QDPR

UniProt ID: P09417
Organism: Homo sapiens
Review Status: INITIALIZED
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Gene Description

QDPR encodes dihydropteridine reductase (DHPR; EC 1.5.1.34), a cytosolic, homodimeric NAD(P)H-dependent oxidoreductase of the short-chain dehydrogenase/reductase (SDR) family. DHPR regenerates the essential cofactor tetrahydrobiopterin (BH4) by reducing the quinonoid form of dihydrobiopterin (q-BH2) back to BH4, completing the pterin cofactor cycle. BH4 is the obligate cofactor of the aromatic amino acid hydroxylases (phenylalanine, tyrosine and tryptophan hydroxylases) and of nitric oxide synthases, so DHPR-mediated BH4 salvage is required for phenylalanine catabolism and for biosynthesis of the monoamine neurotransmitters dopamine and serotonin. The enzyme adopts a Rossmann NAD(P)-binding fold with a catalytic tyrosine (Tyr150), and its structure has been solved by X-ray crystallography. In humans, biallelic loss-of-function variants in QDPR abolish BH4 regeneration and cause autosomal recessive dihydropteridine reductase deficiency, a BH4-deficient ("malignant"/atypical) hyperphenylalaninemia with progressive neurological disease driven by combined hyperphenylalaninemia and monoamine neurotransmitter deficiency.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0004155 6,7-dihydropteridine reductase activity
IBA
GO_REF:0000033 		ACCEPT
Summary: Core molecular function. The phylogenetically inferred (IBA) DHPR activity is the enzyme's defining, experimentally established catalytic function (EC 1.5.1.34): NAD(P)H-dependent reduction of quinonoid dihydropteridine to tetrahydropteridine, regenerating BH4.
Reason: Consistent with direct experimental evidence and UniProt catalytic activity. This term at exactly the right level of specificity for the gene.
Supporting Evidence:
PMID:3033643
Dihydropteridine reductase (DHPR; EC 1.6.99.7) catalyzes the NADH-mediated reduction of quinonoid dihydrobiopterin and is an essential component of the pterin-dependent aromatic amino acid hydroxylating systems.
GO:0005737 cytoplasm
IBA
GO_REF:0000033 		ACCEPT
Summary: DHPR is a soluble cytosolic enzyme; the phylogenetic is_active_in cytoplasm annotation is consistent with the enzyme acting in the cytosolic BH4 salvage cycle that supplies the cytosolic aromatic amino acid hydroxylases.
Reason: Cytoplasmic/cytosolic localization is well supported (IDA to cytoplasm, TAS to cytosol) and is the physiologically relevant compartment. The more specific GO:0005829 cytosol is also annotated and captured in core_functions.
Supporting Evidence:
file:human/QDPR/QDPR-uniprot.txt
Catalyzes the conversion of quinonoid dihydrobiopterin into
GO:0006729 tetrahydrobiopterin biosynthetic process
IBA
GO_REF:0000033 		ACCEPT
Summary: Core biological process. DHPR regenerates BH4 (the reduced, active pterin cofactor) from its quinonoid dihydro form, i.e. the salvage/regeneration arm of BH4 metabolism that this term subsumes.
Reason: Directly supported: DHPR catalyzes formation of tetrahydrobiopterin from quinonoid dihydrobiopterin. This is the enzyme's principal pathway role.
Supporting Evidence:
file:human/QDPR/QDPR-uniprot.txt
Catalyzes the conversion of quinonoid dihydrobiopterin into
PMID:3033643
reduction of quinonoid dihydrobiopterin and is an essential component of the
GO:0070402 NADPH binding
IBA
GO_REF:0000033 		ACCEPT
Summary: DHPR uses reduced nicotinamide cofactor as the hydride donor. UniProt records an NADP(+)/NADPH-dependent catalytic activity (RHEA:17865) in addition to the NADH-dependent reaction, supporting NADPH binding.
Reason: Supported by the annotated NADP(+)-dependent catalytic activity and the Rossmann NAD(P)-binding fold with an NADP-binding region (residues 14-38).
Supporting Evidence:
file:human/QDPR/QDPR-uniprot.txt
Reaction=5,6,7,8-tetrahydropteridine + NADP(+) = 6,7-dihydropteridine +
GO:0070404 NADH binding
IBA
GO_REF:0000033 		ACCEPT
Summary: DHPR is classically the NADH-dependent quinonoid-dihydrobiopterin reductase; NADH is the physiological hydride donor and the crystal structure was solved as an NADH binary complex.
Reason: Directly supported by the NADH-dependent catalytic activity and the structural characterization of the enzyme-NADH complex.
Supporting Evidence:
PMID:3033643
catalyzes the NADH-mediated
PMID:8262916
and kinetic identity to the naturally occurring enzyme has been proven.
GO:0004155 6,7-dihydropteridine reductase activity
IEA
GO_REF:0000120 		ACCEPT
Summary: Automated (ARBA/EC/RHEA) assignment of the DHPR molecular function, mapped from EC 1.5.1.34 and RHEA:17865/17869. Correct and identical to the experimentally supported core function.
Reason: IEA mapping from EC/RHEA is accurate and matches the experimental and IBA DHPR activity annotations.
Supporting Evidence:
file:human/QDPR/QDPR-uniprot.txt
EC=1.5.1.34; Evidence={ECO:0000269|PubMed:3033643,
GO:0042558 pteridine-containing compound metabolic process
IEA
GO_REF:0000117 		KEEP AS NON CORE
Summary: Broad but correct: DHPR acts on the pteridine cofactor biopterin (a pteridine-containing compound). More specific and informative terms (tetrahydrobiopterin biosynthetic process, dihydrobiopterin metabolic process) are also annotated.
Reason: The term is accurate but high-level; it is a parent of the more specific BH4/dihydrobiopterin process terms that better capture the core role.
Supporting Evidence:
file:human/QDPR/QDPR-uniprot.txt
Catalyzes the conversion of quinonoid dihydrobiopterin into
GO:0004155 6,7-dihydropteridine reductase activity
EXP
PMID:8262916
The crystallographic structure of a human dihydropteridine r... 		ACCEPT
Summary: Experimental support for DHPR activity from the enzyme that was purified to homogeneity, shown to have kinetic identity to the naturally occurring enzyme, and crystallized (PDB 1HDR).
Reason: Direct experimental (kinetic/structural) evidence for the DHPR molecular function on the human enzyme.
Supporting Evidence:
PMID:8262916
and kinetic identity to the naturally occurring enzyme has been proven.
GO:0005739 mitochondrion
HTP
PMID:34800366
Quantitative high-confidence human mitochondrial proteome an... 		MARK AS OVER ANNOTATED
Summary: A single high-throughput mitochondrial-proteome mapping study lists QDPR among detected proteins. DHPR is an established soluble cytosolic SDR-family enzyme with no mitochondrial targeting features (initiator Met removed, N-acetylated Ala2, no transit peptide); it functions in the cytosolic BH4 salvage cycle.
Reason: A single HTP proteomic detection is weak evidence for organellar residence of an abundant cytosolic enzyme, which is prone to co-purification. The catalytic function and its substrates/products are cytosolic; there is no corroborating evidence for a mitochondrial pool or function. Do not treat as core; flag as likely over-annotation rather than removing the curated HTP datum outright.
Supporting Evidence:
file:human/QDPR/QDPR-uniprot.txt
Catalyzes the conversion of quinonoid dihydrobiopterin into
GO:0004155 6,7-dihydropteridine reductase activity
IDA
PMID:3033643
Structure and expression of human dihydropteridine reductase... 		ACCEPT
Summary: Direct assay evidence for human DHPR enzymatic activity: the isolated cDNA clone expressed in COS cells produced DHPR enzymatic activity, confirming the molecular function.
Reason: IDA-level experimental confirmation of the core DHPR activity for the human enzyme.
Supporting Evidence:
PMID:3033643
Gene transfer of the recombinant human DHPR into COS cells leads to expression of DHPR enzymatic activity.
GO:0070062 extracellular exosome
HDA
PMID:23533145
In-depth proteomic analyses of exosomes isolated from expres... 		KEEP AS NON CORE
Summary: QDPR was detected in the shotgun proteome of prostatic-secretion (EPS) urinary exosomes. Detection of an abundant cytosolic enzyme in exosome proteomes is common and does not indicate a functional extracellular role.
Reason: Valid HDA proteomic observation but not a site of DHPR catalytic function; the enzyme acts intracellularly in the cytosolic BH4 salvage cycle. Keep as a non-core localization datum.
Supporting Evidence:
PMID:23533145
exosome preparations were characterized by a shotgun proteomics procedure
GO:0070062 extracellular exosome
HDA
PMID:19056867
Large-scale proteomics and phosphoproteomics of urinary exos... 		KEEP AS NON CORE
Summary: QDPR was among the >1000 proteins identified by LC-MS/MS in the proteome of normal human urinary exosomes. As above, this reflects capture of an abundant cytosolic protein in exosomes rather than a functional extracellular localization.
Reason: Corroborating HDA proteomic detection in exosomes; not a functional site of DHPR activity. Retain as non-core.
Supporting Evidence:
PMID:19056867
we used LC-MS/MS to profile the proteome of human urinary exosomes
GO:0005829 cytosol
TAS
Reactome:R-HSA-71130 		ACCEPT
Summary: Cytosolic localization from Reactome, which curates the DHPR reaction (q-dihydrobiopterin + NADH + H+ => tetrahydrobiopterin + NAD+) in the cytosol. This is the physiologically relevant compartment for BH4 salvage.
Reason: Cytosol is the correct, most specific localization for this soluble enzyme and matches the compartment of its substrates/products and the aromatic amino acid hydroxylases it serves.
Supporting Evidence:
file:human/QDPR/QDPR-uniprot.txt
Catalyzes the conversion of quinonoid dihydrobiopterin into
GO:0005737 cytoplasm
IDA
GO_REF:0000054 		ACCEPT
Summary: Direct localization of an expressed fusion protein to the cytoplasm (LIFEdb), consistent with the soluble cytosolic nature of DHPR.
Reason: Consistent with all other localization evidence (cytosol/cytoplasm). The more specific cytosol term is retained in core_functions.
Supporting Evidence:
file:human/QDPR/QDPR-uniprot.txt
Catalyzes the conversion of quinonoid dihydrobiopterin into
GO:0009055 electron transfer activity
TAS
PMID:3033643
Structure and expression of human dihydropteridine reductase... 		MODIFY
Summary: Generic electron-transfer function assigned by TAS. GO:0009055 denotes the directed movement of electrons between molecular entities typically mediated by electron carriers (as in electron transport chains). DHPR does not act as an electron carrier; it is an NAD(P)H-dependent oxidoreductase that transfers a hydride to a quinonoid pteridine (EC 1.5.1.34).
Reason: The essence (a redox activity) is sound but the specific term is the wrong molecular function: DHPR's characterized activity is the 6,7-dihydropteridine reductase reaction, not electron-carrier transfer. The cited reference (PMID:3033643) describes NADH-mediated reduction of quinonoid dihydrobiopterin, i.e. the reductase activity. Replace with the specific reductase term (already annotated) so this generic term is not retained as a distinct function.
Supporting Evidence:
PMID:3033643
catalyzes the NADH-mediated reduction of quinonoid dihydrobiopterin
GO:0004155 6,7-dihydropteridine reductase activity
TAS
PMID:3033643
Structure and expression of human dihydropteridine reductase... 		ACCEPT
Summary: Author-statement (TAS) support for the DHPR molecular function from the human DHPR cloning/expression paper, which describes the enzyme's NADH-mediated reduction of quinonoid dihydrobiopterin.
Reason: Redundant with, and fully consistent with, the IDA/EXP/IBA/IEA DHPR activity annotations. Duplicate GO IDs across evidence codes are acceptable.
Supporting Evidence:
PMID:3033643
catalyzes the NADH-mediated reduction of quinonoid dihydrobiopterin
GO:0051066 dihydrobiopterin metabolic process
TAS
PMID:3033643
Structure and expression of human dihydropteridine reductase... 		ACCEPT
Summary: Accurate process term: DHPR metabolizes dihydrobiopterin, reducing its quinonoid form to BH4. This is a precise description of the enzyme's substrate-level role in the pterin cycle.
Reason: Directly supported by the characterized reaction (quinonoid dihydrobiopterin reduction). Complements the tetrahydrobiopterin biosynthetic process term.
Supporting Evidence:
PMID:3033643
catalyzes the NADH-mediated reduction of quinonoid dihydrobiopterin
GO:0006520 amino acid metabolic process
TAS
PMID:3033643
Structure and expression of human dihydropteridine reductase... 		KEEP AS NON CORE
Summary: Very high-level BP term reflecting DHPR's indirect role in aromatic amino acid metabolism: by regenerating BH4 it supports the aromatic amino acid hydroxylases (notably phenylalanine hydroxylase). DHPR is not itself an amino-acid-metabolizing enzyme; it acts upstream by supplying the cofactor.
Reason: The link to amino acid metabolism is real but indirect and the term is extremely broad. The informative, direct process terms are the BH4/ dihydrobiopterin metabolic terms. Retain as non-core rather than as a core function.
Supporting Evidence:
PMID:3033643
an essential component of the pterin-dependent aromatic amino acid hydroxylating systems

