GSTZ1 is a cytosolic, homodimeric enzyme of the glutathione transferase (GST) superfamily (Zeta class) that is bifunctional. Its principal physiological role is maleylacetoacetate isomerase (MAAI; EC 5.2.1.2), catalysing the glutathione-dependent cis-trans isomerisation of 4-maleylacetoacetate to 4-fumarylacetoacetate. This is the penultimate step of the phenylalanine/tyrosine catabolic pathway that ultimately yields fumarate and acetoacetate, and it is the reaction that gives the gene its "maleylacetoacetate isomerase" name. Glutathione serves as an essential cofactor that is regenerated during the isomerisation. As a canonical member of the GST family the enzyme also possesses classical glutathione S-transferase activity (EC 2.5.1.18), although its conjugating activity toward standard GST substrates is weak, and it has low glutathione peroxidase activity toward organic hydroperoxides. GSTZ1 is the enzyme responsible for the glutathione- dependent biotransformation (dehalogenation/oxygenation) of the drug and environmental contaminant dichloroacetate (DCA) to glyoxylate; DCA inactivates GSTZ1 in a mechanism-based manner, so repeated DCA exposure reduces its own clearance. The enzyme is predominantly cytosolic but is also found in the mitochondrial matrix. It is most highly expressed in liver and kidney (where tyrosine catabolism is active) but is expressed broadly across tissues. Loss of GSTZ1/MAAI function causes maleylacetoacetate isomerase deficiency, a biochemically mild, generally benign inborn error characterised by elevated succinylacetone (and maleylacetone) in blood and urine, usually detected on newborn screening, rather than a severe tyrosinemia.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016034 maleylacetoacetate isomerase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference of maleylacetoacetate isomerase (MAAI) activity, the defining and core molecular function of GSTZ1. This is well supported by direct biochemical characterisation of the human enzyme and by the Zeta-class panel. Reason: MAAI activity is the primary physiological function of GSTZ1 and is directly demonstrated for the human recombinant enzyme (see the IDA annotation from PMID:10739172). The IBA correctly captures the ancestral, conserved function of the Zeta family. Supporting Evidence: PMID:9417084 characterize at the cDNA level the human gene for maleylacetoacetate isomerase (MAAI, EC 5.2.1.2), the only as yet unidentified structural gene of the phenylalanine catabolic pathway |
| GO:0004364 glutathione transferase activity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) inference of glutathione transferase activity, reflecting GSTZ1's membership in the GST superfamily. The activity is genuine but weak toward classical GST substrates and is secondary to the MAAI role. Reason: Recombinant human GSTZ1-1 has only minimal glutathione-conjugating activity with classical GST substrates, so while this activity is real (and mechanistically underlies DCA biotransformation), it is not the enzyme's core evolved function. Supporting Evidence: PMID:9396740 The protein is a dimer composed of 24.2 kDa subunits and has minimal glutathione-conjugating activity with ethacrynic acid and 7-chloro-4-nitrobenz-2-oxa-1, 3-diazole |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) inference of mitochondrial localisation. GSTZ1 is experimentally detected in the mitochondrial matrix as well as the cytosol, but its canonical/primary compartment is the cytosol. Reason: Mitochondrial localisation is experimentally supported (submitochondrial fractionation places GSTZ1 in the matrix; PMID:20884751), but the enzyme is predominantly cytosolic and its core function is exerted in the cytosol. Supporting Evidence: PMID:20884751 an enzyme colocalized in cytosol and mitochondrial matrix |
| GO:0006559 L-phenylalanine catabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference that GSTZ1 participates in the phenylalanine catabolic pathway (the common Phe/Tyr degradation route). This is the core biological process in which the MAAI activity operates. Reason: MAAI is the fifth enzyme of the L-phenylalanine (and tyrosine) degradation pathway that produces fumarate and acetoacetate; this pathway membership is a core aspect of GSTZ1 biology. Supporting Evidence: PMID:9417084 maleylacetoacetate isomerase (MAAI, EC 5.2.1.2), the only as yet unidentified structural gene of the phenylalanine catabolic pathway |
