ortholog and UniProt-SubCell), consistent with the transit peptide and experimental data.
Reason: Correct compartment; agrees with the transit peptide, IDA/HDA/HTP proteomics and the IBA localization.
Supporting Evidence:
file:human/ALDH5A1/ALDH5A1-uniprot.txt
SUBCELLULAR LOCATION: Mitochondrion
GO:0009013 succinate-semialdehyde dehydrogenase [NAD(P)+] activity
IEA
GO_REF:0000002 		MARK AS OVER ANNOTATED
Summary: InterPro2GO electronic assignment of the broader NAD(P)+-utilizing SSADH activity. The human enzyme is specifically NAD+-dependent, so this term is less precise than GO:0004777.
Reason: The human enzyme is an NAD+-specific succinate-semialdehyde dehydrogenase (EC 1.2.1.24; named "NAD(+)-dependent"; kinetically characterized with NAD+), whereas GO:0009013 [NAD(P)+] is the broader family term that also covers NADP+-utilizing (e.g. bacterial EC 1.2.1.16) enzymes. It is not wrong at the family level but over-annotates cofactor breadth; the NAD+-specific GO:0004777 is the correct, more precise molecular function.
Supporting Evidence:
PMID:16199352
The Michaelis constants K(m) for succinic semialdehyde and NAD(+) were 6.3 and 125 microM
file:human/ALDH5A1/ALDH5A1-uniprot.txt
EC=1.2.1.24
GO:0009450 GABA catabolic process
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated electronic assignment of the GABA catabolic process (via InterPro and UniPathway 4-aminobutanoate degradation).
Reason: Correct core process; agrees with the IBA and IDA annotations and the UniProt pathway statement (4-aminobutanoate degradation).
Supporting Evidence:
file:human/ALDH5A1/ALDH5A1-uniprot.txt
Amino-acid degradation; 4-aminobutanoate degradation
GO:0016491 oxidoreductase activity
IEA
GO_REF:0000002 		MARK AS OVER ANNOTATED
Summary: InterPro2GO electronic assignment of the generic oxidoreductase parent term.
Reason: Correct but uninformative; GO:0016491 is a high-level parent of the specific succinate-semialdehyde dehydrogenase (NAD+) activity (GO:0004777) that is already annotated. The specific term should be preferred.
Supporting Evidence:
PMID:7814412
suggesting it is a member of the aldehyde dehydrogenase superfamily of proteins
GO:0006540 GABA shunt
IMP
PMID:15037717
Proton MR spectroscopy in succinic semialdehyde dehydrogenas... 		ACCEPT
Summary: Involvement in the GABA shunt, the pathway (GABA -> succinate semialdehyde -> succinate) that bypasses part of the TCA cycle; supported by the SSADH deficiency phenotype of GABA and GHB accumulation.
Reason: SSADH performs the terminal, committed step of the GABA shunt. The cited case report documents the metabolic consequences of loss of SSADH function (elevated GABA with GHB), consistent with a role in this pathway. GO:0006540 (GABA shunt) is an appropriately specific pathway term for the enzyme's role.
Supporting Evidence:
PMID:15037717
disorder of the CNS catabolism of gamma-aminobutyric acid (GABA), leading to accumulation of the metabolite 4-hydroxybutyrate (GHB)
GO:0005739 mitochondrion
IDA
GO_REF:0000052 		ACCEPT
Summary: Direct immunofluorescence (HPA) localization to the mitochondrion.
Reason: Direct experimental localization consistent with the transit peptide and all other localization evidence.
Supporting Evidence:
file:human/ALDH5A1/ALDH5A1-uniprot.txt
SUBCELLULAR LOCATION: Mitochondrion
GO:0004777 succinate-semialdehyde dehydrogenase (NAD+) activity
EXP
PMID:12208142
Structure of human succinic semialdehyde dehydrogenase gene:... 		ACCEPT
Summary: Experimental determination of NAD+-dependent SSADH catalytic activity for the human enzyme (including characterization of natural variants).
Reason: Core molecular function, experimentally established. This study characterized the human gene, isoforms and variant activities, confirming the NAD+-dependent SSA-to-succinate reaction.
Supporting Evidence:
PMID:12208142
represents the last enzyme in the GABA catabolism and irreversibly oxidizes SSA to succinate
GO:0004777 succinate-semialdehyde dehydrogenase (NAD+) activity
EXP
PMID:14635103
Mutational spectrum of the succinate semialdehyde dehydrogen... 		ACCEPT
Summary: Experimental functional analysis of the human enzyme via expression of wild-type and disease variants, confirming NAD+-dependent SSADH activity.
Reason: Core molecular function, experimentally established; disease-causing missense variants reduced SSADH activity to <5% of normal, directly implicating this activity as the gene's function.
Supporting Evidence:
PMID:14635103
the missense mutations we consider to be causative of SSADH deficiency reduced the SSADH activity to less than 5% of the normal activity in our in vitro expression system
GO:0004777 succinate-semialdehyde dehydrogenase (NAD+) activity
EXP
PMID:19300440
Redox-switch modulation of human SSADH by dynamic catalytic ... 		ACCEPT
Summary: Structural and biochemical characterization of human SSADH, confirming the NAD+-dependent conversion of succinic semialdehyde to succinate.
Reason: Core molecular function, established by crystal structures with product and ADP plus catalytic-residue mutagenesis; the enzyme converts succinic semialdehyde to succinic acid in the mitochondrial matrix.
Supporting Evidence:
PMID:19300440
Succinic semialdehyde dehydrogenase (SSADH) is involved in the final degradation step of the inhibitory neurotransmitter gamma-aminobutyric acid by converting succinic semialdehyde to succinic acid in the mitochondrial matrix
GO:0005739 mitochondrion
HTP
PMID:34800366
Quantitative high-confidence human mitochondrial proteome an... 		ACCEPT
Summary: High-throughput identification of SSADH in the high-confidence human mitochondrial proteome (MitoCoP).
Reason: Consistent with all other localization evidence; SSADH is a bona fide mitochondrial protein captured in a curated high-confidence mitochondrial proteome inventory.
Supporting Evidence:
PMID:34800366
mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP)
GO:0004777 succinate-semialdehyde dehydrogenase (NAD+) activity
IDA
PMID:16199352
High-level expression and characterization of the recombinan... 		ACCEPT
Summary: Direct biochemical characterization of recombinant human SSADH, including kinetic constants for succinic semialdehyde and NAD+.
Reason: Core molecular function, directly measured. Recombinant enzyme was active (specific activity and Km values reported) and forms a homotetramer.
Supporting Evidence:
PMID:16199352
The Michaelis constants K(m) for succinic semialdehyde and NAD(+) were 6.3 and 125 microM