Core Functions

NAD(P)H-dependent regeneration of tetrahydrobiopterin (BH4) by reduction of quinonoid dihydrobiopterin, sustaining the pterin cofactor cycle for the aromatic amino acid hydroxylases.

Molecular Function:
6,7-dihydropteridine reductase activity
Directly Involved In:
tetrahydrobiopterin biosynthetic process dihydrobiopterin metabolic process
Cellular Locations:
cytosol
Supporting Evidence:
  • PMID:3033643
    catalyzes the NADH-mediated reduction of quinonoid dihydrobiopterin and is an essential component of the pterin-dependent aromatic amino acid hydroxylating systems
  • PMID:8262916
    and kinetic identity to the naturally occurring enzyme has been proven.
  • file:human/QDPR/QDPR-uniprot.txt
    Catalyzes the conversion of quinonoid dihydrobiopterin into

Uses reduced nicotinamide cofactor (NADH, and NADPH) as the hydride donor for the reductase reaction, bound in a Rossmann NAD(P)-binding fold.

Molecular Function:
6,7-dihydropteridine reductase activity
Directly Involved In:
tetrahydrobiopterin biosynthetic process
Cellular Locations:
cytosol
Supporting Evidence:
  • PMID:8262916
    The crystallographic structure of a human dihydropteridine reductase NADH binary
  • file:human/QDPR/QDPR-uniprot.txt
    Reaction=5,6,7,8-tetrahydropteridine + NADP(+) = 6,7-dihydropteridine +

References

GO_REF:0000033
Annotation inferences using phylogenetic trees
GO_REF:0000054
Gene Ontology annotation based on curation of intracellular localizations of expressed fusion proteins in living cells
GO_REF:0000117
Electronic Gene Ontology annotations created by ARBA machine learning models
GO_REF:0000120
Combined Automated Annotation using Multiple IEA Methods
PMID:19056867
Large-scale proteomics and phosphoproteomics of urinary exosomes.
PMID:23533145
In-depth proteomic analyses of exosomes isolated from expressed prostatic secretions in urine.
PMID:3033643
Structure and expression of human dihydropteridine reductase.
  • DHPR (EC 1.5.1.34; historically 1.6.99.7) catalyzes the NADH-mediated reduction of quinonoid dihydrobiopterin and is an essential component of the pterin-dependent aromatic amino acid hydroxylating systems.
    "Dihydropteridine reductase (DHPR; EC 1.6.99.7) catalyzes the NADH-mediated reduction of quinonoid dihydrobiopterin and is an essential component of the pterin-dependent aromatic amino acid hydroxylating systems."
  • Recombinant human DHPR expressed in COS cells produces DHPR enzymatic activity, confirming the cloned cDNA encodes an active enzyme.
    "Gene transfer of the recombinant human DHPR into COS cells leads to expression of DHPR enzymatic activity."
PMID:34800366
Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context.
PMID:8262916
The crystallographic structure of a human dihydropteridine reductase NADH binary complex expressed in Escherichia coli by a cDNA constructed from its rat homologue.
  • The human DHPR enzyme was purified to homogeneity with kinetic identity to the natural enzyme and crystallized (PDB 1HDR), providing the first complete structural characterization.
    "and kinetic identity to the naturally occurring enzyme has been proven."
Reactome:R-HSA-71130
q-dihydrobiopterin + NADH + H+ => tetrahydrobiopterin + NAD+

Suggested Questions for Experts

Q: Does the single high-throughput mitochondrial-proteome detection of QDPR reflect a genuine, functionally relevant mitochondrial pool, or co-purification of the abundant cytosolic enzyme?

Q: Which reduced nicotinamide cofactor (NADH vs NADPH) predominates in vivo for human DHPR-mediated BH4 regeneration, and does the preference differ between tissues (e.g. brain vs liver)?

Suggested Experiments

Experiment: Steady-state and pre-steady-state kinetics of purified human DHPR comparing NADH vs NADPH as hydride donors with quinonoid dihydrobiopterin as substrate.

Hypothesis: Human DHPR regenerates BH4 using NADH as the primary physiological hydride donor, with NADPH as an alternative cofactor.

Type: enzyme kinetics

Experiment: Subcellular fractionation with DHPR activity assay, plus endogenous immunofluorescence/immuno-EM, to test for bona fide mitochondrial DHPR activity versus exclusively cytosolic localization.

Hypothesis: QDPR is exclusively cytosolic and the mitochondrial-proteome detection is a co-purification artifact rather than a functional mitochondrial pool.

Type: subcellular fractionation and imaging

Deep Research

Falcon

(QDPR-deep-research-falcon.md)
Comprehensive Research Report: QDPR (Dihydropteridine Reductase) — Human Gene Functional Annotation Falcon Edison Scientific Literature 34 citations 1 artifacts 2026-07-05T15:25:56.164697
Edison artifact artifact-00 (text/markdown)

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Comprehensive Research Report: QDPR (Dihydropteridine Reductase) — Human Gene Functional Annotation

1. Gene and Protein Identity

The human QDPR gene (synonyms: DHPR, SDR33C1; UniProt: P09417) encodes quinoid dihydropteridine reductase (DHPR), classified under EC 1.5.1.34. The gene is located on chromosome 4p15.3, spans over 20 kb, and contains 732 bp of coding sequence distributed across seven exons (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 12-13). DHPR belongs to the short-chain dehydrogenases/reductases (SDR) family and contains a characteristic Rossmann-type dinucleotide-binding fold (varughese1992crystalstructureof pages 1-2, pierson2018discoveryofnovel pages 27-31).