| GO:0006749 glutathione metabolic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) inference of participation in glutathione metabolic process, reflecting the enzyme's use of glutathione as a cofactor/co-substrate. Reason: Glutathione is an obligatory cofactor for the MAAI reaction and a co-substrate for the GST activity, so this process annotation is correct, but it is ancillary to the enzyme's core role in tyrosine/phenylalanine catabolism rather than a primary function in glutathione homeostasis. Supporting Evidence: PMID:10739172 Discovery of a functional polymorphism in human glutathione transferase zeta |
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Root-level InterPro2GO mapping of generic catalytic activity from the GST_Zeta InterPro signature. Uninformative given the specific activities already annotated. Reason: GO:0003824 catalytic activity is a very high-level molecular function term that conveys no specific information. The enzyme's activities are captured precisely by GO:0016034 (MAAI) and GO:0004364 (glutathione transferase), so this generic term is redundant and over-general. |
| GO:0004364 glutathione transferase activity | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Electronic (UniProt/RHEA/EC) mapping of glutathione transferase activity (EC 2.5.1.18, RHEA:16437). Duplicates the IBA/IDA glutathione transferase annotations; genuine but secondary. Reason: Correct GST activity assignment (the enzyme is an EC 2.5.1.18 glutathione transferase) but conjugating activity toward standard substrates is weak and secondary to the MAAI isomerase role. Supporting Evidence: PMID:9396740 has minimal glutathione-conjugating activity with ethacrynic acid and 7-chloro-4-nitrobenz-2-oxa-1, 3-diazole |
| GO:0005737 cytoplasm | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (UniProt-SubCell) mapping of cytoplasmic localisation. Correct but more general than the specific cytosol annotation. Reason: GSTZ1 is a cytoplasmic/cytosolic enzyme (UniProt subcellular location: Cytoplasm). The term is correct though less specific than GO:0005829 cytosol. |
| GO:0009072 aromatic amino acid metabolic process | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping to aromatic amino acid metabolic process, the parent branch encompassing phenylalanine/tyrosine catabolism. Reason: Correct, if general: MAAI acts in the degradation of the aromatic amino acids phenylalanine and tyrosine. Consistent with the more specific L-phenylalanine and L-tyrosine catabolic process annotations. |
| GO:0016034 maleylacetoacetate isomerase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (UniProt/RHEA/EC 5.2.1.2, RHEA:14817) mapping of MAAI activity. Duplicates the IBA/IDA MAAI annotations for the core function. Reason: Correct assignment of the defining EC 5.2.1.2 maleylacetoacetate isomerase activity, independently confirmed experimentally (PMID:10739172). Supporting Evidence: PMID:10739172 Discovery of a functional polymorphism in human glutathione transferase zeta |
| GO:0098869 cellular oxidant detoxification | IEA GO_REF:0000108 | KEEP AS NON CORE | Summary: Inter-ontology (GOC) inference of cellular oxidant detoxification derived from the glutathione peroxidase activity annotation (GO:0004602). Reason: GSTZ1 has low glutathione peroxidase activity toward organic hydroperoxides, so a minor role in oxidant detoxification is plausible, but this is peripheral to its core tyrosine-catabolism function and depends on a weak enzymatic activity. Supporting Evidence: PMID:9396740 GSTZ1-1 has glutathione peroxidase activity with t-butyl and cumene hydroperoxides |
| GO:0005515 protein binding | IPI PMID:17474147 Systematic identification of SH3 domain-mediated human prote... | MARK AS OVER ANNOTATED | Summary: Generic protein binding from a high-throughput SH3-domain peptide array screen. Uninformative and not tied to any established biological function of GSTZ1. Reason: GO:0005515 protein binding is an uninformative molecular function term. This annotation derives from a systematic peptide-array interactome screen (SH3 domains) with no demonstrated functional consequence for GSTZ1. Supporting Evidence: PMID:17474147 Systematic identification of SH3 domain-mediated human protein-protein interactions by peptide array target screening |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: Generic protein binding from the HuRI binary interactome map. Uninformative high-throughput interactor set (e.g. NCK1, QARS1, TRAF2, GORASP2) with no established functional relevance to GSTZ1. Reason: GO:0005515 protein binding is uninformative and this is a systematic all-by-all binary interactome dataset; none of the interactors define a specific molecular function of GSTZ1. Supporting Evidence: PMID:32296183 we present a human 'all-by-all' reference interactome map of human binary protein interactions, or 'HuRI' |