GO:0042802 identical protein binding
IPI
PMID:16199352
High-level expression and characterization of the recombinan... 		KEEP AS NON CORE
Summary: Self-association reflecting the homotetrameric quaternary structure of SSADH (identical protein binding).
Reason: The homotetramer is genuine (purified recombinant SSADH is a tetramer of identical subunits; also stated in UniProt SUBUNIT). "Identical protein binding" is a generic term that captures oligomerization rather than an informative molecular function; it is retained but marked non-core, and is not removed because it is an experimental IPI supported by the quaternary-structure data.
Supporting Evidence:
PMID:16199352
The purified SSADH appears to be a tetramer of identical subunits
file:human/ALDH5A1/ALDH5A1-uniprot.txt
SUBUNIT: Homotetramer
GO:0005739 mitochondrion
HDA
PMID:20833797
Phosphoproteome analysis of functional mitochondria isolated... 		ACCEPT
Summary: High-throughput MS detection of SSADH in isolated functional human skeletal-muscle mitochondria (mitochondrial phosphoproteome).
Reason: Consistent with all other localization evidence; SSADH was identified in a preparation of isolated mitochondria.
Supporting Evidence:
PMID:20833797
phosphoproteomics study of functional mitochondria isolated from human muscle biopsies
GO:0005759 mitochondrial matrix
TAS
Reactome:R-HSA-888548 		ACCEPT
Summary: Traceable assertion (Reactome) placing the SSADH reaction in the mitochondrial matrix.
Reason: Mitochondrial matrix is the correct, more precise subcellular location for this soluble matrix enzyme, consistent with the literature stating the reaction occurs in the mitochondrial matrix.
Supporting Evidence:
Reactome:R-HSA-888548
Mitochondrial succinate semialdehyde dehydrogenase (ALDH5A1) tetramer catalyzes the reaction of succinate semialdehyde (SUCCSA), H2O, and NAD+ to form succinate (SUCCA) and NADH + 2 H+
PMID:19300440
converting succinic semialdehyde to succinic acid in the mitochondrial matrix
GO:0006105 succinate metabolic process
ISS
GO_REF:0000024 		KEEP AS NON CORE
Summary: Sequence-similarity transfer (from mouse ortholog) of a role in succinate metabolism; succinate is the direct product of the SSADH reaction.
Reason: Correct in that succinate is the reaction product that feeds the TCA cycle, but this is a broad process term that is subsumed by the more informative GABA catabolic process / GABA shunt annotations. Retained as a non-core, downstream metabolic role rather than the enzyme's defining function.
Supporting Evidence:
PMID:12208142
irreversibly oxidizes SSA to succinate
GO:0006536 glutamate metabolic process
ISS
GO_REF:0000024 		MARK AS OVER ANNOTATED
Summary: Sequence-similarity transfer (from mouse ortholog) of a role in glutamate metabolism.
Reason: Glutamate is upstream of SSADH in the GABA pathway (glutamate -> GABA via glutamate decarboxylase), not a substrate or product of the SSADH reaction. SSADH acts on succinate semialdehyde and produces succinate; any effect on glutamate metabolism is indirect (through the connected GABA/glutamate shunt). The direct catalytic contribution is to GABA catabolism, so this broader/indirect process term is an over-annotation.
Supporting Evidence:
PMID:19300440
by converting succinic semialdehyde to succinic acid in the mitochondrial matrix
GO:0004777 succinate-semialdehyde dehydrogenase (NAD+) activity
ISS
PMID:7814412
Molecular cloning of the mature NAD(+)-dependent succinic se... 		ACCEPT
Summary: Sequence-similarity assignment of NAD+-dependent SSADH activity based on cloning and characterization of the mammalian enzyme.
Reason: Core molecular function. The cited study cloned mature NAD+-dependent SSADH (EC 1.2.1.24) from rat and human and confirmed activity by bacterial expression, placing it in the aldehyde dehydrogenase superfamily.
Supporting Evidence:
PMID:7814412
encoding succinic semialdehyde dehydrogenase (SSADH; EC 1.2.1.24)
GO:0004777 succinate-semialdehyde dehydrogenase (NAD+) activity
IDA
PMID:9683595
Two exon-skipping mutations as the molecular basis of succin... 		ACCEPT
Summary: Direct assignment of NAD+-dependent SSADH activity in the context of identifying the molecular basis of SSADH deficiency.
Reason: Core molecular function; this study established the human SSADH cDNA and catalytic activity while defining exon-skipping mutations underlying the deficiency.
Supporting Evidence:
PMID:9683595
Succinic semialdehyde dehydrogenase (SSADH) deficiency, a rare metabolic disorder of 4-aminobutyric acid degradation
GO:0007417 central nervous system development
IMP
PMID:9683595
Two exon-skipping mutations as the molecular basis of succin... 		KEEP AS NON CORE
Summary: Assignment of a role in CNS development inferred from the neurological phenotype of SSADH deficiency.
Reason: The neurological features of SSADH deficiency (developmental delay, hypotonia, ataxia, seizures) are downstream consequences of losing GABA catabolism and the resulting GABA/GHB accumulation, rather than evidence that SSADH itself directs CNS developmental programs. This is a pleiotropic, disease- derived process; it is retained as non-core (not removed, as it is an experimental IMP whose full text is not available here), but the enzyme's core role is metabolic (GABA catabolism), not developmental.
Supporting Evidence:
PMID:9683595
Succinic semialdehyde dehydrogenase (SSADH) deficiency, a rare metabolic disorder of 4-aminobutyric acid degradation
GO:0009450 GABA catabolic process
IDA
PMID:9683595
Two exon-skipping mutations as the molecular basis of succin... 		ACCEPT
Summary: Direct assignment of involvement in GABA catabolism; SSADH deficiency disrupts GABA degradation.
Reason: Core biological process. SSADH performs the terminal step of GABA degradation; loss of function causes accumulation of GABA and its shunt product GHB, confirming the enzyme's role in this catabolic pathway.
Supporting Evidence:
PMID:9683595
Succinic semialdehyde dehydrogenase (SSADH) deficiency, a rare metabolic disorder of 4-aminobutyric acid degradation
GO:0005739 mitochondrion
ISS
GO_REF:0000024 		ACCEPT
Summary: Sequence-similarity transfer (from mouse ortholog) of mitochondrial localization.
Reason: Correct compartment; redundant with the direct and phylogenetic mitochondrial localization annotations.
Supporting Evidence:
file:human/ALDH5A1/ALDH5A1-uniprot.txt
SUBCELLULAR LOCATION: Mitochondrion