The following table provides a consolidated summary of QDPR's key properties:

Property Summary
Gene name QDPR; synonyms DHPR, SDR33C1 (eichwald2023tetrahydrobiopterinbeyondits pages 9-10, opladen2020consensusguidelinefor pages 1-2)
Protein name Dihydropteridine reductase; also called quinoid dihydropteridine reductase / DHPR (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 10-12)
UniProt ID P09417 (user-provided target identity)
EC number EC 1.5.1.34 (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 10-12)
Molecular weight Monomer ~25–26 kDa; native enzyme ~50–52 kDa (homodimer) (cutler1986dihydropteridinereductase pages 58-64, breuer2018characterizingthefunction pages 19-24)
Quaternary structure Homodimer; dimer interface formed by a four-helix bundle; Rossmann-type dinucleotide fold in each protomer (varughese1992crystalstructureof pages 4-5, varughese1992crystalstructureof pages 1-2, cutler1986dihydropteridinereductase pages 58-64)
Cofactor Primarily NADH; NADPH can support activity in some in vitro settings but is much less efficient, with strong NADH preference (~160-fold in a characterized homolog) (cutler1986dihydropteridinereductase pages 58-64, lye2002characterizationofquinonoiddihydropteridine pages 6-7, pierson2018discoveryofnovel pages 27-31)
Substrate Physiologic substrate is quinonoid dihydrobiopterin (qBH2) / quinonoid dihydropteridines; enzyme tolerates several substituted quinonoid pteridines (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 10-12, varughese1992crystalstructureof pages 4-5, cutler1986dihydropteridinereductase pages 58-64)
Product Tetrahydrobiopterin (BH4) plus NAD+ (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 10-12)
Reaction type NADH-dependent oxidoreductase reaction in BH4 recycling: qBH2 + NADH + H+ → BH4 + NAD+; reaction proceeds by direct hydride transfer and is effectively irreversible under physiologic conditions (cutler1986dihydropteridinereductase pages 58-64, thony2000tetrahydrobiopterinbiosynthesisregeneration pages 10-12)
Kinetic/mechanistic properties Ordered bi-bi mechanism: NADH binds first, then qBH2; pro-S hydride transferred from the B-face of NADH to substrate N5; free sulfhydryl groups required; example kinetic constants from a well-characterized homolog: Km(NADH) 23.1 ± 3.8 μM, Km(qDMPH2) 36.5 ± 7.1 μM (cutler1986dihydropteridinereductase pages 58-64, lye2002characterizationofquinonoiddihydropteridine pages 6-7, pierson2018discoveryofnovel pages 27-31)
Tissue distribution Widely distributed in animal tissues including brain, adrenal medulla, heart, and lung; functionally important in CNS and liver; recent work also identifies strong enrichment in myelinating oligodendrocytes/myelin (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 12-13, breuer2018characterizingthefunction pages 19-24, siems2025developmentalmaturationand pages 6-8)
Subcellular localization Predominantly cytosolic enzyme in BH4 recycling; in CNS myelin studies, QDPR localizes to oligodendrocyte cell bodies and the adaxonal non-compact compartment of myelin (siems2025developmentalmaturationand pages 6-8, siems2025developmentalmaturationand pages 8-9)
Pathway Tetrahydrobiopterin (BH4) regeneration/recycling pathway: after BH4 is oxidized during aromatic amino acid hydroxylation, PCD/PCBD1 generates qBH2 and QDPR reduces qBH2 back to BH4; this sustains PAH, TH, TPH, and influences NOS-related BH4 homeostasis (breuer2018characterizingthefunction pages 15-19, eichwald2023tetrahydrobiopterinbeyondits pages 5-7, crabtree2011synthesisandrecycling pages 3-4)
Primary biological function Maintains intracellular BH4 availability, thereby supporting phenylalanine hydroxylation, dopamine/serotonin biosynthesis, and nitric-oxide-related biopterin balance (breuer2018characterizingthefunction pages 15-19, breuer2018characterizingthefunction pages 19-24, eichwald2023tetrahydrobiopterinbeyondits pages 5-7)
Additional/putative functions Literature suggests DHPR may help preserve tetrahydrofolate levels in brain where DHFR is low; secondary ferric reductase activity has been reported in nonhuman systems, but its physiologic importance in human QDPR remains unclear (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 12-13, lye2002characterizationofquinonoiddihydropteridine pages 6-7)
Disease associations Dihydropteridine reductase deficiency (DHPRD) / BH4-deficient hyperphenylalaninemia: causes hyperphenylalaninemia, dopamine and serotonin deficiency, developmental delay, hypotonia, dystonia, seizures, microcephaly, and severe neurologic disease; DHPRD accounts for about 33% of HPA-associated BH4 deficiencies in one consensus guideline (eichwald2023tetrahydrobiopterinbeyondits pages 9-10, opladen2020consensusguidelinefor pages 1-2, opladen2020consensusguidelinefor pages 6-7)
Emerging disease links Reduced or dysregulated QDPR/BH4 recycling has been linked to tumor biology: decreased QDPR expression in colorectal cancer is associated with a lower BH4:BH2 ratio and NOS uncoupling; QDPR loss has also been implicated in pancreatic cancer immune suppression in later literature summaries (alam2023uncouplednitricoxide pages 1-2, yan2026overcomingmultidimensionalimmunotherapy pages 5-6)
Chromosomal location Chromosome 4p15.3; human gene spans >20 kb with 7 exons and 732 bp coding sequence (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 12-13)

Table: This table summarizes the core biochemical, structural, localization, pathway, and disease-related properties of human QDPR/dihydropteridine reductase. It is useful as a compact reference for functional annotation of UniProt P09417.

2. Enzymatic Reaction and Substrate Specificity

DHPR catalyzes the NADH-dependent reduction of quinonoid dihydrobiopterin (q-BH2) to tetrahydrobiopterin (BH4), a reaction that is central to the regeneration of this essential cofactor. The overall reaction is:

q-BH2 + NADH + H⁺ → BH4 + NAD⁺

The enzyme exhibits ordered bi-bi kinetics: NADH binds first to form an enzyme–NADH complex, followed by binding of the quinonoid dihydropterin substrate. Products are released in the order BH4 first, then NAD⁺ (cutler1986dihydropteridinereductase pages 58-64, pierson2018discoveryofnovel pages 27-31). The reaction is essentially irreversible under physiological conditions due to differences in reduction potentials (cutler1986dihydropteridinereductase pages 58-64).

DHPR displays a strong cofactor preference for NADH over NADPH, with approximately 160-fold higher efficiency with NADH (lye2002characterizationofquinonoiddihydropteridine pages 6-7). The enzyme transfers the pro-S hydrogen from the B-face of the NADH nicotinamide ring to the N5 position of q-BH2 (cutler1986dihydropteridinereductase pages 58-64, thony2000tetrahydrobiopterinbiosynthesisregeneration pages 10-12, pierson2018discoveryofnovel pages 27-31). Kinetic parameters determined for a well-characterized homolog include Km(NADH) = 23.1 ± 3.8 μM and Km(qDMPH₂) = 36.5 ± 7.1 μM, with a Vmax of 2550 ± 145 μmol/min/mg protein (lye2002characterizationofquinonoiddihydropteridine pages 6-7). The enzyme requires free sulfhydryl groups for activity and does not contain detectable prosthetic groups such as flavin or metal ions (cutler1986dihydropteridinereductase pages 58-64, thony2000tetrahydrobiopterinbiosynthesisregeneration pages 10-12).

DHPR is relatively substrate-tolerant with respect to quinonoid dihydropterins, accepting various 2-substituted and 6-substituted dihydropteridine analogs, though it exhibits very low activity toward non-quinonoid pteridines (approximately 66,000-fold lower activity with H₂B compared to quinonoid substrates) (lye2002characterizationofquinonoiddihydropteridine pages 6-7, lye2002characterizationofquinonoiddihydropteridine pages 1-1, varughese1992crystalstructureof pages 4-5). Methotrexate acts as a competitive inhibitor with respect to the pterin substrate, while NAD⁺ is a competitive inhibitor with respect to NADH (cutler1986dihydropteridinereductase pages 58-64). A secondary NADH-dependent ferric reductase activity has been described, though at much lower specific activity than the primary reaction (lye2002characterizationofquinonoiddihydropteridine pages 6-7).

3. Protein Structure

The crystal structure of DHPR was solved at 2.3 Å resolution from rat liver, revealing an α/β protein with a central β-sheet (strands A–H) flanked by major α-helices (varughese1992crystalstructureof pages 4-5, varughese1992crystalstructureof pages 1-2). The enzyme functions as a homodimer with a total molecular weight of approximately 50–52 kDa, comprising two identical subunits of ~25–26 kDa each (cutler1986dihydropteridinereductase pages 58-64). The dimer interface is stabilized by a four-helix bundle motif composed of two α-helices (αE and αF) contributed by each protomer, with an unusual right-handed twist (varughese1992crystalstructureof pages 4-5, varughese1992crystalstructureof pages 1-2).

The active site is a U-shaped surface channel formed by three extended loops connecting β-strands D–E, E–F, and F–G, located at the carboxyl-terminal edge of the dinucleotide fold (varughese1992crystalstructureof pages 4-5). Within this cleft, the quinonoid substrate binds in a stacked configuration between the nicotinamide ring of the NADH cofactor and the indole side chain of Trp-86, positioning the pteridine ring nearly parallel to the nicotinamide plane. The N5 of q-BH2 is positioned approximately 3.4 Å from the C4 of bound nicotinamide, enabling direct hydride transfer (varughese1992crystalstructureof pages 4-5). Structurally, DHPR is distinct from dihydrofolate reductase and more closely resembles NAD-requiring flavin-dependent enzymes such as glutathione reductase, despite lacking flavin prosthetic groups (varughese1992crystalstructureof pages 1-2).

4. Biochemical Pathway Context: BH4 Recycling

DHPR functions as the terminal enzyme of the tetrahydrobiopterin (BH4) recycling pathway. BH4 is an essential cofactor for three aromatic amino acid hydroxylases—phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH), and tryptophan hydroxylase (TPH)—as well as nitric oxide synthase (NOS) and alkylglycerol monooxygenase (breuer2018characterizingthefunction pages 15-19, breuer2018characterizingthefunction pages 19-24, eichwald2023tetrahydrobiopterinbeyondits pages 5-7).

