BCAT2 encodes the mitochondrial branched-chain-amino-acid aminotransferase, a pyridoxal 5'-phosphate (PLP)-dependent class-IV aminotransferase that reversibly transfers the alpha-amino group of leucine, isoleucine and valine to 2-oxoglutarate, producing glutamate and the corresponding branched-chain 2-oxo acids. This transamination is the first step of branched-chain amino acid catabolism; the 2-oxo acid products are then oxidatively decarboxylated by the mitochondrial branched-chain 2-oxo acid dehydrogenase complex. The enzyme is a homodimer whose active site lies near the dimer interface, with PLP bound as a Schiff base to an active-site lysine and a redox-sensitive CXXC motif that positions substrate for catalysis. Mammalian orthologs carry a 27-residue N-terminal mitochondrial presequence that is removed on import. The horse protein A0A9L0TSN4 is a gene model of BCAT2 (PANTHER subfamily PTHR11825:SF39) whose residues 60-452 correspond to the full conserved mammalian enzyme, including the presequence; residues 1-59 are a Gly/Arg/Pro-rich extension absent from reviewed mammalian orthologs.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | MODIFY | Summary: Correct but uninformative root-level MF from InterPro class-IV aminotransferase signatures. The specific supported activity is branched-chain-amino-acid:2-oxoglutarate transaminase activity. Reason: The horse protein is a member of PANTHER subfamily PTHR11825:SF39 (mitochondrial BCAT) and retains the PLP Schiff-base lysine, the CXXC motif and all curated substrate-binding residues of human BCAT2, whose branched-chain aminotransferase activity is experimentally established. The general term is therefore replaced by the specific activity GO:0004084, which this gene already carries. Proposed replacements: branched-chain-amino-acid:2-oxoglutarate transaminase activity Supporting Evidence: file:HORSE/BCAT2/BCAT2-uniprot.txt PTHR11825:SF39; BRANCHED-CHAIN-AMINO-ACID AMINOTRANSFERASE, MITOCHONDRIAL file:HORSE/BCAT2/BCAT2-bioinformatics/RESULTS-rereview.md | A0A9L0TSN4 | K/K | C/C | C/C | R/R | Y/Y | Y/Y | V/V | V/V | T/T | |
| GO:0004084 branched-chain-amino-acid:2-oxoglutarate transaminase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Core molecular function. Branched-chain amino acid transamination is experimentally established for the human and rat orthologs in the same PANTHER subfamily (SF39), and the PAINT root node of PTHR11825 (PTN000214535) carries the L-leucine, L-isoleucine and L-valine 2-oxoglutarate transaminase activities, so the horse protein inherits them by descent. All catalytic and substrate-binding residues are retained. Reason: Evolutionary grounding: the horse protein is in PTHR11825:SF39 together with human O15382 and rat O35854, both with experimental activity; PAINT placed the three branched-chain transaminase activities (GO:0052654, GO:0052655, GO:0052656, all is_a GO:0004084) at PTN000214535, the family root, seeded by human, rat, fly and fission yeast data (interpro/panther/PTHR11825/PTHR11825-paint.tsv). Sequence: residues 60-452 align to the full human enzyme at 83.4% identity with the PLP lysine (human K229), CXXC cysteines (human C342/C345) and curated substrate contacts conserved. The 59-residue N-terminal extension lies outside the catalytic domains and does not affect this inference. Supporting Evidence: file:human/BCAT2/BCAT2-uniprot.txt Reaction=L-leucine + 2-oxoglutarate = 4-methyl-2-oxopentanoate + L- PMID:9165094 When rat BCAT(m) is overexpressed in COS-1 cells, the protein exhibits BCAT activity PMID:17050531 X-ray crystallography revealed that the CXXC motif, through its network of hydrogen bonds, plays a crucial role in orienting the substrate optimally for catalysis. file:HORSE/BCAT2/BCAT2-bioinformatics/RESULTS-rereview.md | A0A9L0TSN4 | K/K | C/C | C/C | R/R | Y/Y | Y/Y | V/V | V/V | T/T | file:HORSE/BCAT2/BCAT2-uniprot.txt PTHR11825:SF39; BRANCHED-CHAIN-AMINO-ACID AMINOTRANSFERASE, MITOCHONDRIAL |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | ACCEPT | Summary: BCAT2 is the mitochondrial isozyme: the PAINT eukaryotic node PTN000214536 carries mitochondrion, and the horse protein is in the mitochondrial subfamily SF39 and retains the region corresponding to the 27-residue mammalian presequence (horse residues 60-86). The location holds for the gene product; whether this particular gene model, with 59 extra upstream residues, is a real translation product is unverified. Reason: The previous review left this UNDECIDED because of the N-terminal extension. On re-examination the location claim