agxta (alanine--glyoxylate aminotransferase a) is one of two zebrafish co-orthologs of vertebrate AGXT, retained from the teleost-specific genome duplication; its paralog is agxtb. It encodes a pyridoxal 5'-phosphate dependent class-V aminotransferase that is predicted to transaminate glyoxylate with L-alanine to glycine and pyruvate, and L-serine with pyruvate to hydroxypyruvate and L-alanine. In vertebrates this enzyme detoxifies glyoxylate, the precursor of oxalate, and its organelle location (peroxisomes, mitochondria or both) varies between species through an N-terminal mitochondrial targeting sequence and a C-terminal peroxisomal targeting signal. agxta keeps the catalytic lysine and substrate-binding arginine, ends in a C-terminal tripeptide (SRV) that resembles a weak peroxisomal targeting signal, and has no N-terminal extension or upstream start codon, so it is predicted to be the peroxisomal copy; agxtb carries the mitochondrial-type N-terminal signal instead. agxta is expressed in larval and adult liver, pronephric duct and kidney, intestine and spleen. No functional study of the zebrafish protein has been published.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004760 L-serine:pyruvate transaminase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Serine:pyruvate aminotransferase activity propagated from the AGT/SPT node PTN000475663 (human AGXT, rat Agxt, Arabidopsis AGT1, fly Spat and others). Reason: The vertebrate AGT is the same enzyme as serine:pyruvate aminotransferase, and purified fish liver AGT uses L-serine with pyruvate. agxta is inside the AGT clade and keeps the PLP-binding lysine and substrate-binding arginine of human AGXT. Nothing suggests loss of this sub-activity. Accepted, as for agxtb. Supporting Evidence: PMID:6697688 It was specific for L-alanine and L-serine with glyoxylate and for L-serine with pyruvate as amino acceptor. file:DANRE/agxta/agxta-bioinformatics/RESULTS.md Both copies keep the PLP-binding lysine (human K209) and the substrate-binding arginine (human R360), so both are predicted to be active aminotransferases. |
| GO:0004760 L-serine:pyruvate transaminase activity | IEA GO_REF:0000003 | ACCEPT | Summary: Serine:pyruvate aminotransferase activity mapped from EC 2.6.1.51 on the UniProt record. Reason: Same activity as the IBA row; the EC assignment comes from the PIRNR family rule, which fits a full-length AGT with conserved catalytic residues. Supporting Evidence: file:DANRE/agxta/agxta-bioinformatics/RESULTS.md Both copies keep the PLP-binding lysine (human K209) and the substrate-binding arginine (human R360), so both are predicted to be active aminotransferases. |
| GO:0005777 peroxisome | IBA GO_REF:0000033 | ACCEPT | Summary: Peroxisome propagated from node PTN000475539, seeded by human AGXT and rat Agxt. Reason: Of the two zebrafish copies, agxta is the one that fits this location. It ends in SRV, which a survey of the zebrafish peroxisomal proteome classed as a weak PTS1, and it has no N-terminal extension; its transcript has an in-frame stop 16 codons upstream of the start, so it cannot make an MTS-bearing isoform. Freshwater-fish liver AGT is found partly in peroxisomes. The call is a prediction from sequence; zebrafish Agxta has not been localized, and C-terminal tripeptides of this kind (SRV) have not been tested as PTS1 in fish. Supporting Evidence: PMID:35295584 Zebrafish also contains a putative peroxisomal alanine:glyoxylate aminotransferase (AGT) (Agxta, F1QY24 _DANRE) with a weak PTS1 (SRV), a key enzyme to prevent oxalate accumulation (Table 1 and Supplementary Table S3). PMID:8954944 The present report describes that hepatic alanine:glyoxylate aminotransferase is located both in the peroxisomes and in the mitochondria in fresh water fish file:DANRE/agxta/agxta-bioinformatics/RESULTS.md agxta therefore cannot make an MTS-bearing isoform from this locus. |
| GO:0006545 glycine biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: Glycine biosynthetic process propagated from the AGT node PTN000475663. Reason: The AGT reaction forms glycine from glyoxylate and alanine, so the enzyme catalyses a step of glycine formation. This is the direct product of the conserved activity and applies to both zebrafish copies. The more specific term for glycine formation by transamination of glyoxylate (GO:0019265) is obsolete, so this term is the appropriate one. Supporting Evidence: PMID:41781394 The enzyme alanine:glyoxylate aminotransferase (AGT) converts glyoxylate to glycine, preventing the formation of harmful calcium oxalate crystals. |
| GO:0008453 L-alanine:glyoxylate transaminase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Alanine:glyoxylate aminotransferase activity propagated from PTN000475663; the defining activity of AGXT. Reason: Core activity of the family, measured for purified fish liver AGT (mackerel and other species) and for mammalian AGXT. agxta keeps the catalytic residues. Core function. Supporting Evidence: PMID:6697688 Alanine:glyoxylate aminotransferase was highly purified and characterized from mackerel liver. file:DANRE/agxta/agxta-bioinformatics/RESULTS.md Both copies keep the PLP-binding lysine (human K209) and the substrate-binding arginine (human R360), so both are predicted to be active aminotransferases. |
| GO:0008453 L-alanine:glyoxylate transaminase activity | IEA GO_REF:0000003 | ACCEPT | Summary: Alanine:glyoxylate aminotransferase activity from EC 2.6.1.44. Reason: Same activity as the IBA row. Supporting Evidence: file:DANRE/agxta/agxta-bioinformatics/RESULTS.md Both copies keep the PLP-binding lysine (human K209) and the substrate-binding arginine (human R360), so both are predicted to be active aminotransferases. |
| GO:0009436 glyoxylate catabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: Glyoxylate catabolic process propagated from PTN000475663. Reason: Removing glyoxylate before it is oxidized to oxalate is the physiological role of AGT. In fish, glyoxylate is also the end product of hepatic purine breakdown in peroxisomes, which fits a peroxisomal copy. Core process, shared with agxtb (which would act on mitochondrial glyoxylate). Supporting Evidence: PMID:8954944 The end products of purine degradation are urea and glyoxylate in fish. PMID:35295584 This prevents the conversion of glyoxylate into oxalate, which is toxic. |
Loading supporting content…
Download this section (compressed HTML)Q: Is Agxta peroxisomal in zebrafish hepatocytes, and is the C-terminal SRV sufficient for import by zebrafish Pex5, or does it need internal ancillary signals as human AGT does?
Q: Did the split of targeting signals between agxta (PTS1-like end only) and agxtb (MTS only) happen once before the teleost radiation, or several times? The arowana agxta orthologue still has an Arg-rich N-terminal extension.
Q: Does loss of agxta cause hyperoxaluria or kidney calcium oxalate deposits in zebrafish, and does agxtb compensate?
Experiment: Express GFP-tagged Agxta (C-terminal tag left free by tagging at the N-terminus) and Agxtb (tag at the C-terminus) in zebrafish liver or cultured fish cells with peroxisomal (SKL) and mitochondrial markers; swap the C-terminal tripeptides and N-terminal extensions between the copies.
Hypothesis: The two copies are targeted to different organelles.
Type: subcellular localization
Experiment: Generate agxta, agxtb and double mutants (preferably promoter-deletion alleles to avoid transcriptional adaptation) and measure urinary/whole-larva oxalate, glycolate and glycine, and kidney crystal deposition after a glycolate or hydroxyproline load.
Hypothesis: Peroxisomal Agxta is needed to prevent oxalate formation.
Type: knockout phenotyping
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)