| GO:0042802 identical protein binding | IPI PMID:16189514 Towards a proteome-scale map of the human protein-protein in... | KEEP AS NON CORE | Summary: Identical protein binding (self-interaction) from a proteome-scale interactome screen, consistent with the enzyme being a homodimer. Reason: GSTZ1 is a homodimer (established by crystallography, PMID:11327815), so self-interaction is genuine. It is retained as a non-core structural property rather than a core molecular function. Supporting Evidence: PMID:16189514 Towards a proteome-scale map of the human protein-protein interaction network |
| GO:0042802 identical protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | KEEP AS NON CORE | Summary: Identical protein binding (self-interaction) from a proteome-scale interactome map, consistent with the homodimeric quaternary structure. Reason: Self-interaction reflects the known homodimeric assembly of GSTZ1; genuine but a structural property rather than a core molecular function. Supporting Evidence: PMID:25416956 A proteome-scale map of the human interactome network |
| GO:0042802 identical protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | KEEP AS NON CORE | Summary: Identical protein binding (self-interaction) from the HuRI binary interactome map, consistent with the homodimeric structure of GSTZ1. Reason: Self-interaction reflects the homodimeric assembly established by crystallography (PMID:11327815); retained as a non-core structural property. Supporting Evidence: PMID:32296183 a human 'all-by-all' reference interactome map of human binary protein interactions, or 'HuRI' |
| GO:0006559 L-phenylalanine catabolic process | IEA GO_REF:0000041 | ACCEPT | Summary: UniPathway-based electronic annotation to the L-phenylalanine degradation pathway (UPA00139), matching the UniProt PATHWAY statement (step 5/6). Reason: Correct pathway membership: MAAI catalyses step 5 of L-phenylalanine degradation to acetoacetate and fumarate, per the UniProt UniPathway annotation. Supporting Evidence: PMID:9417084 the only as yet unidentified structural gene of the phenylalanine catabolic pathway |
| GO:0005739 mitochondrion | IDA PMID:20884751 Mitochondrion as a novel site of dichloroacetate biotransfor... | KEEP AS NON CORE | Summary: Direct assay (immunodetection, LC-MS/MS, submitochondrial fractionation) placing active GSTZ1 in liver mitochondria, where it also biotransforms DCA. A genuine secondary localisation alongside the predominant cytosolic pool. Reason: Experimentally supported mitochondrial localisation and activity, but GSTZ1 is predominantly cytosolic; the mitochondrial pool is secondary to the core cytosolic MAAI function. Defer to the experimental annotation (do not remove). Supporting Evidence: PMID:20884751 Immunoreactive GSTZ1 was detected in liver mitochondria from humans and rats, and its identity was confirmed by liquid chromatography/tandem mass spectrometry |
| GO:0005829 cytosol | IDA PMID:20884751 Mitochondrion as a novel site of dichloroacetate biotransfor... | ACCEPT | Summary: Direct assay demonstrating catalytically active GSTZ1 in the cytosol, the predominant and canonical compartment for the enzyme. Reason: Cytosol is the primary subcellular location where GSTZ1 exerts its MAAI and GST activities; DCA dechlorination activity is 2.5-3-fold higher in cytosol than in whole mitochondria. Supporting Evidence: PMID:20884751 The specific activity of GSTZ1-catalyzed dechlorination of DCA was 2.5- to 3-fold higher in cytosol than in whole mitochondria |
| GO:1990748 cellular detoxification | IDA PMID:20884751 Mitochondrion as a novel site of dichloroacetate biotransfor... | KEEP AS NON CORE | Summary: Direct-assay annotation of cellular detoxification, reflecting GSTZ1-catalysed glutathione-dependent biotransformation of dichloroacetate (a drug/xenobiotic) to glyoxylate. Reason: GSTZ1 genuinely detoxifies/biotransforms dichloroacetate, a pharmacologically and environmentally relevant halogenated compound. This xenobiotic-handling role is a real but non-core function relative to tyrosine catabolism. Supporting Evidence: PMID:20884751 a cytosolic enzyme that converts DCA to glyoxylate |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | KEEP AS NON CORE | Summary: High-throughput mitochondrial proteomics detection of GSTZ1, corroborating the secondary mitochondrial pool. Reason: Consistent with the IDA mitochondrial localisation; a genuine but secondary compartment for the predominantly cytosolic enzyme. Supporting Evidence: PMID:34800366 Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-9011595 | KEEP AS NON CORE | Summary: Reactome (TAS) placement of GSTZ1 in the mitochondrial matrix, associated with the DCA dehalogenation reaction. Matches submitochondrial fractionation data. Reason: Matrix localisation is supported by rat submitochondrial fractionation (PMID:20884751), but is secondary to the predominant cytosolic pool. Supporting Evidence: PMID:20884751 Study of rat submitochondrial fractions revealed GSTZ1 to be localized in the mitochondrial matrix |
