SLC27A5 (bile acyl-CoA synthetase, BACS; fatty acid transport protein 5, FATP5) is a liver-enriched member of the ATP-dependent AMP-binding (acyl-CoA synthetase) family anchored in the endoplasmic reticulum membrane by an N-terminal hydrophobic region, with a large cytoplasmic catalytic domain. Its main role is the ATP-dependent activation of C24 bile acids (cholate, chenodeoxycholate and the secondary bile acids deoxycholate and lithocholate) to their CoA thioesters, the step that precedes glycine or taurine conjugation, so that bile acids returning to the liver through the enterohepatic circulation are re-activated and re-conjugated. It also activates long-chain and very-long-chain fatty acids (C18 to C26) to acyl-CoAs and contributes to hepatic long-chain fatty acid uptake; in mouse liver it is also detected at the basal (sinusoidal) plasma membrane of hepatocytes, and loss of the mouse gene leaves most gallbladder bile acids unconjugated and reduces hepatocyte fatty acid uptake. The donkey protein A0A9L0JNL2 belongs to the same PANTHER subfamily as human, mouse and rat SLC27A5 (PTHR43107:SF25) and is 81% identical to human SLC27A5 over the full human length, with a 10-residue N-terminal extension that the horse gene model shares.
Existing Annotations Review
GO Term
Evidence
Action
Reason
GO:0000038 very long-chain fatty acid metabolic process
IEA GO_REF:0000107
ACCEPT
Summary: Very long-chain fatty acid metabolic process, by Ensembl Compara from human IDA (PMID:10479480).
Reason: Human SLC27A5 activates C24:0 and C26:0; the enzyme performs this metabolic step.
For all substrates tested (C18:0, C20:0, C24:0, C26:0)
GO:0001676 long-chain fatty acid metabolic process
IEA GO_REF:0000118
ACCEPT
Summary: Long-chain fatty acid metabolic process, from TreeGrafter PANTHER node PTN009166993. Human SLC27A5 activates C18:0 and C20:0 (and very-long-chain) fatty acids to acyl-CoAs.
Reason: The enzyme itself catalyses the activation step of long-chain fatty acid metabolism; consistent with the human review, which accepts this term.
Summary: Long-chain fatty acid-CoA ligase activity, from the combined pipeline (EC 6.2.1.3 and PANTHER node). Human SLC27A5 has acyl-CoA synthetase activity with long-chain fatty acids.
Reason: Experimentally demonstrated for the human ortholog; the donkey protein conserves the AMP-binding catalytic domain (PROSITE AMP_BINDING) with 81% identity. Core activity alongside bile acid-CoA ligase.
Summary: Long-chain fatty acid transmembrane transporter activity, from the combined pipeline (human and PANTHER).
Reason: SLC27A5 is said to "mediate" LCFA import, but a transporter activity separate from its acyl-CoA synthetase activity has not been demonstrated; uptake is plausibly driven by acyl-CoA formation. The PAINT IBD for this term is placed at the family root node PTN001945407 with human SLC27A5 among its seeds, and is negated (IRD) only in the SLC27A3 clade, so it is a deliberate phylogenetic assertion that the donkey SF25 member inherits; it is retained, not removed. Kept as non-core, consistent with the human review.
Supporting Evidence:
file:human/SLC27A5/SLC27A5-uniprot.txt
FUNCTION: May mediate the import of long-chain fatty acids (LCFA) by
Summary: Endoplasmic reticulum, by Ensembl Compara from human IDA (PMID:10479480).
Reason: True (human SLC27A5 is associated with the ER and not with peroxisomes), but endoplasmic reticulum membrane (GO:0005789), which this review accepts as the principal location of this ER-anchored enzyme, already entails it. Re-review change from ACCEPT, because the earlier reason itself argued that the ER membrane term is the more specific location.
Summary: Endoplasmic reticulum membrane, from the combined pipeline (human annotation and PANTHER).
Reason: Principal location; human SLC27A5 is an ER membrane protein with a cytoplasmic catalytic domain, and the donkey N-terminal hydrophobic region is conserved.
Summary: Plasma membrane, from the combined pipeline (UniProt subcellular-location rule and PANTHER node). Mouse Slc27a5 has a plasma-membrane pool associated with fatty-acid uptake; human SLC27A5 is mainly in the ER membrane.
Reason: The plasma-membrane pool is real in mouse hepatocytes, but it is specifically the basal (sinusoidal) plasma membrane, and basal plasma membrane (GO:0009925, a part_of descendant of plasma membrane) is retained in this review as a non-core location; this broader row adds nothing. Re-review change from KEEP_AS_NON_CORE.
Summary: Bile acid biosynthetic process, by Ensembl Compara from human IDA (PMID:11980911). SLC27A5 activates THCA (the C27 de novo precursor) in vitro, but its main physiological role is re-activation of returning C24 bile acids before re-conjugation.
Reason: The enzyme catalyses a CoA-activation step that bile acid synthesis and conjugation require, and THCA activation is a de novo pathway step in vitro; however the authors and mouse data place its main role in recycling rather than de novo synthesis. Kept as non-core, with bile acid metabolic process as the core process.
