Human reference: https://www.uniprot.org/uniprotkb/O95992/entry
Here, we report the cloning of cholesterol
25-hydroxylase cDNAs from the mouse and human. The encoded enzymes are polytopic
membrane proteins of 298 and 272 amino acids, respectively, which contain
clusters of histidine residues that are essential for catalytic activity. Unlike
most other sterol hydroxylases, cholesterol 25-hydroxylase is not a cytochrome
P450, but rather it is a member of a small family of enzymes that utilize diiron
cofactors to catalyze the hydroxylation of hydrophobic substrates. The
cholesterol 25-hydroxylase gene lacks introns, and in the human it is located on
chromosome 10q23. The murine gene is expressed at low levels in multiple
tissues. Expression of cholesterol 25-hydroxylase in transfected cells reduces
the biosynthesis of cholesterol from acetate and suppresses the cleavage of
sterol regulatory element binding protein-1 and -2. The data suggest that
cholesterol 25-hydroxylase has the capacity to play an important role in
regulating lipid metabolism by synthesizing a co-repressor that blocks sterol
regulatory element binding protein processing and ultimately leads to inhibition
of gene transcription.
The horse sequence comparison is in CH25H-bioinformatics/RESULTS.md. Transfer is assessed for this exact accession; human biochemical evidence is not equine experimental validation.
ProtNLM2 files were not opened or used.
Deep research. just deep-research-falcon HORSE CH25H --fallback perplexity-lite failed
(falcon: "Failed to authenticate"; perplexity-lite: HTTP 401 insufficient quota). No
deep-research file was created. A web search found no equine-specific experimental study of
CH25H; the review relies on the human/mouse literature cached in publications/
(PMID:9852097, PMID:32944968, PMID:33239446) and the reviewed human UniProt entry.
Identity of the benchmark accession. F6T000 (TrEMBL, unreviewed, 270 aa; RefSeq
XP_001503057.1; Ensembl ENSECAP00000003097) is the full-length horse CH25H product, not a
fragment or isoform. It aligns over 100% of its length to human O95992 (272 aa) at 83.3%
identity; the only length difference is the C-terminus (horse ...LRSAHAK vs human
...LRTASVPAR). The gene is intronless in mammals PMID:9852097, so splice isoforms are not expected. No reviewed horse CH25H entry exists.
Evolutionary grounding. PANTHER PTHR11863 (sterol desaturase), a functionally
heterogeneous family. Human, mouse and rat CH25H are in subfamily SF213 (CHOLESTEROL
25-HYDROXYLASE) per the PANTHER 19.0 member index; F6T000 has no subfamily call in UniProt
(horse is not a PANTHER reference genome), but its GO:0001567 IBA comes from PTN002627316
(Euteleostomi), the CH25H-clade node seeded by human CH25H and mouse Ch25h, which places it in
that clade. The C-4 methylsterol oxidase (PTN000221246, family root) and zymosterol
biosynthesis (PTN002275159, seeded only by mouse Msmo1) IBAs are deep-node propagations from
the MSMO1/ERG25 branch; removed as functional divergence. The PTHR11863 family review
(interpro/panther/PTHR11863/PTHR11863-review.yaml) agrees that C-4 methylsterol oxidase
activity is not supported for CH25H; no subfamily-placement disagreement. Note: panther.obo
also has SF34 named "CHOLESTEROL 25-HYDROXYLASE"; the characterised orthologs are in SF213.
Histidine boxes. New script CH25H-bioinformatics/hisbox_rereview.py (output
RESULTS-rereview.md, results-rereview.json) maps the three human UniProt histidine-box
MOTIF features: box-1 WHLLH->WHVLH (142-146), box-2 HKVHH->HKMHH (157-161), box-3
HHDLHHS->HHDLHHS (238-244); every histidine is conserved. The earlier RESULTS.md had an empty
feature list because it only mapped METAL/BINDING features.
Actions. ACCEPT: cholesterol 25-hydroxylase activity (IBA, IEA), ER membrane (IBA), iron
ion binding, cholesterol metabolic process. MODIFY: steroid hydroxylase activity ->
GO:0001567; membrane -> ER membrane. KEEP_AS_NON_CORE: lipid biosynthetic process (no oxysterol
biosynthesis term exists), response to type I interferon, B cell chemotaxis, negative
regulation of cholesterol metabolic process, negative regulation of virus-host plasma
membrane fusion. REMOVE: C-4 methylsterol oxidase activity, zymosterol biosynthetic process.
The human review (genes/human/CH25H) uses MARK_AS_OVER_ANNOTATED for the same two IBA rows;
REMOVE is used here because they assert a specific reaction/pathway the CH25H clade does not
perform. Harmonising the human review is a possible follow-up.
Relative. Human CH25H (O95992), one-to-one ortholog in PANTHER SF213 / PAINT node
PTN002627316; reviewed at genes/human/CH25H/CH25H-ai-review.yaml (all rows actioned). Modules
listing CH25H: modules/bile_acid_biosynthesis.yaml, modules/bile_acid_synthesis_ring_modification.yaml;
modules/srebp_signaling.yaml mentions its product 25-hydroxycholesterol (SREBP regulation) but not CH25H.