Core Functions

NAD+-dependent oxidation of succinate semialdehyde to succinate, the terminal step of GABA catabolism, feeding the carbon skeleton of GABA into the TCA cycle

Molecular Function:
succinate-semialdehyde dehydrogenase (NAD+) activity
Directly Involved In:
GABA catabolic process
Cellular Locations:
mitochondrial matrix
Supporting Evidence:
  • PMID:19300440
    Succinic semialdehyde dehydrogenase (SSADH) is involved in the final degradation step of the inhibitory neurotransmitter gamma-aminobutyric acid by converting succinic semialdehyde to succinic acid in the mitochondrial matrix
  • PMID:12208142
    represents the last enzyme in the GABA catabolism and irreversibly oxidizes SSA to succinate
  • file:human/ALDH5A1/ALDH5A1-uniprot.txt
    Reaction=succinate semialdehyde + NAD(+) + H2O = succinate + NADH + 2

References

GO_REF:0000002
Gene Ontology annotation through association of InterPro records with GO terms
GO_REF:0000024
Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence similarity
GO_REF:0000033
Annotation inferences using phylogenetic trees
GO_REF:0000052
Gene Ontology annotation based on curation of immunofluorescence data
GO_REF:0000120
Combined Automated Annotation using Multiple IEA Methods
PMID:12208142
Structure of human succinic semialdehyde dehydrogenase gene: identification of promoter region and alternatively processed isoforms.
PMID:14635103
Mutational spectrum of the succinate semialdehyde dehydrogenase (ALDH5A1) gene and functional analysis of 27 novel disease-causing mutations in patients with SSADH deficiency.
PMID:15037717
Proton MR spectroscopy in succinic semialdehyde dehydrogenase deficiency.
PMID:16199352
High-level expression and characterization of the recombinant enzyme, and tissue distribution of human succinic semialdehyde dehydrogenase.
PMID:19300440
Redox-switch modulation of human SSADH by dynamic catalytic loop.
PMID:20833797
Phosphoproteome analysis of functional mitochondria isolated from resting human muscle reveals extensive phosphorylation of inner membrane protein complexes and enzymes.
PMID:34800366
Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context.
PMID:7814412
Molecular cloning of the mature NAD(+)-dependent succinic semialdehyde dehydrogenase from rat and human. cDNA isolation, evolutionary homology, and tissue expression.
PMID:9683595
Two exon-skipping mutations as the molecular basis of succinic semialdehyde dehydrogenase deficiency (4-hydroxybutyric aciduria).
Reactome:R-HSA-888548
ALDH5A1 dehydrogenates SUCCSA to SUCCA
file:human/ALDH5A1/ALDH5A1-uniprot.txt
UniProtKB P51649 (SSDH_HUMAN), Succinate-semialdehyde dehydrogenase, mitochondrial

Suggested Questions for Experts

Q: Beyond the neurological phenotype, does SSADH activity contribute quantitatively to cellular energy metabolism in non-neural tissues (liver, kidney, skeletal muscle) where it is highly expressed?

Q: To what extent is SSADH activity regulated in vivo by the redox-sensitive Cys340-Cys342 catalytic-loop disulfide, and does this modulate GABA and GHB levels under oxidative stress?

Suggested Experiments

Experiment: Isotope tracing (e.g. 13C-GABA) in patient-derived or knockout cells to quantify flux through SSADH into the TCA cycle and confirm the metabolic (versus purely developmental) consequences of loss of function.

Experiment: Structure-guided assays testing whether physiological oxidants shift SSADH between active and inhibited states in intact mitochondria, linking the redox switch to GABA shunt output.

📚 Additional Documentation

Notes

(ALDH5A1-notes.md)

ALDH5A1 (SSADH) review notes

UniProtKB: P51649 (SSDH_HUMAN). HGNC:408. EC 1.2.1.24. 535 aa precursor with an
N-terminal mitochondrial transit peptide (1..47); mature chain 48..535.

Core biology

ALDH5A1 encodes succinate-semialdehyde dehydrogenase, mitochondrial (SSADH), the
NAD+-dependent enzyme catalysing the final step of GABA degradation (the GABA shunt):
oxidation of succinate semialdehyde -> succinate, which then enters the TCA cycle,
routing GABA carbon into central energy metabolism.