Summary: Bile acid metabolic process, from TreeGrafter PANTHER node PTN009166993; the PAINT IBD for this term sits on PTN008678717, a node whose experimental seeds are SLC27A2 and SLC27A5 (human Q9Y2P5 among them), so the term is correctly inherited by the SLC27A5 subfamily (PTHR43107:SF25). SLC27A5 activates bile acids to CoA thioesters for re-conjugation.
Reason: Correct but general. The specific process in which SLC27A5 does the work is bile acid conjugation (GO:0002152, is_a bile acid metabolic process), whose GO definition comment names cholate-CoA ligase activity (GO:0047747) as the first step of the process, before glycine N-choloyltransferase (BAAT). SLC27A5 therefore performs a step of conjugation itself, rather than merely supplying a substrate, and the mouse knockout leaves most gallbladder bile acids unconjugated. Re-review change from ACCEPT.
Summary: Basal plasma membrane, by Ensembl Compara from mouse Slc27a5 (basal/sinusoidal hepatocyte membrane).
Reason: Mouse Slc27a5 (same PANTHER subfamily PTHR43107:SF25) is localized to the basal plasma membrane of hepatocytes, consistent with a role in fatty-acid uptake from the circulation; human SLC27A5 is reported mainly in the ER membrane, which remains the location of the core bile acid-activating activity. Kept as a non-core location.
Summary: Fatty acid transmembrane transporter activity, from the combined pipeline (mouse and PANTHER).
Reason: Parent of long-chain fatty acid transmembrane transporter activity, which is retained in this review (as non-core); this broader row adds nothing.
GO:0031957 very long-chain fatty acid-CoA ligase activity
IEA GO_REF:0000107
ACCEPT
Summary: Very long-chain fatty acid-CoA ligase activity, by Ensembl Compara from human IDA (PMID:10479480).
Reason: Human SLC27A5 activates lignocerate (C24:0) and hexacosanoate (C26:0); the donkey catalytic domain is conserved.
Summary: Long-chain fatty acid import into cell, from TreeGrafter PANTHER node; mouse knockout data (via human UniProt by similarity) indicate a role in hepatic fatty acid uptake.
Reason: FATP-family uptake is thought to be coupled to acyl-CoA synthetase activity (vectorial acylation) rather than to a demonstrated transport mechanism; kept as non-core, consistent with the human review.
Supporting Evidence:
file:human/SLC27A5/SLC27A5-uniprot.txt
Plays an important role in hepatic fatty acid uptake and bile acid
Summary: Cholate-CoA ligase activity, from the combined pipeline (human and mouse experimental annotations). Human SLC27A5 (hVLCS-H2) is a cholate:CoA ligase.
Reason: Defining core activity (bile acyl-CoA synthetase); demonstrated for the human ortholog, which also activates chenodeoxycholate, deoxycholate and lithocholate.
this enzyme also activates chenodeoxycholate, the secondary bile acids
Core Functions
ER-membrane bile acyl-CoA synthetase that activates C24 bile acids (cholate, chenodeoxycholate, deoxycholate, lithocholate) to CoA thioesters for re-conjugation with glycine or taurine.
Q: Is the 10-residue N-terminal extension of the donkey SLC27A5 model a real coding start, and does donkey liver show the same bile acid-CoA ligase substrate range as human?
Q: GO:0002152 bile acid conjugation is currently annotated only to BAAT, although its definition comment names cholate-CoA ligase activity as the first step; should the bile acid-CoA ligase (SLC27A5) also carry this process term in human and mouse?
Suggested Experiments
Experiment: Assay donkey liver microsomes and recombinant donkey SLC27A5 for cholate, chenodeoxycholate and lignocerate CoA-ligase activity.
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
Assessment of 1 ProtNLM2 GO prediction for donkey SLC27A5 (A0A9L0JNL2), judged against the donkey GOA record (long-chain, very-long-chain and cholate CoA-ligase activities; long-chain fatty acid, very long-chain fatty acid and bile acid metabolic processes; ER membrane and others), the human ortholog Q9Y2P5 with its enzyme characterization (PMID:10749848, PMID:10479480), and a donkey-human alignment (81% identity over the full human length). Lipid metabolic process is correct but much less specific than the curated process annotations (LSP). Non-GO ProtNLM2 outputs, not scored: subcellular locations 'Endoplasmic reticulum membrane' (agrees with the human experimental localization) and 'Membrane' (true but uninformative). No function text was emitted for this accession.
LSP — Less precise than existing annotation Review score: 2/2
Prediction method: ProtNLM2 · Version: UniProt API snapshot 2026-10-14
Review rationale: SLC27A5 activates bile acids and long- and very-long-chain fatty acids to their CoA thioesters (human data, PMID:10749848 and PMID:10479480), and the donkey protein retains the AMP-binding catalytic domain at 81% identity, so participation in lipid metabolism is correct. Donkey GOA already carries the more specific descendants long-chain fatty acid metabolic process, very long-chain fatty acid metabolic process and bile acid metabolic process, so the prediction is correct but less precise than existing annotations. Training-set membership is unknown.
Supporting Evidence:
PMID:10749848: "exhibited cholate:CoA ligase (choloyl-CoA synthetase) activity with both"
PMID:10479480: "For all substrates tested (C18:0, C20:0, C24:0, C26:0)"