| GO:0005829 cytosol | TAS Reactome:R-HSA-176054 | ACCEPT | Summary: Reactome (TAS) annotation of cytosolic localisation associated with GST-mediated glutathione conjugation. Consistent with the canonical compartment of GSTZ1. Reason: Cytosol is the primary compartment of GSTZ1; this Reactome TAS annotation agrees with direct experimental evidence. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-71173 | ACCEPT | Summary: Reactome (TAS) annotation of cytosolic localisation associated with the 4-MAA isomerisation reaction (GSTZ1 isomerizes 4-MAA), consistent with the enzyme's canonical cytosolic MAAI activity. Reason: Reflects the cytosolic MAAI reaction in tyrosine catabolism; cytosol is the primary compartment for this core function. |
| GO:0004364 glutathione transferase activity | IDA PMID:10739172 Discovery of a functional polymorphism in human glutathione ... | KEEP AS NON CORE | Summary: Direct-assay characterisation of glutathione transferase activity of recombinant human GSTZ1 allelic variants, including 3.6-fold higher DCA activity for the GSTZ1a-1a form. Reason: Genuine, directly demonstrated GST activity (the enzyme is EC 2.5.1.18) that mechanistically underlies DCA biotransformation, but conjugating activity toward standard substrates is weak and secondary to the MAAI role. Supporting Evidence: PMID:10739172 GSTZ1a-1a displayed differences in activity towards several substrates compared with GSTZ1b-1b and GSTZ1c-1c, including 3.6-fold higher activity towards dichloroacetate |
| GO:0006749 glutathione metabolic process | IDA PMID:10739172 Discovery of a functional polymorphism in human glutathione ... | KEEP AS NON CORE | Summary: Direct-assay annotation of participation in glutathione metabolic process, reflecting glutathione use during GSTZ1 catalysis. Reason: Correct given the obligatory role of glutathione as cofactor/co-substrate, but ancillary to the core tyrosine-catabolism function rather than a primary role in glutathione homeostasis. Supporting Evidence: PMID:10739172 human glutathione transferase zeta |
| GO:0016034 maleylacetoacetate isomerase activity | IDA PMID:10739172 Discovery of a functional polymorphism in human glutathione ... | ACCEPT | Summary: Direct-assay demonstration of maleylacetoacetate isomerase activity for recombinant human GSTZ1. This is the experimental anchor for the enzyme's core molecular function. Reason: MAAI activity is directly demonstrated for the human enzyme and is the defining, core evolved function of GSTZ1 in tyrosine/phenylalanine catabolism. The variant proteins were purified and assayed for catalytic activity toward multiple substrates. Supporting Evidence: PMID:10739172 The variant proteins were expressed in Escherichia coli as 6X His-tagged proteins and purified by Ni-agarose column chromatography |
| GO:0042803 protein homodimerization activity | IPI PMID:10739172 Discovery of a functional polymorphism in human glutathione ... | KEEP AS NON CORE | Summary: Protein homodimerization activity: GSTZ1 functions as a homodimer, the active quaternary form of the enzyme. Reason: Homodimerisation is genuine and structurally confirmed (crystal structure of the homodimer, PMID:11327815), forming the functional enzyme, but it is a structural property rather than the core catalytic molecular function. Supporting Evidence: PMID:10739172 human glutathione transferase zeta |
| GO:0005739 mitochondrion | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Sequence-similarity (ISS) transfer of mitochondrial localisation from the mouse orthologue (Q9WVL0). Consistent with experimentally observed mitochondrial pool. Reason: Corroborates the secondary mitochondrial localisation seen experimentally; the predominant compartment remains the cytosol. |
