GSTZ1 (O43708, MAAI_HUMAN) review notes

Identity

• Human glutathione S-transferase zeta 1 (GSTZ1-1) = maleylacetoacetate isomerase (MAAI).
• Bifunctional cytosolic enzyme of the GST superfamily, Zeta family (216 aa, homodimer).
• EC 5.2.1.2 (MAAI isomerase) and EC 2.5.1.18 (GST). Requires glutathione as cofactor for MAAI activity.
• HGNC:4643; chromosome 14q24.3. PDB 1FW1, 8E8P.

Core physiological role: MAAI, step 5 of Phe/Tyr catabolism

• Catalyzes cis-trans isomerization of 4-maleylacetoacetate -> 4-fumarylacetoacetate,
  the penultimate step of tyrosine/phenylalanine degradation (glutathione-dependent).
• PMID:9417084 — human homologue of A. nidulans maiA; MAAI activity when expressed in E. coli.
• PMID:10373324 — gene expressed ubiquitously though pathway restricted to liver/kidney; "suggests a possible second role."
• UniProt PATHWAY: "L-phenylalanine degradation; acetoacetate and fumarate from L-phenylalanine: step 5/6." UniProt KW: Tyrosine catabolism, Phenylalanine catabolism.
• UniProt DR includes GO:0006572 L-tyrosine catabolic process (IEA UniProtKB-KW) and Reactome R-HSA-8963684 Tyrosine catabolism.

GST / xenobiotic role

• PMID:9396740 — defines Zeta class GST; low GST + low GPx activity.
• PMID:10739172 — functional polymorphism (GSTZ1*A K32/R42 vs variants); GSTZ1a-1a has 3.6-fold higher DCA activity. UniProt FUNCTION cites this for both catalytic activities and DCA oxygenation.
• Crystal structure with GSH PMID:11327815 "molecular basis for its remarkable catalytic promiscuity"; homodimer.

Dichloroacetate (DCA) biotransformation — clinically important

• GSTZ1 dehalogenates/biotransforms DCA to glyoxylate; DCA inactivates GSTZ1 (mechanism-based) -> reduced clearance on repeat dosing.
• [PMID:20884751 "Repeated doses of DCA result in reduced drug clearance, probably through inhibition of glutathione transferase zeta1 (GSTZ1), a cytosolic enzyme that converts DCA to glyoxylate"; "the mitochondrion is a novel site of DCA biotransformation catalyzed by GSTZ1, an enzyme colocalized in cytosol and mitochondrial matrix"].
• Original DCA oxygenation report: PMID:9531472 (not cached) "Glutathione transferase zeta catalyses the oxygenation of the carcinogen dichloroacetic acid to glyoxylic acid."

Localization

• Primary = cytosol (UniProt SUBCELLULAR LOCATION: Cytoplasm). GO:0005829 cytosol IDA (FlyBase, PMID:20884751), plus Reactome TAS.
• Also mitochondrial matrix: IDA (PMID:20884751 submitochondrial fractionation, rat), HTP (PMID:34800366 mito proteome), TAS Reactome, ISS, IBA. Real but secondary/non-canonical -> KEEP_AS_NON_CORE.

Deficiency (MAAID)

• [UniProt DISEASE] Maleylacetoacetate isomerase deficiency (MAAID) [MIM:617596], autosomal recessive; mild elevations of succinylacetone in blood/urine, usually caught by newborn screening; normal liver function; "MAAID is a benign disorder." {PubMed:27876694}. Variants Met-99, Val-150 have decreased MAAI activity.
• Consistent with biochemically mild phenotype (succinylacetone, maleylacetone excretion) rather than severe tyrosinemia.