During hydroxylase-catalyzed reactions, BH4 donates electrons and is oxidized to pterin-4a-carbinolamine. This intermediate is first dehydrated by pterin-4a-carbinolamine dehydratase (PCD/PCBD1) to yield quinonoid dihydrobiopterin (q-BH2). DHPR then catalyzes the final recycling step, reducing q-BH2 back to BH4 using NADH (pierson2018discoveryofnovel pages 14-18, eichwald2023tetrahydrobiopterinbeyondits pages 5-7, crabtree2011synthesisandrecycling pages 3-4). This recycling is critical because BH4 must be continuously regenerated to sustain the catalytic cycles of the hydroxylases and NOS. If q-BH2 is not rapidly reduced by DHPR, it can undergo non-enzymatic rearrangement to 7,8-dihydrobiopterin (BH2), which must then be salvaged by dihydrofolate reductase (DHFR) through an alternative, less efficient pathway (breuer2018characterizingthefunction pages 15-19).

By maintaining BH4 homeostasis, DHPR indirectly supports:
- Phenylalanine metabolism (via PAH: phenylalanine → tyrosine)
- Dopamine biosynthesis (via TH: tyrosine → L-DOPA)
- Serotonin biosynthesis (via TPH: tryptophan → 5-hydroxytryptophan)
- Nitric oxide production (via NOS)

Additionally, DHPR may contribute to maintaining tetrahydrofolate levels in the brain, where DHFR concentrations are low, suggesting a secondary role in folate metabolism (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 12-13).

5. Subcellular Localization and Tissue Expression

DHPR is a cytosolic enzyme that is widely distributed across mammalian tissues. It is found at relatively high levels in brain, adrenal medulla, heart, and lung (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 12-13). Its presence in brain and adrenal medulla is consistent with its role in supporting tyrosine and tryptophan hydroxylation. However, its abundance in tissues with minimal aromatic amino acid hydroxylase activity, such as heart and lung, suggests additional metabolic functions that remain incompletely understood (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 12-13).

A landmark recent finding by Siems et al. (2025) demonstrated that QDPR is a highly specific marker for mature myelinating oligodendrocytes and CNS myelin. Using quantitative mass spectrometry of purified myelin fractions, the study showed that QDPR abundance increases markedly during developmental maturation of myelin (between postnatal day 18 and P75) and remains stable thereafter (siems2025developmentalmaturationand pages 6-8, siems2025developmentalmaturationand pages 1-2). Immunohistochemistry confirmed that over 95% of QDPR-positive cells co-express the oligodendrocyte marker CA2 in both corpus callosum and cortex (siems2025developmentalmaturationand pages 6-8). Ultrastructural analysis by immunogold electron microscopy revealed that QDPR localizes specifically to the adaxonal, non-compact compartment of CNS myelin (siems2025developmentalmaturationand pages 6-8, siems2025developmentalmaturationand pages 8-9). This expression pattern is conserved between mouse and human myelinating oligodendrocytes (siems2025developmentalmaturationand pages 8-9, siems2025developmentalmaturationand pages 4-6). The functional significance of QDPR in oligodendrocytes likely relates to its role in monoamine metabolism, with mice lacking QDPR developing hyperphenylalaninemia, brain monoamine deficiency, and enhanced fear responses (siems2025developmentalmaturationand pages 11-12).

6. Clinical Significance: QDPR Deficiency

Dihydropteridine reductase deficiency (DHPRD) is an autosomal recessive disorder that accounts for approximately 33% of hyperphenylalaninemia (HPA)-associated BH4 deficiencies, making it the second most common form after 6-pyruvoyltetrahydropterin synthase (PTPS) deficiency (opladen2020consensusguidelinefor pages 1-2). The severe form predominates (267 of 303 reported patients) and represents one of the most devastating BH4 deficiency disorders (eichwald2023tetrahydrobiopterinbeyondits pages 9-10).

Clinical manifestations of DHPRD include progressive mental retardation secondary to extensive neuronal loss, hypotonia, developmental delay, movement disorders (particularly dystonia), and parkinsonism/hypokinetic rigid syndrome in approximately 10% of patients (opladen2020consensusguidelinefor pages 6-7). A distinctive feature is the high susceptibility to epileptic seizures compared to other BH4 disorders. Up to 25% of DHPRD patients present with microcephaly, a notably higher frequency than in other BH4 deficiencies (opladen2020consensusguidelinefor pages 6-7). Additional neuropathological findings include basal ganglia calcification, abnormal vascular proliferation in the brain, and risk of sudden death (eichwald2023tetrahydrobiopterinbeyondits pages 9-10).

The pathophysiology centers on severe depletion of monoamine neurotransmitters (dopamine, serotonin, norepinephrine) in the CNS, reflected by decreased levels of their metabolites homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) in cerebrospinal fluid (breuer2018characterizingthefunction pages 19-24, opladen2020consensusguidelinefor pages 1-2). Treatment involves supplementation with neurotransmitter precursors (L-DOPA with a decarboxylase inhibitor and 5-hydroxytryptophan), folinic acid, and a phenylalanine-restricted diet (eichwald2023tetrahydrobiopterinbeyondits pages 9-10). Recent research has demonstrated that DHPR patient-derived iPSCs can be rescued using CRISPR/Cas9 gene therapy, restoring normal BH4 and tyrosine hydroxylase protein levels in differentiated cells (breuer2018characterizingthefunction pages 19-24).

7. Emerging Roles in Cancer and Other Diseases

Recent research has expanded understanding of QDPR beyond inherited metabolic disease into oncology and metabolic disorders.

Colorectal cancer: Alam et al. (2023) reported that human stage 1 colon tumors exhibit significantly decreased QDPR expression, leading to a reduced BH4:BH2 ratio and uncoupled nitric oxide synthase (NOS) activity. This uncoupling generates increased reactive oxygen/nitrogen species that promote pro-inflammatory and proliferative pathways driving cancer progression. Treatment with sepiapterin, which restores the BH4:BH2 ratio, inhibited colon cancer cell proliferation and reduced tumor growth in mouse models (alam2023uncouplednitricoxide pages 1-2).

Pancreatic cancer: In pancreatic ductal adenocarcinoma (PDAC), QDPR knockout in mouse models led to accumulation of BH2, reduced BH4/BH2 ratios, and resistance to immune checkpoint blockade, indicating that QDPR deficiency creates an immunosuppressive tumor microenvironment (yan2026overcomingmultidimensionalimmunotherapy pages 5-6).

Type 2 diabetes and diabetic kidney disease: A recent study identified the QDPR rs3733570 polymorphism as potentially associated with increased susceptibility to type 2 diabetes mellitus and, in the presence of dyslipidemia, to diabetic kidney disease in the Chinese Han population, further underscoring the broader metabolic relevance of BH4 homeostasis maintained by QDPR.

Traumatic brain injury: Bioinformatics and molecular studies have identified QDPR as a key gene associated with oxidative stress in TBI, with overexpression of QDPR confirmed in TBI models by RT-qPCR and Western blot.

8. Summary

QDPR encodes dihydropteridine reductase, the essential terminal enzyme of the tetrahydrobiopterin (BH4) recycling pathway. Through its NADH-dependent reduction of quinonoid dihydrobiopterin back to BH4, DHPR maintains the availability of this critical cofactor for aromatic amino acid hydroxylases (PAH, TH, TPH) and nitric oxide synthase. The enzyme is a cytosolic homodimer of the SDR family with a well-characterized crystal structure featuring a Rossmann fold and a U-shaped active site channel. DHPR is broadly expressed across tissues, with particular importance in the brain for neurotransmitter synthesis and newly recognized enrichment in myelinating oligodendrocytes. Its deficiency causes a severe neurometabolic disorder characterized by hyperphenylalaninemia and monoamine neurotransmitter depletion, while emerging evidence implicates QDPR dysregulation in cancer biology, where reduced QDPR expression contributes to NOS uncoupling and immune evasion in the tumor microenvironment.

References

  1. (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 12-13): Beat THÖNY, Günter AUERBACH, and Nenad BLAU. Tetrahydrobiopterin biosynthesis, regeneration and functions. The Biochemical journal, 347 Pt 1:1-16, Apr 2000. URL: https://doi.org/10.1042/bj3470001, doi:10.1042/bj3470001. This article has 1116 citations.

  2. (varughese1992crystalstructureof pages 1-2): K. Varughese, Matthew M. Skinner, John M. WHITELEYt, David A. MATrHEWS, and N. Xuong. Crystal structure of rat liver dihydropteridine reductase. Proceedings of the National Academy of Sciences of the United States of America, 89 13:6080-4, Jul 1992. URL: https://doi.org/10.1073/pnas.89.13.6080, doi:10.1073/pnas.89.13.6080. This article has 210 citations and is from a highest quality peer-reviewed journal.

  3. (pierson2018discoveryofnovel pages 27-31): Yann Mathieu Pierson. Discovery of novel competitive dihydropteridine reductase inhibitors via high throughput screening and their effect on metabolic pterin balance of fibroblast. ArXiv, Jan 2018. URL: https://doi.org/10.5075/epfl-thesis-8371, doi:10.5075/epfl-thesis-8371. This article has 0 citations.

  4. (eichwald2023tetrahydrobiopterinbeyondits pages 9-10): Tuany Eichwald, Lucila de Bortoli da da Silva, Ananda Christina Staats Staats Pires, Laís Niero, Erick Schnorrenberger, Clovis Colpani Filho, Gisele Espíndola, Wei-Lin Huang, Gilles J. Guillemin, José E. Abdenur, and Alexandra Latini. Tetrahydrobiopterin: beyond its traditional role as a cofactor. Antioxidants, 12:1037, May 2023. URL: https://doi.org/10.3390/antiox12051037, doi:10.3390/antiox12051037. This article has 92 citations.