| GO:0004364 glutathione transferase activity | TAS PMID:9396740 Zeta, a novel class of glutathione transferases in a range o... | KEEP AS NON CORE | Summary: Author-stated (TAS) glutathione transferase activity from the original characterisation of the Zeta class of GSTs. Reason: Correct classification as a GST, but the conjugating activity is explicitly minimal with standard substrates; secondary to the MAAI role. Supporting Evidence: PMID:9396740 has minimal glutathione-conjugating activity with ethacrynic acid and 7-chloro-4-nitrobenz-2-oxa-1, 3-diazole |
| GO:0004602 glutathione peroxidase activity | TAS PMID:9396740 Zeta, a novel class of glutathione transferases in a range o... | KEEP AS NON CORE | Summary: Author-stated (TAS) glutathione peroxidase activity toward organic hydroperoxides, reported to be low relative to other GSTs. Reason: A genuine but low-level activity (with t-butyl and cumene hydroperoxides) that is peripheral to the enzyme's core MAAI function; underlies the derived oxidant detoxification annotation. Supporting Evidence: PMID:9396740 Although low in comparison with other GSTs, GSTZ1-1 has glutathione peroxidase activity with t-butyl and cumene hydroperoxides |
| GO:0006559 L-phenylalanine catabolic process | TAS PMID:9417084 Characterization of a fungal maleylacetoacetate isomerase ge... | ACCEPT | Summary: Author-stated (TAS) participation in the phenylalanine catabolic pathway, from the paper that identified the human MAAI gene as the last structural gene of that pathway. Reason: MAAI is a bona fide enzyme of the phenylalanine/tyrosine degradation pathway; this is a core biological process for GSTZ1. Supporting Evidence: PMID:9417084 the only as yet unidentified structural gene of the phenylalanine catabolic pathway |
| GO:0016034 maleylacetoacetate isomerase activity | TAS PMID:10373324 Gene structure, chromosomal location, and expression pattern... | ACCEPT | Summary: Author-stated (TAS) maleylacetoacetate isomerase activity, from the paper reporting the human MAAI gene structure and expression. Reinforces the core molecular function. Reason: Reaffirms the defining EC 5.2.1.2 MAAI activity of GSTZ1, the core molecular function. Supporting Evidence: PMID:10373324 The gene for maleylacetoacetate isomerase (MAAI) (EC 5.2.1.2) was the last gene in the mammalian phenylalanine/tyrosine catabolic pathway to be cloned |
| GO:0006572 L-tyrosine catabolic process | TAS PMID:10373324 Gene structure, chromosomal location, and expression pattern... | NEW | Summary: L-tyrosine catabolic process is NOT present in the current GOA snapshot for GSTZ1 but is the biological process most directly served by it: MAAI catalyses the penultimate step of tyrosine degradation. This is a reviewer-proposed (NEW) annotation, not an existing GOA row; it is supported by the MAAI-defining paper (PMID:10373324) and consistent with the UniProt "Tyrosine catabolism" keyword. Reason: Tyrosine catabolism is the specific pathway in which the maleylacetoacetate isomerase reaction operates (maleylacetoacetate and fumarylacetoacetate are tyrosine-degradation intermediates). This process should be annotated for GSTZ1 and is used in the core function; the phenylalanine catabolic process annotations capture the shared upstream route but not the tyrosine-specific one. Supporting Evidence: PMID:10373324 The gene for maleylacetoacetate isomerase (MAAI) (EC 5.2.1.2) was the last gene in the mammalian phenylalanine/tyrosine catabolic pathway to be cloned |
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Download this section (compressed HTML)Q: Given that MAAI is expressed ubiquitously whereas the full phenylalanine/tyrosine catabolic pathway is restricted to liver and kidney, does GSTZ1 have a distinct physiological role in non-hepatic tissues?
Q: What is the relative contribution of the cytosolic versus mitochondrial matrix pools of GSTZ1 to dichloroacetate biotransformation and to tyrosine catabolism in vivo?
Experiment: Metabolic flux and metabolomic profiling of GSTZ1-knockout cells and MAAID patient samples to quantify accumulation of maleylacetoacetate/maleylacetone and downstream succinylacetone.
Hypothesis: Loss of GSTZ1 MAAI activity diverts tyrosine catabolism to produce maleylacetone and succinylacetone without causing severe fumarylacetoacetate-driven toxicity, consistent with the benign MAAID phenotype.
Experiment: Compartment-specific (cytosol vs mitochondrial matrix) activity assays and targeting experiments to determine how GSTZ1 reaches the mitochondrial matrix and whether the matrix pool contributes meaningfully to DCA clearance.
Hypothesis: The mitochondrial matrix pool of GSTZ1 arises from dual localisation of the same cytosolic protein and contributes a minor fraction of total DCA biotransformation.
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