  • Catalytic activity (UniProt/Rhea RHEA:13217): succinate semialdehyde + NAD(+) + H2O =
    succinate + NADH + 2 H(+); EC 1.2.1.24 [file:human/ALDH5A1/ALDH5A1-uniprot.txt].
  • "Mitochondrial NAD(+)-dependent succinic semialdehyde dehydrogenase (ALDH5A1, SSADH)
    represents the last enzyme in the GABA catabolism and irreversibly oxidizes SSA to
    succinate" PMID:12208142.
  • "Succinic semialdehyde dehydrogenase (SSADH) is involved in the final degradation step
    of the inhibitory neurotransmitter gamma-aminobutyric acid by converting succinic
    semialdehyde to succinic acid in the mitochondrial matrix" PMID:19300440.
  • Homotetramer of identical subunits [PMID:16199352; UniProt SUBUNIT].
  • Km(SSA) 6.3 uM, Km(NAD+) 125 uM; recombinant enzyme characterised PMID:16199352.
  • Redox-regulated via a reversible Cys340-Cys342 disulfide on a dynamic catalytic loop;
    inhibited under oxidizing conditions / by H2O2 PMID:19300440.
  • Member of the aldehyde dehydrogenase superfamily PMID:7814412.

Localization

Mitochondrion / mitochondrial matrix. TransitPeptide 1..47. UniProt subcellular location:
Mitochondrion. Reactome places the reaction in the mitochondrial matrix (R-HSA-888548).
Confirmed by IDA (HPA), HDA/HTP proteomics (PMID:20833797 muscle mito phosphoproteome;
PMID:34800366 MitoCoP high-confidence mito proteome), and IBA.

Disease

SSADH deficiency (SSADHD; MIM:271980) = 4-hydroxybutyric (gamma-hydroxybutyric, GHB)
aciduria. Autosomal recessive; accumulation of GABA and GHB; developmental delay,
hypotonia, intellectual disability, ataxia, seizures, behavioural disturbance
[PMID:9683595, PMID:12208142, PMID:14635103, PMID:15037717].

Annotation review decisions (summary)

  • MF GO:0004777 succinate-semialdehyde dehydrogenase (NAD+) activity: CORE. Multiple EXP
    (12208142, 14635103, 19300440), IDA (16199352, 9683595), ISS, IEA, IBA all converge.
    All ACCEPT.
  • MF GO:0009013 succinate-semialdehyde dehydrogenase [NAD(P)+] activity (IEA/InterPro):
    human enzyme is NAD+-specific (name, EC 1.2.1.24, kinetics). NAD(P)+ term is broader/
    the bacterial (EC 1.2.1.16) flavour. MARK_AS_OVER_ANNOTATED (not wrong at family level
    but less precise than the NAD+-specific term).
  • MF GO:0016491 oxidoreductase activity (IEA): correct but generic parent of GO:0004777.
    MARK_AS_OVER_ANNOTATED.
  • MF GO:0042802 identical protein binding (IPI, 16199352): supported (homotetramer) but
    bare binding term; per policy do not REMOVE experimental IPI -> KEEP_AS_NON_CORE.
  • BP GO:0009450 GABA catabolic process: CORE. IBA + IDA (9683595). ACCEPT.
  • BP GO:0006540 GABA shunt (IMP, 15037717): correct process; ACCEPT (arguably the more
    informative pathway term). 15037717 is a case report of SSADHD showing GABA/GHB
    accumulation, supporting the pathway role.
  • BP GO:0006105 succinate metabolic process (ISS): product is succinate; correct but
    broad/generic vs the GABA catabolic framing. KEEP_AS_NON_CORE.
  • BP GO:0006536 glutamate metabolic process (ISS): weaker — glutamate is upstream of GABA
    (glutamate -> GABA via GAD), not a direct substrate/product of SSADH. The ISS is a
    transfer from mouse ortholog. MARK_AS_OVER_ANNOTATED (indirect at best).
  • BP GO:0007417 central nervous system development (IMP, 9683595): 9683595 identifies
    splicing mutations causing SSADHD; the neurological disease phenotype is a downstream
    consequence of loss of GABA catabolism, not evidence that SSADH drives CNS development.
    This is a pleiotropic/disease-derived process. KEEP_AS_NON_CORE (do not REMOVE an
    experimental IMP whose full text we cannot read).
  • CC GO:0005739 mitochondrion (IBA, IEA, IDA/HPA, HDA, HTP) and GO:0005759 mitochondrial
    matrix (TAS Reactome): all ACCEPT; matrix is the more precise, correct location.