  5. (opladen2020consensusguidelinefor pages 1-2): Thomas Opladen, Eduardo López-Laso, Elisenda Cortès-Saladelafont, Toni S. Pearson, H. Serap Sivri, Yilmaz Yildiz, Birgit Assmann, Manju A. Kurian, Vincenzo Leuzzi, Simon Heales, Simon Pope, Francesco Porta, Angeles García-Cazorla, Tomáš Honzík, Roser Pons, Luc Regal, Helly Goez, Rafael Artuch, Georg F. Hoffmann, Gabriella Horvath, Beat Thöny, Sabine Scholl-Bürgi, Alberto Burlina, Marcel M. Verbeek, Mario Mastrangelo, Jennifer Friedman, Tessa Wassenberg, Kathrin Jeltsch, Jan Kulhánek, and Oya Kuseyri Hübschmann. Consensus guideline for the diagnosis and treatment of tetrahydrobiopterin (bh4) deficiencies. Orphanet Journal of Rare Diseases, May 2020. URL: https://doi.org/10.1186/s13023-020-01379-8, doi:10.1186/s13023-020-01379-8. This article has 182 citations and is from a peer-reviewed journal.

  6. (thony2000tetrahydrobiopterinbiosynthesisregeneration pages 10-12): Beat THÖNY, Günter AUERBACH, and Nenad BLAU. Tetrahydrobiopterin biosynthesis, regeneration and functions. The Biochemical journal, 347 Pt 1:1-16, Apr 2000. URL: https://doi.org/10.1042/bj3470001, doi:10.1042/bj3470001. This article has 1116 citations.

  7. (cutler1986dihydropteridinereductase pages 58-64): Dihydropteridine reductase This article has 1 citations.

  8. (breuer2018characterizingthefunction pages 19-24): Maximilian Breuer. Characterizing the function and role of three dihydropteridine reductase homologs qdpra, qdprb1 and qdprb2 in the embryonic development of danio rerio. Text, Jan 2018. URL: https://doi.org/10.11588/heidok.00024357, doi:10.11588/heidok.00024357. This article has 0 citations and is from a peer-reviewed journal.

  9. (varughese1992crystalstructureof pages 4-5): K. Varughese, Matthew M. Skinner, John M. WHITELEYt, David A. MATrHEWS, and N. Xuong. Crystal structure of rat liver dihydropteridine reductase. Proceedings of the National Academy of Sciences of the United States of America, 89 13:6080-4, Jul 1992. URL: https://doi.org/10.1073/pnas.89.13.6080, doi:10.1073/pnas.89.13.6080. This article has 210 citations and is from a highest quality peer-reviewed journal.

  10. (lye2002characterizationofquinonoiddihydropteridine pages 6-7): Lon-Fye Lye, Mark L. Cunningham, and Stephen M. Beverley. Characterization of quinonoid-dihydropteridine reductase (qdpr) from the lower eukaryote leishmania major *. The Journal of Biological Chemistry, 277:38245-38253, Oct 2002. URL: https://doi.org/10.1074/jbc.m206543200, doi:10.1074/jbc.m206543200. This article has 37 citations.

  11. (siems2025developmentalmaturationand pages 6-8): Sophie B. Siems, Vasiliki‐Ilya Gargareta, Leonie C. Schadt, Vinicius Daguano Gastaldi, Ramona B. Jung, Lars Piepkorn, Patrizia Casaccia, Ting Sun, Olaf Jahn, and Hauke B. Werner. Developmental maturation and regional heterogeneity but no sexual dimorphism of the murine cns myelin proteome. Glia, 73:38-56, Sep 2025. URL: https://doi.org/10.1002/glia.24614, doi:10.1002/glia.24614. This article has 12 citations and is from a domain leading peer-reviewed journal.

  12. (siems2025developmentalmaturationand pages 8-9): Sophie B. Siems, Vasiliki‐Ilya Gargareta, Leonie C. Schadt, Vinicius Daguano Gastaldi, Ramona B. Jung, Lars Piepkorn, Patrizia Casaccia, Ting Sun, Olaf Jahn, and Hauke B. Werner. Developmental maturation and regional heterogeneity but no sexual dimorphism of the murine cns myelin proteome. Glia, 73:38-56, Sep 2025. URL: https://doi.org/10.1002/glia.24614, doi:10.1002/glia.24614. This article has 12 citations and is from a domain leading peer-reviewed journal.

  13. (breuer2018characterizingthefunction pages 15-19): Maximilian Breuer. Characterizing the function and role of three dihydropteridine reductase homologs qdpra, qdprb1 and qdprb2 in the embryonic development of danio rerio. Text, Jan 2018. URL: https://doi.org/10.11588/heidok.00024357, doi:10.11588/heidok.00024357. This article has 0 citations and is from a peer-reviewed journal.

  14. (eichwald2023tetrahydrobiopterinbeyondits pages 5-7): Tuany Eichwald, Lucila de Bortoli da da Silva, Ananda Christina Staats Staats Pires, Laís Niero, Erick Schnorrenberger, Clovis Colpani Filho, Gisele Espíndola, Wei-Lin Huang, Gilles J. Guillemin, José E. Abdenur, and Alexandra Latini. Tetrahydrobiopterin: beyond its traditional role as a cofactor. Antioxidants, 12:1037, May 2023. URL: https://doi.org/10.3390/antiox12051037, doi:10.3390/antiox12051037. This article has 92 citations.

  15. (crabtree2011synthesisandrecycling pages 3-4): Mark J. Crabtree and Keith M. Channon. Synthesis and recycling of tetrahydrobiopterin in endothelial function and vascular disease. Nitric oxide : biology and chemistry, 25 2:81-8, Aug 2011. URL: https://doi.org/10.1016/j.niox.2011.04.004, doi:10.1016/j.niox.2011.04.004. This article has 273 citations.

  16. (opladen2020consensusguidelinefor pages 6-7): Thomas Opladen, Eduardo López-Laso, Elisenda Cortès-Saladelafont, Toni S. Pearson, H. Serap Sivri, Yilmaz Yildiz, Birgit Assmann, Manju A. Kurian, Vincenzo Leuzzi, Simon Heales, Simon Pope, Francesco Porta, Angeles García-Cazorla, Tomáš Honzík, Roser Pons, Luc Regal, Helly Goez, Rafael Artuch, Georg F. Hoffmann, Gabriella Horvath, Beat Thöny, Sabine Scholl-Bürgi, Alberto Burlina, Marcel M. Verbeek, Mario Mastrangelo, Jennifer Friedman, Tessa Wassenberg, Kathrin Jeltsch, Jan Kulhánek, and Oya Kuseyri Hübschmann. Consensus guideline for the diagnosis and treatment of tetrahydrobiopterin (bh4) deficiencies. Orphanet Journal of Rare Diseases, May 2020. URL: https://doi.org/10.1186/s13023-020-01379-8, doi:10.1186/s13023-020-01379-8. This article has 182 citations and is from a peer-reviewed journal.

  17. (alam2023uncouplednitricoxide pages 1-2): Asim Alam, Steven C. Smith, Sundaresan Gobalakrishnan, Mina McGinn, Vasily A. Yakovlev, and Christopher S. Rabender. Uncoupled nitric oxide synthase activity promotes colorectal cancer progression. Frontiers in Oncology, Mar 2023. URL: https://doi.org/10.3389/fonc.2023.1165326, doi:10.3389/fonc.2023.1165326. This article has 12 citations.

  18. (yan2026overcomingmultidimensionalimmunotherapy pages 5-6): Jin Yan, Huiyi Ou, Shuai Wang, Kaizhou Jin, Xianjun Yu, and Weiding Wu. Overcoming multidimensional immunotherapy resistance in pdac: from microenvironment to clinic. Frontiers in Immunology, Mar 2026. URL: https://doi.org/10.3389/fimmu.2026.1782960, doi:10.3389/fimmu.2026.1782960. This article has 1 citations and is from a peer-reviewed journal.

  19. (lye2002characterizationofquinonoiddihydropteridine pages 1-1): Lon-Fye Lye, Mark L. Cunningham, and Stephen M. Beverley. Characterization of quinonoid-dihydropteridine reductase (qdpr) from the lower eukaryote leishmania major *. The Journal of Biological Chemistry, 277:38245-38253, Oct 2002. URL: https://doi.org/10.1074/jbc.m206543200, doi:10.1074/jbc.m206543200. This article has 37 citations.

  20. (pierson2018discoveryofnovel pages 14-18): Yann Mathieu Pierson. Discovery of novel competitive dihydropteridine reductase inhibitors via high throughput screening and their effect on metabolic pterin balance of fibroblast. ArXiv, Jan 2018. URL: https://doi.org/10.5075/epfl-thesis-8371, doi:10.5075/epfl-thesis-8371. This article has 0 citations.

  21. (siems2025developmentalmaturationand pages 1-2): Sophie B. Siems, Vasiliki‐Ilya Gargareta, Leonie C. Schadt, Vinicius Daguano Gastaldi, Ramona B. Jung, Lars Piepkorn, Patrizia Casaccia, Ting Sun, Olaf Jahn, and Hauke B. Werner. Developmental maturation and regional heterogeneity but no sexual dimorphism of the murine cns myelin proteome. Glia, 73:38-56, Sep 2025. URL: https://doi.org/10.1002/glia.24614, doi:10.1002/glia.24614. This article has 12 citations and is from a domain leading peer-reviewed journal.

  22. (siems2025developmentalmaturationand pages 4-6): Sophie B. Siems, Vasiliki‐Ilya Gargareta, Leonie C. Schadt, Vinicius Daguano Gastaldi, Ramona B. Jung, Lars Piepkorn, Patrizia Casaccia, Ting Sun, Olaf Jahn, and Hauke B. Werner. Developmental maturation and regional heterogeneity but no sexual dimorphism of the murine cns myelin proteome. Glia, 73:38-56, Sep 2025. URL: https://doi.org/10.1002/glia.24614, doi:10.1002/glia.24614. This article has 12 citations and is from a domain leading peer-reviewed journal.

