HSD17B4 encodes the peroxisomal multifunctional enzyme type 2 (MFE-2 / MFP-2), also
called D-bifunctional protein (DBP) and historically "17-beta-hydroxysteroid
dehydrogenase type 4 (17β-HSD4)". It is a 736-aa peroxisomal-matrix enzyme that catalyzes
steps 2 and 3 of peroxisomal fatty-acid beta-oxidation with D (R) stereochemistry.
The protein is multidomain (UniProt P51659 FT):
- N-terminal (3R)-hydroxyacyl-CoA dehydrogenase domain (aa 1–305; SDR/Rossmann fold, NAD+
binding), historically the "17β-HSD" part [file:human/HSD17B4/HSD17B4-uniprot.txt "REGION 1..305 /note=\"(3R)-hydroxyacyl-CoA dehydrogenase\""].
- Central enoyl-CoA hydratase 2 / D-3-hydroxyacyl-CoA dehydratase domain (aa 322–622;
MaoC-like hot-dog fold) [file:human/HSD17B4/HSD17B4-uniprot.txt "REGION 322..622 /note=\"Enoyl-CoA hydratase 2\""].
- C-terminal SCP2 sterol-carrier / lipid-transfer domain (aa 624–736)
[file:human/HSD17B4/HSD17B4-uniprot.txt "DOMAIN 624..736 /note=\"SCP2\""].
The mature protein carries a C-terminal peroxisomal targeting motif (aa 734–736, "AKL")
[file:human/HSD17B4/HSD17B4-uniprot.txt "MOTIF 734..736 /note=\"Microbody targeting signal\""] and is imported
into the peroxisomal matrix. In vivo it is processed by the peroxisomal protease TYSND1 into
separate dehydrogenase and hydratase chains (PRO_0000400082 aa 1–311, PRO_0000400083 aa
312–736), but the full-length protein is bifunctional PMID:9089413.
DBP performs the 2nd (hydration) and 3rd (dehydrogenation) reactions of each beta-oxidation
cycle, in D/R stereochemistry:
The 3-oxoacyl-CoA product is handed to the peroxisomal thiolase step (SCPx/ACAA1) to complete
the cycle. UniProt FUNCTION: "Catalyzes two of the four reactions in fatty acid degradation:
hydration of 2-enoyl-CoA (trans-2-enoyl-CoA) to produce (3R)-3-hydroxyacyl-CoA, and
dehydrogenation of (3R)-3-hydroxyacyl-CoA to produce 3-ketoacyl-CoA (3-oxoacyl-CoA), which is
further metabolized by SCPx." [file:human/HSD17B4/HSD17B4-uniprot.txt "Catalyzes two of the four reactions in fatty"].
The bifunctional purified enzyme has both activities; a proteolytic C-terminal fragment
retains only the dehydratase PMID:9089413.
Original cloning identified the cDNA as identical to 17β-HSD IV PMID:9089413.
Although first characterized as a 17β-hydroxysteroid dehydrogenase (from a porcine
17β-estradiol dehydrogenase probe), DBP's physiological substrates are fatty acyl-CoAs,
not steroids. It is the main bifunctional enzyme for:
- Very-long-chain fatty acids (VLCFAs) — DBP deficiency accumulates straight-chain VLCFA
PMID:9482850;
the D enzyme is suspected to be the main VLCFA beta-oxidation enzyme PMID:9482850.
- 2-methyl-branched-chain (pristanic) acids and C27 bile-acid intermediates (DHCA/THCA)
PMID:9482850.
- Long-chain dicarboxylic acids — DBP is among the main beta-oxidation enzymes for C16DCA
PMID:15060085.
- DHA (C22:6n-3) biosynthesis — peroxisomal beta-oxidation of C24:6n-3 to C22:6n-3 needs DBP
PMID:11734571.
The 1995 cloning paper reports only a "specific unidirectional oxidative 17β-HSD activity"
when over-expressed PMID:7487879;
this is the source of the estradiol/androgen/estrogen-dehydrogenase annotations. Physiologically
these steroid activities are minor/secondary side reactions of the fatty-acid oxidoreductase;
UniProt's RecName is "Peroxisomal multifunctional enzyme type 2", and the FUNCTION section
describes only fatty-acid/bile-acid metabolism, not steroid metabolism. The steroid MF/BP
annotations (GO:0004303, GO:0008209, GO:0008210) are therefore over-annotations of the
enzyme's core evolved role, but they rest on an IDA experiment (PMID:7487879) and are not
removed per curation policy.
Peroxisome / peroxisomal matrix. UniProt: "SUBCELLULAR LOCATION: Peroxisome"
[file:human/HSD17B4/HSD17B4-uniprot.txt "SUBCELLULAR LOCATION: Peroxisome"]. Supported by IDA
immunofluorescence (PMID:15599942, HPA GO_REF:0000052), Reactome peroxisomal-matrix TAS, and
the C-terminal PTS1-like microbody targeting signal. Cytosol/membrane/peroxisomal-membrane
annotations reflect the import route (PTS1 cargo transits cytosol before import; Reactome
R-HSA-9033235/9033236 model the PEX5 import machinery) or proteomics co-fractionation, not the
functional site of catalysis.
Homodimer [file:human/HSD17B4/HSD17B4-uniprot.txt "SUBUNIT: Homodimer"]. Crystal structures
solved for each domain: SCP2-like domain (1IKT), hydratase 2 (1S9C), dehydrogenase+NAD (1ZBQ).
The hydratase-2 crystal structure shows a hot-dog-fold dimer able to accommodate bulky C26,
pristanic and DHCA/THCA CoA esters PMID:15644212.
Homodimerization annotations (GO:0042803) are IDA-supported (PMID:15644212, PMID:9089413).
Two catalytic MFs constitute the core evolved function:
1. GO:0080023 (2E)-enoyl-CoA hydratase activity — definition "a (3R)-3-hydroxyacyl-CoA = a
(2E)-enoyl-CoA + H2O", i.e. the D-specific hydratase 2 (EC 4.2.1.119). (The GOA IDA
annotations use the less specific parent GO:0004300 enoyl-CoA hydratase activity and
GO:0018812 3-hydroxyacyl-CoA dehydratase activity; GO:0080023 is the D-specific,
stereochemistry-correct term.)
2. GO:0106386 (3R)-3-hydroxyacyl-CoA dehydrogenase (NAD+) activity — EC 1.1.1.n12.
Directly involved in: GO:0006635 fatty acid beta-oxidation; bile acid biosynthesis. Location:
GO:0005782 peroxisomal matrix.