📄 View Raw YAML

 
id: P51649
gene_symbol: ALDH5A1
product_type: PROTEIN
status: INITIALIZED
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: Succinate-semialdehyde dehydrogenase, mitochondrial (SSADH; ALDH5A1)
  is a mitochondrial NAD+-dependent oxidoreductase (EC 1.2.1.24) that catalyzes the
  final step of gamma-aminobutyric acid (GABA) degradation, the GABA shunt. It
  irreversibly oxidizes succinate semialdehyde to succinate (using NAD+ and water,
  producing NADH), thereby routing the carbon skeleton of the inhibitory
  neurotransmitter GABA into the tricarboxylic acid (TCA) cycle and central energy
  metabolism. The protein is synthesized as a precursor with an N-terminal transit
  peptide, imported into the mitochondrion, and assembles into a homotetramer that
  resides in the mitochondrial matrix; its activity is redox-regulated through a
  reversible disulfide bond on a dynamic catalytic loop. It is a member of the
  aldehyde dehydrogenase superfamily and is expressed broadly, including in brain,
  liver, kidney and skeletal muscle. Loss-of-function variants cause succinic
  semialdehyde dehydrogenase deficiency (SSADH deficiency; 4-hydroxybutyric /
  gamma-hydroxybutyric aciduria), an autosomal recessive disorder in which GABA and
  4-hydroxybutyrate (GHB) accumulate, producing developmental delay, hypotonia,
  intellectual disability, ataxia, seizures and behavioural disturbances.
alternative_products:
- name: '1'
  id: P51649-1
- name: '2'
  id: P51649-2
  sequence_note: VSP_045231
existing_annotations:
- term:
    id: GO:0004777
    label: succinate-semialdehyde dehydrogenase (NAD+) activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: Phylogenetic (IBA) assignment of the enzyme's core NAD+-dependent
      succinate-semialdehyde dehydrogenase activity, consistent with the family and
      with direct experimental evidence for the human protein.
    action: ACCEPT
    reason: This is the well-established core molecular function of SSADH, catalyzing
      the NAD+-dependent oxidation of succinate semialdehyde to succinate (EC
      1.2.1.24). The IBA is corroborated by multiple experimental annotations for the
      human enzyme (PMID:12208142, PMID:14635103, PMID:19300440, PMID:16199352,
      PMID:9683595) and the UniProt catalytic activity statement.
    supported_by:
    - reference_id: PMID:19300440
      supporting_text: "Succinic semialdehyde dehydrogenase (SSADH) is involved in the final degradation \nstep of the inhibitory neurotransmitter gamma-aminobutyric acid by converting \nsuccinic semialdehyde to succinic acid in the mitochondrial matrix"
    - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
      supporting_text: "Reaction=succinate semialdehyde + NAD(+) + H2O = succinate + NADH + 2"
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: Phylogenetic assignment of mitochondrial localization, consistent with
      the N-terminal mitochondrial transit peptide and multiple experimental
      localization datasets.
    action: ACCEPT
    reason: SSADH is a nuclear-encoded mitochondrial matrix enzyme with an N-terminal
      transit peptide; mitochondrial localization is well supported. The more precise
      location (mitochondrial matrix, GO:0005759) is captured by a separate
      annotation.
    supported_by:
    - reference_id: PMID:19300440
      supporting_text: "converting \nsuccinic semialdehyde to succinic acid in the mitochondrial matrix"
    - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
      supporting_text: "SUBCELLULAR LOCATION: Mitochondrion"
- term:
    id: GO:0009450
    label: GABA catabolic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: Phylogenetic assignment of the GABA catabolic process, the pathway in
      which SSADH performs the terminal step.
    action: ACCEPT
    reason: SSADH catalyzes the last committed step of GABA degradation (succinate
      semialdehyde to succinate). This is a core biological process for the gene,
      independently supported by IDA (PMID:9683595) and the UniProt/literature
      pathway descriptions.
    supported_by:
    - reference_id: PMID:12208142
      supporting_text: "represents the last enzyme in the GABA catabolism and irreversibly \noxidizes SSA to succinate"
- term:
    id: GO:0004777
    label: succinate-semialdehyde dehydrogenase (NAD+) activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: Automated electronic assignment (ARBA/RHEA/EC mapping) of the core
      NAD+-dependent SSADH activity, matching EC 1.2.1.24 and RHEA:13217.
    action: ACCEPT
    reason: The electronic mapping (EC:1.2.1.24, RHEA:13217) reproduces the correct
      core molecular function and agrees with the experimental annotations.
    supported_by:
    - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
      supporting_text: "Reaction=succinate semialdehyde + NAD(+) + H2O = succinate + NADH + 2"
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: Automated electronic assignment of mitochondrial localization (via mouse
      ortholog and UniProt-SubCell), consistent with the transit peptide and
      experimental data.
    action: ACCEPT
    reason: Correct compartment; agrees with the transit peptide, IDA/HDA/HTP
      proteomics and the IBA localization.
    supported_by:
    - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
      supporting_text: "SUBCELLULAR LOCATION: Mitochondrion"
- term:
    id: GO:0009013
    label: succinate-semialdehyde dehydrogenase [NAD(P)+] activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro2GO electronic assignment of the broader NAD(P)+-utilizing SSADH
      activity. The human enzyme is specifically NAD+-dependent, so this term is less
      precise than GO:0004777.
    action: MARK_AS_OVER_ANNOTATED
    reason: The human enzyme is an NAD+-specific succinate-semialdehyde dehydrogenase
      (EC 1.2.1.24; named "NAD(+)-dependent"; kinetically characterized with NAD+),
      whereas GO:0009013 [NAD(P)+] is the broader family term that also covers
      NADP+-utilizing (e.g. bacterial EC 1.2.1.16) enzymes. It is not wrong at the
      family level but over-annotates cofactor breadth; the NAD+-specific GO:0004777
      is the correct, more precise molecular function.
    proposed_replacement_terms:
    - id: GO:0004777
      label: succinate-semialdehyde dehydrogenase (NAD+) activity
    supported_by:
    - reference_id: PMID:16199352
      supporting_text: "The Michaelis \nconstants K(m) for succinic semialdehyde and NAD(+) were 6.3 and 125 microM"
    - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
      supporting_text: "EC=1.2.1.24"
- term:
    id: GO:0009450