  23. (siems2025developmentalmaturationand pages 11-12): Sophie B. Siems, Vasiliki‐Ilya Gargareta, Leonie C. Schadt, Vinicius Daguano Gastaldi, Ramona B. Jung, Lars Piepkorn, Patrizia Casaccia, Ting Sun, Olaf Jahn, and Hauke B. Werner. Developmental maturation and regional heterogeneity but no sexual dimorphism of the murine cns myelin proteome. Glia, 73:38-56, Sep 2025. URL: https://doi.org/10.1002/glia.24614, doi:10.1002/glia.24614. This article has 12 citations and is from a domain leading peer-reviewed journal.

Artifacts

Citations

  1. thony2000tetrahydrobiopterinbiosynthesisregeneration pages 12-13
  2. thony2000tetrahydrobiopterinbiosynthesisregeneration pages 10-12
  3. cutler1986dihydropteridinereductase pages 58-64
  4. lye2002characterizationofquinonoiddihydropteridine pages 6-7
  5. varughese1992crystalstructureof pages 4-5
  6. varughese1992crystalstructureof pages 1-2
  7. breuer2018characterizingthefunction pages 15-19
  8. siems2025developmentalmaturationand pages 6-8
  9. siems2025developmentalmaturationand pages 11-12
  10. opladen2020consensusguidelinefor pages 1-2
  11. eichwald2023tetrahydrobiopterinbeyondits pages 9-10
  12. opladen2020consensusguidelinefor pages 6-7
  13. breuer2018characterizingthefunction pages 19-24
  14. alam2023uncouplednitricoxide pages 1-2
  15. yan2026overcomingmultidimensionalimmunotherapy pages 5-6
  16. pierson2018discoveryofnovel pages 27-31
  17. siems2025developmentalmaturationand pages 8-9
  18. eichwald2023tetrahydrobiopterinbeyondits pages 5-7
  19. crabtree2011synthesisandrecycling pages 3-4
  20. lye2002characterizationofquinonoiddihydropteridine pages 1-1
  21. pierson2018discoveryofnovel pages 14-18
  22. siems2025developmentalmaturationand pages 1-2
  23. siems2025developmentalmaturationand pages 4-6
  24. https://doi.org/10.1042/bj3470001,
  25. https://doi.org/10.1073/pnas.89.13.6080,
  26. https://doi.org/10.5075/epfl-thesis-8371,
  27. https://doi.org/10.3390/antiox12051037,
  28. https://doi.org/10.1186/s13023-020-01379-8,
  29. https://doi.org/10.11588/heidok.00024357,
  30. https://doi.org/10.1074/jbc.m206543200,
  31. https://doi.org/10.1002/glia.24614,
  32. https://doi.org/10.1016/j.niox.2011.04.004,
  33. https://doi.org/10.3389/fonc.2023.1165326,
  34. https://doi.org/10.3389/fimmu.2026.1782960,

📚 Additional Documentation

Notes

(QDPR-notes.md)

QDPR (Dihydropteridine reductase / DHPR) — review notes

UniProtKB:P09417, HGNC:9752, EC 1.5.1.34. Human, NCBITaxon:9606.

Deep research provenance

just deep-research-falcon human QDPR was attempted (2026-07-05) but the falcon/Edison
run timed out (600s) and exited with code 1; no QDPR-deep-research-falcon.md was produced.
Per project policy I did NOT fabricate a -deep-research-*.md. This review is grounded in
the cached UniProt record, the seeded GOA, cached publications/PMID_*.md, and the dismech
BH4-deficiency disorder file (DHPR Deficiency subtype).

Verified core biology

DHPR is the cytosolic NAD(P)H-dependent enzyme that regenerates tetrahydrobiopterin (BH4)
by reducing quinonoid-dihydrobiopterin (q-BH2) back to BH4. It is an essential component of
the aromatic amino acid hydroxylating systems (PAH, TH, TPH), which use BH4 as cofactor.

  • PMID:3033643 — establishes MF (q-BH2 → BH4, NADH-dependent), dihydrobiopterin metabolism, and the link to aromatic-amino-acid hydroxylation. (Note the abstract uses the older EC 1.6.99.7; current is EC 1.5.1.34.)
  • UniProt FUNCTION: "Catalyzes the conversion of quinonoid dihydrobiopterin into tetrahydrobiopterin." (ECO:0000269|PubMed:3033643, ECO:0000269|PubMed:8262916)
  • UniProt CATALYTIC ACTIVITY: 5,6,7,8-tetrahydropteridine + NAD(+) = 6,7-dihydropteridine + NADH + H(+); RHEA:17869; EC 1.5.1.34. Physiological direction right-to-left (i.e. q-dihydropteridine + NADH → tetrahydropteridine). Also NADP(+) variant (RHEA:17865). So both NADH and NADPH are cofactors.
  • PMID:8262916 — first structural characterization (PDB 1HDR); confirms activity/catalysis (EXP annotation to GO:0004155). Homodimer; SDR family.
  • UniProt: SDR (short-chain dehydrogenase/reductase) family; SDR33C1. Rossmann NAD(P)-binding fold; ACT_SITE Tyr150 (proton acceptor); NADP binding residues 14-38.

Disease

DHPR deficiency (HPABH4C, MIM:261630; MONDO:0009862; Orphanet 226) — autosomal recessive
BH4-deficient ("malignant"/atypical) hyperphenylalaninemia. Second most common HPA-associated
BH4 deficiency. Loss of BH4 regeneration → hyperphenylalaninemia + dopamine/serotonin
(monoamine neurotransmitter) deficiency + secondary cerebral folate deficiency; NOT corrected
by dietary Phe restriction alone; lethal if untreated.
- UniProt DISEASE (HPABH4C): "attributable to depletion of the neurotransmitters dopamine and serotonin, whose syntheses are controlled by tryptophan and tyrosine hydroxylases that use BH-4 as cofactor. Patients do not respond to phenylalanine-restricted diet."
- dismech Tetrahydrobiopterin_Deficiency.yaml DHPR Deficiency subtype: PMID:32022462.

Localization

  • Cytosol/cytoplasm: well established. GO:0005829 cytosol (TAS Reactome R-HSA-71130); GO:0005737 cytoplasm (IBA; IDA LIFEdb). This is the physiologically relevant compartment (BH4 salvage cycle serves cytosolic aromatic-amino-acid hydroxylases).
  • GO:0005739 mitochondrion (HTP, PMID:34800366): large-scale MS mitochondrial-proteome mapping. DHPR is a soluble cytosolic enzyme in the SDR family with no mitochondrial targeting features (INIT_MET removed, N-acetyl-Ala2; no transit peptide). Likely a co-purifying/cytosolic contaminant or low-confidence assignment — mark as over-annotated / non-core, do not assert core mitochondrial localization.
  • GO:0070062 extracellular exosome (HDA x2, urinary/prostatic exosome proteomics): typical of abundant cytosolic proteins captured in exosome proteomes; not a site of catalytic function. Keep as non-core / over-annotated.

Term scrutiny

  • GO:0009055 electron transfer activity (TAS PMID:3033643): the term is the ETC electron-carrier function ("directed movement of electrons from one molecular entity to another, typically mediated by electron carriers"). DHPR performs NAD(P)H-dependent hydride transfer to a pteridine (an oxidoreductase reaction), not electron-carrier shuttling. This is a generic/imprecise over-annotation; MODIFY to the specific GO:0004155 (already present) — the reference PMID:3033643 supports the reductase activity, not a distinct electron-carrier function.
  • GO:0006520 amino acid metabolic process (TAS PMID:3033643): DHPR is not itself an amino-acid-metabolizing enzyme; it regenerates the BH4 cofactor used by the aromatic-amino-acid hydroxylases. The role is indirect/upstream. Very high-level term; keep as non-core (the biologically informative BP terms are BH4 biosynthesis/regeneration and pteridine metabolism). PMID:3033643 supports the link to aromatic-amino-acid hydroxylation.
  • GO:0006559 L-phenylalanine catabolic process (IBA; in UniProt DR line, NOT in GOA tsv) — DHPR supports PAH-mediated Phe→Tyr via BH4 regeneration; not in the GOA snapshot so not reviewed as an existing annotation.

Core functions

  • MF: GO:0004155 6,7-dihydropteridine reductase activity (EC 1.5.1.34) — with NADH binding GO:0070404 and NADPH binding GO:0070402.
  • BP: GO:0006729 tetrahydrobiopterin biosynthetic process (BH4 regeneration/salvage).
  • CC: GO:0005829 cytosol.