concerns the BCAT2 gene product, and the evolutionary evidence is strong: PAINT placed mitochondrion at PTN000214536 (taxon Eukaryota) seeded by human, mouse, rat, budding yeast and fission yeast data, and rat BCAT2 is processed at its mitochondrial targeting sequence when expressed in cells. The horse sequence beginning at Met60 matches the conserved mammalian N terminus that contains the presequence, and the 59-residue upstream extension is found in no reviewed mammalian ortholog and in only some horse gene models (A0A3Q2HXS7 and Ensembl transcripts), which suggests a gene-model artefact from an upstream in-frame start rather than a functional divergence. Whether a protein carrying this extension would be imported is untested; that caveat is recorded in the notes and does not overturn the gene-level location. Supporting Evidence: PMID:9165094 When rat BCAT(m) is overexpressed in COS-1 cells, the protein exhibits BCAT activity and correct processing of the mitochondrial targeting sequence. file:human/BCAT2/BCAT2-uniprot.txt CC -!- SUBCELLULAR LOCATION: Mitochondrion {ECO:0000250|UniProtKB:O35854}. file:HORSE/BCAT2/BCAT2-bioinformatics/RESULTS-rereview.md A0A9L0TSN4 residues 60-452 (393 aa) begin at the Met aligned to human Met1. file:HORSE/BCAT2/BCAT2-bioinformatics/RESULTS-rereview.md First 20 residues found in reviewed orthologs: none. |
| GO:0009081 branched-chain amino acid metabolic process | IEA GO_REF:0000002 | MODIFY | Summary: True but general. In mammals the mitochondrial isozyme catalyses the first step of branched-chain amino acid breakdown, so the specific process is branched-chain amino acid catabolic process. Reason: The general metabolic term is entailed by GO:0009083, which this gene already carries and which PAINT supports at the family root through its L-isoleucine and L-valine catabolic process assertions. Replace with the catabolic term. Proposed replacements: branched-chain amino acid catabolic process Supporting Evidence: file:human/BCAT2/BCAT2-uniprot.txt CC -!- FUNCTION: Catalyzes the first reaction in the catabolism of the |
| GO:0009083 branched-chain amino acid catabolic process | IEA GO_REF:0000117 | ACCEPT | Summary: Core process. The enzyme performs the first (transamination) step of leucine, isoleucine and valine catabolism, so it directly participates in the process. Reason: BCAT2 itself catalyses a step of this pathway, which meets the participation test. The PAINT root node PTN000214535 carries L-isoleucine catabolic process (GO:0006550) and L-valine catabolic process (GO:0006574), both children of this term, and the horse protein is within that clade. The ARBA source of this IEA is not used as support; the evidence is the experimentally characterised orthologs and the phylogeny. Supporting Evidence: file:human/BCAT2/BCAT2-uniprot.txt CC -!- FUNCTION: Catalyzes the first reaction in the catabolism of the file:HORSE/BCAT2/BCAT2-uniprot.txt PTHR11825:SF39; BRANCHED-CHAIN-AMINO-ACID AMINOTRANSFERASE, MITOCHONDRIAL |
| GO:0030170 pyridoxal phosphate binding | ISS PMID:11264579 The structure of human mitochondrial branched-chain aminotra... | NEW | Summary: PLP is the covalently bound cofactor of BCAT2 (Schiff base to the active-site lysine), shown structurally for human BCAT2; the corresponding lysine is conserved in the horse protein. Reason: PLP binding is an intrinsic part of the enzyme's catalytic mechanism and is supported by structural data for the human ortholog in the same subfamily, plus conservation of the Schiff-base lysine (human K229) in the horse sequence. It is not an ancestor or descendant of the transaminase term, so it adds cofactor information rather than redundancy. Supporting Evidence: PMID:11264579 The 4'-aldehyde of the PLP cofactor is covalently linked to the epsilon-amino group of the active-site lysine file:HORSE/BCAT2/BCAT2-bioinformatics/RESULTS-rereview.md | A0A9L0TSN4 | K/K | C/C | C/C | R/R | Y/Y | Y/Y | V/V | V/V | T/T | |
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Download this section (compressed HTML)Q: Is the 59-residue Gly/Arg/Pro-rich upstream extension of horse model A0A9L0TSN4 ever translated, or does translation initiate at Met60 as in other mammals? If translated, is the extended protein still imported into mitochondria and processed?
Experiment: Map the horse BCAT2 translation start by 5'-RACE plus ribosome profiling or N-terminal proteomics of equine muscle or liver, and test mitochondrial import of tagged full-length (1-452) versus Met60-initiated constructs.
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