    label: GABA catabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: Automated electronic assignment of the GABA catabolic process (via
      InterPro and UniPathway 4-aminobutanoate degradation).
    action: ACCEPT
    reason: Correct core process; agrees with the IBA and IDA annotations and the
      UniProt pathway statement (4-aminobutanoate degradation).
    supported_by:
    - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
      supporting_text: "Amino-acid degradation; 4-aminobutanoate degradation"
- term:
    id: GO:0016491
    label: oxidoreductase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro2GO electronic assignment of the generic oxidoreductase parent
      term.
    action: MARK_AS_OVER_ANNOTATED
    reason: Correct but uninformative; GO:0016491 is a high-level parent of the
      specific succinate-semialdehyde dehydrogenase (NAD+) activity (GO:0004777) that
      is already annotated. The specific term should be preferred.
    proposed_replacement_terms:
    - id: GO:0004777
      label: succinate-semialdehyde dehydrogenase (NAD+) activity
    supported_by:
    - reference_id: PMID:7814412
      supporting_text: "suggesting it is a member of \nthe aldehyde dehydrogenase superfamily of proteins"
- term:
    id: GO:0006540
    label: GABA shunt
  evidence_type: IMP
  original_reference_id: PMID:15037717
  qualifier: involved_in
  review:
    summary: Involvement in the GABA shunt, the pathway (GABA -> succinate semialdehyde
      -> succinate) that bypasses part of the TCA cycle; supported by the SSADH
      deficiency phenotype of GABA and GHB accumulation.
    action: ACCEPT
    reason: SSADH performs the terminal, committed step of the GABA shunt. The cited
      case report documents the metabolic consequences of loss of SSADH function
      (elevated GABA with GHB), consistent with a role in this pathway. GO:0006540
      (GABA shunt) is an appropriately specific pathway term for the enzyme's role.
    supported_by:
    - reference_id: PMID:15037717
      supporting_text: "disorder of the CNS catabolism of gamma-aminobutyric acid (GABA), leading to \naccumulation of the metabolite 4-hydroxybutyrate (GHB)"
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Direct immunofluorescence (HPA) localization to the mitochondrion.
    action: ACCEPT
    reason: Direct experimental localization consistent with the transit peptide and
      all other localization evidence.
    supported_by:
    - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
      supporting_text: "SUBCELLULAR LOCATION: Mitochondrion"
- term:
    id: GO:0004777
    label: succinate-semialdehyde dehydrogenase (NAD+) activity
  evidence_type: EXP
  original_reference_id: PMID:12208142
  qualifier: enables
  review:
    summary: Experimental determination of NAD+-dependent SSADH catalytic activity
      for the human enzyme (including characterization of natural variants).
    action: ACCEPT
    reason: Core molecular function, experimentally established. This study
      characterized the human gene, isoforms and variant activities, confirming the
      NAD+-dependent SSA-to-succinate reaction.
    supported_by:
    - reference_id: PMID:12208142
      supporting_text: "represents the last enzyme in the GABA catabolism and irreversibly \noxidizes SSA to succinate"
- term:
    id: GO:0004777
    label: succinate-semialdehyde dehydrogenase (NAD+) activity
  evidence_type: EXP
  original_reference_id: PMID:14635103
  qualifier: enables
  review:
    summary: Experimental functional analysis of the human enzyme via expression of
      wild-type and disease variants, confirming NAD+-dependent SSADH activity.
    action: ACCEPT
    reason: Core molecular function, experimentally established; disease-causing
      missense variants reduced SSADH activity to <5% of normal, directly implicating
      this activity as the gene's function.
    supported_by:
    - reference_id: PMID:14635103
      supporting_text: "the missense mutations we \nconsider to be causative of SSADH deficiency reduced the SSADH activity to less \nthan 5% of the normal activity in our in vitro expression system"
- term:
    id: GO:0004777
    label: succinate-semialdehyde dehydrogenase (NAD+) activity
  evidence_type: EXP
  original_reference_id: PMID:19300440
  qualifier: enables
  review:
    summary: Structural and biochemical characterization of human SSADH, confirming
      the NAD+-dependent conversion of succinic semialdehyde to succinate.
    action: ACCEPT
    reason: Core molecular function, established by crystal structures with product
      and ADP plus catalytic-residue mutagenesis; the enzyme converts succinic
      semialdehyde to succinic acid in the mitochondrial matrix.
    supported_by:
    - reference_id: PMID:19300440
      supporting_text: "Succinic semialdehyde dehydrogenase (SSADH) is involved in the final degradation \nstep of the inhibitory neurotransmitter gamma-aminobutyric acid by converting \nsuccinic semialdehyde to succinic acid in the mitochondrial matrix"
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  qualifier: located_in
  review:
    summary: High-throughput identification of SSADH in the high-confidence human
      mitochondrial proteome (MitoCoP).
    action: ACCEPT
    reason: Consistent with all other localization evidence; SSADH is a bona fide
      mitochondrial protein captured in a curated high-confidence mitochondrial
      proteome inventory.
    supported_by:
    - reference_id: PMID:34800366
      supporting_text: "mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP)"
- term:
    id: GO:0004777
    label: succinate-semialdehyde dehydrogenase (NAD+) activity
  evidence_type: IDA
  original_reference_id: PMID:16199352
  qualifier: enables
  review:
    summary: Direct biochemical characterization of recombinant human SSADH,
      including kinetic constants for succinic semialdehyde and NAD+.
    action: ACCEPT
    reason: Core molecular function, directly measured. Recombinant enzyme was active
      (specific activity and Km values reported) and forms a homotetramer.
    supported_by:
    - reference_id: PMID:16199352
      supporting_text: "The Michaelis \nconstants K(m) for succinic semialdehyde and NAD(+) were 6.3 and 125 microM"
- term:
    id: GO:0042802
    label: identical protein binding
  evidence_type: IPI
  original_reference_id: PMID:16199352
  qualifier: enables
  review:
    summary: Self-association reflecting the homotetrameric quaternary structure of
      SSADH (identical protein binding).
    action: KEEP_AS_NON_CORE
    reason: The homotetramer is genuine (purified recombinant SSADH is a tetramer of
      identical subunits; also stated in UniProt SUBUNIT). "Identical protein binding"