📄 View Raw YAML

 
id: P09417
gene_symbol: QDPR
product_type: PROTEIN
status: INITIALIZED
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  QDPR encodes dihydropteridine reductase (DHPR; EC 1.5.1.34), a cytosolic,
  homodimeric NAD(P)H-dependent oxidoreductase of the short-chain
  dehydrogenase/reductase (SDR) family. DHPR regenerates the essential cofactor
  tetrahydrobiopterin (BH4) by reducing the quinonoid form of dihydrobiopterin
  (q-BH2) back to BH4, completing the pterin cofactor cycle. BH4 is the obligate
  cofactor of the aromatic amino acid hydroxylases (phenylalanine, tyrosine and
  tryptophan hydroxylases) and of nitric oxide synthases, so DHPR-mediated BH4
  salvage is required for phenylalanine catabolism and for biosynthesis of the
  monoamine neurotransmitters dopamine and serotonin. The enzyme adopts a
  Rossmann NAD(P)-binding fold with a catalytic tyrosine (Tyr150), and its
  structure has been solved by X-ray crystallography. In humans, biallelic
  loss-of-function variants in QDPR abolish BH4 regeneration and cause
  autosomal recessive dihydropteridine reductase deficiency, a BH4-deficient
  ("malignant"/atypical) hyperphenylalaninemia with progressive neurological
  disease driven by combined hyperphenylalaninemia and monoamine
  neurotransmitter deficiency.
alternative_products:
- name: '1'
  id: P09417-1
- name: '2'
  id: P09417-2
  sequence_note: VSP_054356
existing_annotations:
- term:
    id: GO:0004155
    label: 6,7-dihydropteridine reductase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      Core molecular function. The phylogenetically inferred (IBA) DHPR activity
      is the enzyme's defining, experimentally established catalytic function
      (EC 1.5.1.34): NAD(P)H-dependent reduction of quinonoid dihydropteridine
      to tetrahydropteridine, regenerating BH4.
    action: ACCEPT
    reason: >-
      Consistent with direct experimental evidence and UniProt catalytic
      activity. This term at exactly the right level of specificity for the gene.
    supported_by:
      - reference_id: PMID:3033643
        supporting_text: "Dihydropteridine reductase (DHPR; EC 1.6.99.7) catalyzes the NADH-mediated \nreduction of quinonoid dihydrobiopterin and is an essential component of the \npterin-dependent aromatic amino acid hydroxylating systems."
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      DHPR is a soluble cytosolic enzyme; the phylogenetic is_active_in cytoplasm
      annotation is consistent with the enzyme acting in the cytosolic BH4
      salvage cycle that supplies the cytosolic aromatic amino acid hydroxylases.
    action: ACCEPT
    reason: >-
      Cytoplasmic/cytosolic localization is well supported (IDA to cytoplasm,
      TAS to cytosol) and is the physiologically relevant compartment. The more
      specific GO:0005829 cytosol is also annotated and captured in core_functions.
    supported_by:
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "Catalyzes the conversion of quinonoid dihydrobiopterin into"
- term:
    id: GO:0006729
    label: tetrahydrobiopterin biosynthetic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      Core biological process. DHPR regenerates BH4 (the reduced, active pterin
      cofactor) from its quinonoid dihydro form, i.e. the salvage/regeneration
      arm of BH4 metabolism that this term subsumes.
    action: ACCEPT
    reason: >-
      Directly supported: DHPR catalyzes formation of tetrahydrobiopterin from
      quinonoid dihydrobiopterin. This is the enzyme's principal pathway role.
    supported_by:
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "Catalyzes the conversion of quinonoid dihydrobiopterin into"
      - reference_id: PMID:3033643
        supporting_text: "reduction of quinonoid dihydrobiopterin and is an essential component of the"
- term:
    id: GO:0070402
    label: NADPH binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      DHPR uses reduced nicotinamide cofactor as the hydride donor. UniProt
      records an NADP(+)/NADPH-dependent catalytic activity (RHEA:17865) in
      addition to the NADH-dependent reaction, supporting NADPH binding.
    action: ACCEPT
    reason: >-
      Supported by the annotated NADP(+)-dependent catalytic activity and the
      Rossmann NAD(P)-binding fold with an NADP-binding region (residues 14-38).
    supported_by:
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "Reaction=5,6,7,8-tetrahydropteridine + NADP(+) = 6,7-dihydropteridine +"
- term:
    id: GO:0070404
    label: NADH binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      DHPR is classically the NADH-dependent quinonoid-dihydrobiopterin
      reductase; NADH is the physiological hydride donor and the crystal
      structure was solved as an NADH binary complex.
    action: ACCEPT
    reason: >-
      Directly supported by the NADH-dependent catalytic activity and the
      structural characterization of the enzyme-NADH complex.
    supported_by:
      - reference_id: PMID:3033643
        supporting_text: "catalyzes the NADH-mediated"
      - reference_id: PMID:8262916
        supporting_text: "and kinetic identity to the naturally occurring enzyme has been proven."
- term:
    id: GO:0004155
    label: 6,7-dihydropteridine reductase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated (ARBA/EC/RHEA) assignment of the DHPR molecular function,
      mapped from EC 1.5.1.34 and RHEA:17865/17869. Correct and identical to the
      experimentally supported core function.
    action: ACCEPT
    reason: >-
      IEA mapping from EC/RHEA is accurate and matches the experimental and IBA
      DHPR activity annotations.
    supported_by:
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "EC=1.5.1.34; Evidence={ECO:0000269|PubMed:3033643,"
- term:
    id: GO:0042558
    label: pteridine-containing compound metabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: involved_in
  review:
    summary: >-
      Broad but correct: DHPR acts on the pteridine cofactor biopterin
      (a pteridine-containing compound). More specific and informative terms
      (tetrahydrobiopterin biosynthetic process, dihydrobiopterin metabolic
      process) are also annotated.
    action: KEEP_AS_NON_CORE
    reason: >-
      The term is accurate but high-level; it is a parent of the more specific
      BH4/dihydrobiopterin process terms that better capture the core role.
    supported_by:
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "Catalyzes the conversion of quinonoid dihydrobiopterin into"
- term:
    id: GO:0004155
    label: 6,7-dihydropteridine reductase activity
  evidence_type: EXP
  original_reference_id: PMID:8262916
  qualifier: enables
  review:
    summary: >-
      Experimental support for DHPR activity from the enzyme that was purified
      to homogeneity, shown to have kinetic identity to the naturally occurring
      enzyme, and crystallized (PDB 1HDR).
    action: ACCEPT
    reason: >-
      Direct experimental (kinetic/structural) evidence for the DHPR molecular
      function on the human enzyme.
    supported_by:
      - reference_id: PMID:8262916
        supporting_text: "and kinetic identity to the naturally occurring enzyme has been proven."
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  qualifier: located_in
  review:
    summary: >-
      A single high-throughput mitochondrial-proteome mapping study lists QDPR
      among detected proteins. DHPR is an established soluble cytosolic SDR-family
      enzyme with no mitochondrial targeting features (initiator Met removed,
      N-acetylated Ala2, no transit peptide); it functions in the cytosolic BH4
      salvage cycle.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      A single HTP proteomic detection is weak evidence for organellar residence
      of an abundant cytosolic enzyme, which is prone to co-purification. The
      catalytic function and its substrates/products are cytosolic; there is no
      corroborating evidence for a mitochondrial pool or function. Do not treat
      as core; flag as likely over-annotation rather than removing the curated
      HTP datum outright.
    supported_by:
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "Catalyzes the conversion of quinonoid dihydrobiopterin into"
- term:
    id: GO:0004155
    label: 6,7-dihydropteridine reductase activity
  evidence_type: IDA
  original_reference_id: PMID:3033643
  qualifier: enables
  review:
    summary: >-
      Direct assay evidence for human DHPR enzymatic activity: the isolated cDNA
      clone expressed in COS cells produced DHPR enzymatic activity, confirming
      the molecular function.
    action: ACCEPT
    reason: >-
      IDA-level experimental confirmation of the core DHPR activity for the human
      enzyme.
    supported_by:
      - reference_id: PMID:3033643
        supporting_text: "Gene transfer of the recombinant human DHPR into COS cells \nleads to expression of DHPR enzymatic activity."
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:23533145
  qualifier: located_in
  review:
    summary: >-
      QDPR was detected in the shotgun proteome of prostatic-secretion (EPS)
      urinary exosomes. Detection of an abundant cytosolic enzyme in exosome
      proteomes is common and does not indicate a functional extracellular role.
    action: KEEP_AS_NON_CORE
    reason: >-
      Valid HDA proteomic observation but not a site of DHPR catalytic function;
      the enzyme acts intracellularly in the cytosolic BH4 salvage cycle. Keep
      as a non-core localization datum.
    supported_by:
      - reference_id: PMID:23533145
        supporting_text: "exosome preparations were \ncharacterized by a shotgun proteomics procedure"
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:19056867
  qualifier: located_in
  review:
    summary: >-
      QDPR was among the >1000 proteins identified by LC-MS/MS in the proteome
      of normal human urinary exosomes. As above, this reflects capture of an
      abundant cytosolic protein in exosomes rather than a functional
      extracellular localization.
    action: KEEP_AS_NON_CORE
    reason: >-
      Corroborating HDA proteomic detection in exosomes; not a functional site
      of DHPR activity. Retain as non-core.
    supported_by:
      - reference_id: PMID:19056867
        supporting_text: "we used \nLC-MS/MS to profile the proteome of human urinary exosomes"
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-71130
  qualifier: located_in
  review:
    summary: >-
      Cytosolic localization from Reactome, which curates the DHPR reaction
      (q-dihydrobiopterin + NADH + H+ => tetrahydrobiopterin + NAD+) in the
      cytosol. This is the physiologically relevant compartment for BH4 salvage.
    action: ACCEPT
    reason: >-
      Cytosol is the correct, most specific localization for this soluble enzyme
      and matches the compartment of its substrates/products and the aromatic
      amino acid hydroxylases it serves.
    supported_by:
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "Catalyzes the conversion of quinonoid dihydrobiopterin into"
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IDA
  original_reference_id: GO_REF:0000054
  qualifier: located_in
  review:
    summary: >-