Protein-binding annotations

• GO:0005515 protein binding (IPI, PMID:17474147 SH3 peptide array; PMID:32296183 HuRI binary interactome) and GO:0042802 identical protein binding (PMID:16189514, 25416956, 32296183 HuRI). Interactors NCK1, QARS1, TRAF2, GORASP2, PLEKHG4, TCP11, ZMYND12, CLVS2 are high-throughput binary-interactome hits with no established functional consequence -> uninformative protein binding; MARK_AS_OVER_ANNOTATED.
• GO:0042802 identical protein binding + GO:0042803 protein homodimerization: the enzyme IS a homodimer (crystal structure PMID:11327815; PMID:10739172 IPI self). Homodimerization is a genuine, structurally-supported property -> KEEP_AS_NON_CORE (homodimer is the functional unit but not the "core function" MF).

Action plan for GOA lines

• GO:0016034 MAAI activity (IBA, IEA, IDA PMID:10739172, TAS PMID:10373324): core MF -> ACCEPT (IDA is the anchor).
• GO:0004364 glutathione transferase activity (IBA, IEA, IDA PMID:10739172, TAS PMID:9396740): real but low/secondary -> KEEP_AS_NON_CORE (biochemically genuine; the GST activity toward classic substrates is weak).
• GO:0004602 glutathione peroxidase activity (TAS PMID:9396740): low activity -> KEEP_AS_NON_CORE.
• GO:0003824 catalytic activity (IEA InterPro): too general -> MARK_AS_OVER_ANNOTATED (or MODIFY to MAAI). Root-ish MF; mark over-annotated.
• GO:0006559 L-phenylalanine catabolic process (IBA, IEA UniPathway, TAS PMID:9417084): correct pathway -> ACCEPT (this is the Phe/Tyr common pathway).
• GO:0006749 glutathione metabolic process (IBA, IDA PMID:10739172): GSH is cofactor/substrate -> KEEP_AS_NON_CORE.
• GO:0009072 aromatic amino acid metabolic process (IEA InterPro): broader parent of Phe/Tyr catabolism -> ACCEPT (correct, if general).
• GO:0098869 cellular oxidant detoxification (IEA GOC from GO:0004602): tied to weak GPx activity -> KEEP_AS_NON_CORE.
• GO:1990748 cellular detoxification (IDA PMID:20884751): DCA detox -> KEEP_AS_NON_CORE.
• CC cytosol (IDA, TAS): core location -> ACCEPT.
• CC cytoplasm (IEA): correct but general -> ACCEPT.
• CC mitochondrion / mitochondrial matrix (IBA, IDA, HTP, TAS, ISS): real secondary loc -> KEEP_AS_NON_CORE.
• protein binding / identical protein binding (IPI HuRI/SH3): uninformative -> MARK_AS_OVER_ANNOTATED.
• protein homodimerization activity (IPI PMID:10739172): genuine homodimer -> KEEP_AS_NON_CORE.

2026-10-24 re-review (status COMPLETE)

• Both bare GO:0005515 rows (SH3 peptide array; HuRI) are now REMOVE under the
  protein-binding policy. Neither paper supports a more informative term; the homodimer
  is already captured by GO:0042802/GO:0042803.
• Glutathione transferase activity (all four rows) and cellular detoxification (IDA)
  are now ACCEPT, as the second core function. That function is the
  glutathione-dependent dehalogenation of dichloroacetate, directly assayed
  PMID:10739172 and localized to the cytosol and mitochondrial matrix PMID:20884751.
  Activity toward classical CDNB-type substrates stays noted as minimal PMID:9396740.
• Cytoplasm is now MODIFY -> cytosol. Catalytic activity and aromatic amino acid
  metabolic process are now TOO_GENERAL.
• Weak title-fragment quotes were replaced with curated UniProt statements (MAAI
  reaction with ECO:0000269; GSH cofactor note; homodimer from PubMed:11327815) and with
  PAINT rows.

2026-11-01 review follow-up (PR 5356)

• GO:1902000 homogentisate catabolic process is now asserted uniformly for all nine
  homogentisate-pathway exemplars (KT2440 hmgA/hmgC/hmgB, human HGD/GSTZ1/FAH,
  A. nidulans hmgA/maiA/fahA), as NEW where GOA lacks it: each enzyme performs one of
  the three catalytic steps of homogentisate breakdown.