      is a generic term that captures oligomerization rather than an informative
      molecular function; it is retained but marked non-core, and is not removed
      because it is an experimental IPI supported by the quaternary-structure data.
    supported_by:
    - reference_id: PMID:16199352
      supporting_text: "The purified SSADH appears to be a tetramer of identical \nsubunits"
    - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
      supporting_text: "SUBUNIT: Homotetramer"
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HDA
  original_reference_id: PMID:20833797
  qualifier: located_in
  review:
    summary: High-throughput MS detection of SSADH in isolated functional human
      skeletal-muscle mitochondria (mitochondrial phosphoproteome).
    action: ACCEPT
    reason: Consistent with all other localization evidence; SSADH was identified in a
      preparation of isolated mitochondria.
    supported_by:
    - reference_id: PMID:20833797
      supporting_text: "phosphoproteomics study of functional mitochondria isolated from human muscle biopsies"
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-888548
  qualifier: located_in
  review:
    summary: Traceable assertion (Reactome) placing the SSADH reaction in the
      mitochondrial matrix.
    action: ACCEPT
    reason: Mitochondrial matrix is the correct, more precise subcellular location for
      this soluble matrix enzyme, consistent with the literature stating the reaction
      occurs in the mitochondrial matrix.
    supported_by:
    - reference_id: Reactome:R-HSA-888548
      supporting_text: "Mitochondrial succinate semialdehyde dehydrogenase (ALDH5A1) tetramer catalyzes the reaction of succinate semialdehyde (SUCCSA), H2O, and NAD+ to form succinate (SUCCA) and NADH + 2 H+"
    - reference_id: PMID:19300440
      supporting_text: "converting \nsuccinic semialdehyde to succinic acid in the mitochondrial matrix"
- term:
    id: GO:0006105
    label: succinate metabolic process
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: involved_in
  review:
    summary: Sequence-similarity transfer (from mouse ortholog) of a role in succinate
      metabolism; succinate is the direct product of the SSADH reaction.
    action: KEEP_AS_NON_CORE
    reason: Correct in that succinate is the reaction product that feeds the TCA cycle,
      but this is a broad process term that is subsumed by the more informative GABA
      catabolic process / GABA shunt annotations. Retained as a non-core, downstream
      metabolic role rather than the enzyme's defining function.
    supported_by:
    - reference_id: PMID:12208142
      supporting_text: "irreversibly \noxidizes SSA to succinate"
- term:
    id: GO:0006536
    label: glutamate metabolic process
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: involved_in
  review:
    summary: Sequence-similarity transfer (from mouse ortholog) of a role in glutamate
      metabolism.
    action: MARK_AS_OVER_ANNOTATED
    reason: Glutamate is upstream of SSADH in the GABA pathway (glutamate -> GABA via
      glutamate decarboxylase), not a substrate or product of the SSADH reaction.
      SSADH acts on succinate semialdehyde and produces succinate; any effect on
      glutamate metabolism is indirect (through the connected GABA/glutamate
      shunt). The direct catalytic contribution is to GABA catabolism, so this
      broader/indirect process term is an over-annotation.
    supported_by:
    - reference_id: PMID:19300440
      supporting_text: "by converting \nsuccinic semialdehyde to succinic acid in the mitochondrial matrix"
- term:
    id: GO:0004777
    label: succinate-semialdehyde dehydrogenase (NAD+) activity
  evidence_type: ISS
  original_reference_id: PMID:7814412
  qualifier: enables
  review:
    summary: Sequence-similarity assignment of NAD+-dependent SSADH activity based on
      cloning and characterization of the mammalian enzyme.
    action: ACCEPT
    reason: Core molecular function. The cited study cloned mature NAD+-dependent
      SSADH (EC 1.2.1.24) from rat and human and confirmed activity by bacterial
      expression, placing it in the aldehyde dehydrogenase superfamily.
    supported_by:
    - reference_id: PMID:7814412
      supporting_text: "encoding succinic semialdehyde dehydrogenase (SSADH; EC 1.2.1.24)"
- term:
    id: GO:0004777
    label: succinate-semialdehyde dehydrogenase (NAD+) activity
  evidence_type: IDA
  original_reference_id: PMID:9683595
  qualifier: enables
  review:
    summary: Direct assignment of NAD+-dependent SSADH activity in the context of
      identifying the molecular basis of SSADH deficiency.
    action: ACCEPT
    reason: Core molecular function; this study established the human SSADH cDNA and
      catalytic activity while defining exon-skipping mutations underlying the
      deficiency.
    supported_by:
    - reference_id: PMID:9683595
      supporting_text: "Succinic semialdehyde \ndehydrogenase (SSADH) deficiency, a rare metabolic \ndisorder of 4-aminobutyric acid degradation"
- term:
    id: GO:0007417
    label: central nervous system development
  evidence_type: IMP
  original_reference_id: PMID:9683595
  qualifier: involved_in
  review:
    summary: Assignment of a role in CNS development inferred from the neurological
      phenotype of SSADH deficiency.
    action: KEEP_AS_NON_CORE
    reason: The neurological features of SSADH deficiency (developmental delay,
      hypotonia, ataxia, seizures) are downstream consequences of losing GABA
      catabolism and the resulting GABA/GHB accumulation, rather than evidence that
      SSADH itself directs CNS developmental programs. This is a pleiotropic, disease-
      derived process; it is retained as non-core (not removed, as it is an
      experimental IMP whose full text is not available here), but the enzyme's core
      role is metabolic (GABA catabolism), not developmental.
    supported_by:
    - reference_id: PMID:9683595
      supporting_text: "Succinic semialdehyde \ndehydrogenase (SSADH) deficiency, a rare metabolic \ndisorder of 4-aminobutyric acid degradation"
- term:
    id: GO:0009450
    label: GABA catabolic process
  evidence_type: IDA
  original_reference_id: PMID:9683595
  qualifier: involved_in
  review:
    summary: Direct assignment of involvement in GABA catabolism; SSADH deficiency
      disrupts GABA degradation.
    action: ACCEPT
    reason: Core biological process. SSADH performs the terminal step of GABA
      degradation; loss of function causes accumulation of GABA and its shunt product
      GHB, confirming the enzyme's role in this catabolic pathway.
    supported_by:
    - reference_id: PMID:9683595