      Direct localization of an expressed fusion protein to the cytoplasm
      (LIFEdb), consistent with the soluble cytosolic nature of DHPR.
    action: ACCEPT
    reason: >-
      Consistent with all other localization evidence (cytosol/cytoplasm). The
      more specific cytosol term is retained in core_functions.
    supported_by:
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "Catalyzes the conversion of quinonoid dihydrobiopterin into"
- term:
    id: GO:0009055
    label: electron transfer activity
  evidence_type: TAS
  original_reference_id: PMID:3033643
  qualifier: enables
  review:
    summary: >-
      Generic electron-transfer function assigned by TAS. GO:0009055 denotes the
      directed movement of electrons between molecular entities typically
      mediated by electron carriers (as in electron transport chains). DHPR does
      not act as an electron carrier; it is an NAD(P)H-dependent oxidoreductase
      that transfers a hydride to a quinonoid pteridine (EC 1.5.1.34).
    action: MODIFY
    reason: >-
      The essence (a redox activity) is sound but the specific term is the wrong
      molecular function: DHPR's characterized activity is the 6,7-dihydropteridine
      reductase reaction, not electron-carrier transfer. The cited reference
      (PMID:3033643) describes NADH-mediated reduction of quinonoid
      dihydrobiopterin, i.e. the reductase activity. Replace with the specific
      reductase term (already annotated) so this generic term is not retained as
      a distinct function.
    proposed_replacement_terms:
      - id: GO:0004155
        label: 6,7-dihydropteridine reductase activity
    supported_by:
      - reference_id: PMID:3033643
        supporting_text: "catalyzes the NADH-mediated \nreduction of quinonoid dihydrobiopterin"
- term:
    id: GO:0004155
    label: 6,7-dihydropteridine reductase activity
  evidence_type: TAS
  original_reference_id: PMID:3033643
  qualifier: enables
  review:
    summary: >-
      Author-statement (TAS) support for the DHPR molecular function from the
      human DHPR cloning/expression paper, which describes the enzyme's
      NADH-mediated reduction of quinonoid dihydrobiopterin.
    action: ACCEPT
    reason: >-
      Redundant with, and fully consistent with, the IDA/EXP/IBA/IEA DHPR
      activity annotations. Duplicate GO IDs across evidence codes are acceptable.
    supported_by:
      - reference_id: PMID:3033643
        supporting_text: "catalyzes the NADH-mediated \nreduction of quinonoid dihydrobiopterin"
- term:
    id: GO:0051066
    label: dihydrobiopterin metabolic process
  evidence_type: TAS
  original_reference_id: PMID:3033643
  qualifier: involved_in
  review:
    summary: >-
      Accurate process term: DHPR metabolizes dihydrobiopterin, reducing its
      quinonoid form to BH4. This is a precise description of the enzyme's
      substrate-level role in the pterin cycle.
    action: ACCEPT
    reason: >-
      Directly supported by the characterized reaction (quinonoid dihydrobiopterin
      reduction). Complements the tetrahydrobiopterin biosynthetic process term.
    supported_by:
      - reference_id: PMID:3033643
        supporting_text: "catalyzes the NADH-mediated \nreduction of quinonoid dihydrobiopterin"
- term:
    id: GO:0006520
    label: amino acid metabolic process
  evidence_type: TAS
  original_reference_id: PMID:3033643
  qualifier: involved_in
  review:
    summary: >-
      Very high-level BP term reflecting DHPR's indirect role in aromatic amino
      acid metabolism: by regenerating BH4 it supports the aromatic amino acid
      hydroxylases (notably phenylalanine hydroxylase). DHPR is not itself an
      amino-acid-metabolizing enzyme; it acts upstream by supplying the cofactor.
    action: KEEP_AS_NON_CORE
    reason: >-
      The link to amino acid metabolism is real but indirect and the term is
      extremely broad. The informative, direct process terms are the BH4/
      dihydrobiopterin metabolic terms. Retain as non-core rather than as a core
      function.
    supported_by:
      - reference_id: PMID:3033643
        supporting_text: "an essential component of the \npterin-dependent aromatic amino acid hydroxylating systems"
core_functions:
  - description: >-
      NAD(P)H-dependent regeneration of tetrahydrobiopterin (BH4) by reduction of
      quinonoid dihydrobiopterin, sustaining the pterin cofactor cycle for the
      aromatic amino acid hydroxylases.
    molecular_function:
      id: GO:0004155
      label: 6,7-dihydropteridine reductase activity
    directly_involved_in:
      - id: GO:0006729
        label: tetrahydrobiopterin biosynthetic process
      - id: GO:0051066
        label: dihydrobiopterin metabolic process
    locations:
      - id: GO:0005829
        label: cytosol
    supported_by:
      - reference_id: PMID:3033643
        supporting_text: "catalyzes the NADH-mediated \nreduction of quinonoid dihydrobiopterin and is an essential component of the \npterin-dependent aromatic amino acid hydroxylating systems"
      - reference_id: PMID:8262916
        supporting_text: "and kinetic identity to the naturally occurring enzyme has been proven."
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "Catalyzes the conversion of quinonoid dihydrobiopterin into"
  - description: >-
      Uses reduced nicotinamide cofactor (NADH, and NADPH) as the hydride donor
      for the reductase reaction, bound in a Rossmann NAD(P)-binding fold.
    molecular_function:
      id: GO:0004155
      label: 6,7-dihydropteridine reductase activity
    directly_involved_in:
      - id: GO:0006729
        label: tetrahydrobiopterin biosynthetic process
    locations:
      - id: GO:0005829
        label: cytosol
    supported_by:
      - reference_id: PMID:8262916
        supporting_text: "The crystallographic structure of a human dihydropteridine reductase NADH \nbinary"
      - reference_id: file:human/QDPR/QDPR-uniprot.txt
        supporting_text: "Reaction=5,6,7,8-tetrahydropteridine + NADP(+) = 6,7-dihydropteridine +"
proposed_new_terms: []
suggested_questions:
  - question: >-
      Does the single high-throughput mitochondrial-proteome detection of QDPR
      reflect a genuine, functionally relevant mitochondrial pool, or
      co-purification of the abundant cytosolic enzyme?
  - question: >-
      Which reduced nicotinamide cofactor (NADH vs NADPH) predominates in vivo
      for human DHPR-mediated BH4 regeneration, and does the preference differ
      between tissues (e.g. brain vs liver)?
suggested_experiments:
  - hypothesis: >-
      Human DHPR regenerates BH4 using NADH as the primary physiological hydride
      donor, with NADPH as an alternative cofactor.
    description: >-
      Steady-state and pre-steady-state kinetics of purified human DHPR
      comparing NADH vs NADPH as hydride donors with quinonoid dihydrobiopterin
      as substrate.
    experiment_type: enzyme kinetics
  - hypothesis: >-
      QDPR is exclusively cytosolic and the mitochondrial-proteome detection is a
      co-purification artifact rather than a functional mitochondrial pool.
    description: >-
      Subcellular fractionation with DHPR activity assay, plus endogenous
      immunofluorescence/immuno-EM, to test for bona fide mitochondrial DHPR
      activity versus exclusively cytosolic localization.
    experiment_type: subcellular fractionation and imaging
references:
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      GO_Central PAN-GO phylogenetic pipeline; the IBA annotations it supports
      (DHPR activity, cytoplasm, tetrahydrobiopterin biosynthetic process,
      NAD(P)H binding) are all consistent with the experimentally characterized
      function of this well-studied enzyme.
- id: GO_REF:0000054
  title: Gene Ontology annotation based on curation of intracellular localizations
    of expressed fusion proteins in living cells
  findings: []
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:19056867
  title: Large-scale proteomics and phosphoproteomics of urinary exosomes.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      Genuine urinary-exosome proteomics dataset that includes QDPR; supports a
      non-core extracellular-exosome localization, not a functional role.
- id: PMID:23533145
  title: In-depth proteomic analyses of exosomes isolated from expressed prostatic
    secretions in urine.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      Exosome shotgun proteomics from expressed prostatic secretions; supports a
      non-core extracellular-exosome localization datum.
- id: PMID:3033643
  title: Structure and expression of human dihydropteridine reductase.
  findings:
    - statement: >-
        DHPR (EC 1.5.1.34; historically 1.6.99.7) catalyzes the NADH-mediated
        reduction of quinonoid dihydrobiopterin and is an essential component of
        the pterin-dependent aromatic amino acid hydroxylating systems.
      supporting_text: "Dihydropteridine reductase (DHPR; EC 1.6.99.7) catalyzes the NADH-mediated \nreduction of quinonoid dihydrobiopterin and is an essential component of the \npterin-dependent aromatic amino acid hydroxylating systems."
    - statement: Recombinant human DHPR expressed in COS cells produces DHPR enzymatic activity, confirming the cloned cDNA encodes an active enzyme.
      supporting_text: "Gene transfer of the recombinant human DHPR into COS cells \nleads to expression of DHPR enzymatic activity."
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Primary paper cloning/expressing human DHPR and establishing its NADH-
      dependent reductase function and role in the aromatic amino acid
      hydroxylating systems. Abstract uses the older EC 1.6.99.7; the current EC
      is 1.5.1.34 (same activity).
- id: PMID:34800366
  title: Quantitative high-confidence human mitochondrial proteome and its dynamics
    in cellular context.
  findings: []
  reference_review:
    relevance: LOW
    correctness: LOW_QUALITY
    review_notes: >-
      High-throughput mitochondrial-proteome mapping in which QDPR is detected;
      weak, single-source evidence for organellar residence of an abundant
      cytosolic SDR enzyme lacking mitochondrial targeting features. Treated as
      over-annotation, not core localization.
- id: PMID:8262916
  title: The crystallographic structure of a human dihydropteridine reductase NADH
    binary complex expressed in Escherichia coli by a cDNA constructed from its rat
    homologue.
  findings:
    - statement: The human DHPR enzyme was purified to homogeneity with kinetic identity to the natural enzyme and crystallized (PDB 1HDR), providing the first complete structural characterization.
      supporting_text: "and kinetic identity to the naturally occurring enzyme has been proven."
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Structural/kinetic characterization of human DHPR as an NADH binary
      complex; supports the EXP molecular-function annotation and NADH binding.
- id: Reactome:R-HSA-71130
  title: q-dihydrobiopterin + NADH + H+ => tetrahydrobiopterin + NAD+
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Reactome reaction for the DHPR-catalyzed regeneration of BH4 in the
      cytosol; supports cytosolic localization and the core reductase reaction.