      supporting_text: "Succinic semialdehyde \ndehydrogenase (SSADH) deficiency, a rare metabolic \ndisorder of 4-aminobutyric acid degradation"
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: Sequence-similarity transfer (from mouse ortholog) of mitochondrial
      localization.
    action: ACCEPT
    reason: Correct compartment; redundant with the direct and phylogenetic
      mitochondrial localization annotations.
    supported_by:
    - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
      supporting_text: "SUBCELLULAR LOCATION: Mitochondrion"
core_functions:
- description: NAD+-dependent oxidation of succinate semialdehyde to succinate, the
    terminal step of GABA catabolism, feeding the carbon skeleton of GABA into the
    TCA cycle
  molecular_function:
    id: GO:0004777
    label: succinate-semialdehyde dehydrogenase (NAD+) activity
  directly_involved_in:
  - id: GO:0009450
    label: GABA catabolic process
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  supported_by:
  - reference_id: PMID:19300440
    supporting_text: "Succinic semialdehyde dehydrogenase (SSADH) is involved in the final degradation \nstep of the inhibitory neurotransmitter gamma-aminobutyric acid by converting \nsuccinic semialdehyde to succinic acid in the mitochondrial matrix"
  - reference_id: PMID:12208142
    supporting_text: "represents the last enzyme in the GABA catabolism and irreversibly \noxidizes SSA to succinate"
  - reference_id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
    supporting_text: "Reaction=succinate semialdehyde + NAD(+) + H2O = succinate + NADH + 2"
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs
    by curator judgment of sequence similarity
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:12208142
  title: 'Structure of human succinic semialdehyde dehydrogenase gene: identification
    of promoter region and alternatively processed isoforms.'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Primary study of the human ALDH5A1 gene; states SSADH is the last
      enzyme of GABA catabolism and irreversibly oxidizes SSA to succinate. Supports
      core MF and BP.
- id: PMID:14635103
  title: Mutational spectrum of the succinate semialdehyde dehydrogenase (ALDH5A1)
    gene and functional analysis of 27 novel disease-causing mutations in patients
    with SSADH deficiency.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Functional expression of human variants; disease-causing missense
      alleles reduce SSADH activity to <5% of normal, directly linking the activity to
      the gene.
- id: PMID:15037717
  title: Proton MR spectroscopy in succinic semialdehyde dehydrogenase deficiency.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Case report of SSADH deficiency; documents that the disorder is one
      of CNS GABA catabolism with GABA and GHB accumulation, supporting the GABA
      shunt / catabolic role.
- id: PMID:16199352
  title: High-level expression and characterization of the recombinant enzyme, and
    tissue distribution of human succinic semialdehyde dehydrogenase.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Biochemical characterization of recombinant human SSADH (kinetics,
      homotetramer). Supports both the NAD+ activity and the identical-protein-binding
      (oligomerization) annotation.
- id: PMID:19300440
  title: Redox-switch modulation of human SSADH by dynamic catalytic loop.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Crystal structures and mutagenesis of human SSADH; explicitly states
      the enzyme converts succinic semialdehyde to succinic acid in the mitochondrial
      matrix as the final step of GABA degradation. Anchors core MF, BP and location.
- id: PMID:20833797
  title: Phosphoproteome analysis of functional mitochondria isolated from resting
    human muscle reveals extensive phosphorylation of inner membrane protein complexes
    and enzymes.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Large-scale mitochondrial phosphoproteomics; supports mitochondrial
      localization only (SSADH detected in isolated mitochondria).
- id: PMID:34800366
  title: Quantitative high-confidence human mitochondrial proteome and its dynamics
    in cellular context.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: MitoCoP high-confidence mitochondrial proteome inventory; supports
      mitochondrial localization.
- id: PMID:7814412
  title: Molecular cloning of the mature NAD(+)-dependent succinic semialdehyde dehydrogenase
    from rat and human. cDNA isolation, evolutionary homology, and tissue expression.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Cloning of mature NAD+-dependent SSADH (EC 1.2.1.24) from rat and
      human; assigns the enzyme to the aldehyde dehydrogenase superfamily.
- id: PMID:9683595
  title: Two exon-skipping mutations as the molecular basis of succinic semialdehyde
    dehydrogenase deficiency (4-hydroxybutyric aciduria).
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Establishes the genetic basis of SSADH deficiency; source for the
      GABA catabolic process (IDA) and CNS development (IMP) annotations.
- id: Reactome:R-HSA-888548
  title: ALDH5A1 dehydrogenates SUCCSA to SUCCA
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Reactome reaction; places the SSADH-catalyzed conversion of succinate
      semialdehyde to succinate in the mitochondrial matrix.
- id: file:human/ALDH5A1/ALDH5A1-uniprot.txt
  title: UniProtKB P51649 (SSDH_HUMAN), Succinate-semialdehyde dehydrogenase, mitochondrial
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: UniProt record; source for catalytic activity (RHEA:13217, EC
      1.2.1.24), mitochondrial subcellular location, homotetramer, 4-aminobutanoate
      degradation pathway, and SSADH deficiency disease association.
suggested_questions:
- question: Beyond the neurological phenotype, does SSADH activity contribute
    quantitatively to cellular energy metabolism in non-neural tissues (liver, kidney,
    skeletal muscle) where it is highly expressed?
- question: To what extent is SSADH activity regulated in vivo by the redox-sensitive
    Cys340-Cys342 catalytic-loop disulfide, and does this modulate GABA and GHB levels
    under oxidative stress?
suggested_experiments:
- description: Isotope tracing (e.g. 13C-GABA) in patient-derived or knockout cells to
    quantify flux through SSADH into the TCA cycle and confirm the metabolic (versus
    purely developmental) consequences of loss of function.
- description: Structure-guided assays testing whether physiological oxidants shift
    SSADH between active and inhibited states in intact mitochondria, linking the redox
    switch to GABA shunt output.