Sepiapterin reductase (SPR) is a short-chain dehydrogenase/reductase (SDR) family enzyme that catalyzes the terminal reduction steps in the de novo and salvage biosynthesis of tetrahydrobiopterin (BH4). In the de novo pathway, SPR reduces 6-pyruvoyl-tetrahydropterin through 1'-oxo and 2'-oxo intermediates to BH4. In the salvage pathway, it reduces sepiapterin to 7,8-dihydrobiopterin (BH2). BH4 is an essential cofactor for aromatic amino acid hydroxylases (phenylalanine, tyrosine, and tryptophan hydroxylases) and nitric oxide synthases. SPR deficiency causes DOPA-responsive dystonia with monoamine neurotransmitter deficiency without hyperphenylalaninemia. The enzyme is a cytosolic homodimer that uses NADPH as cofactor.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004757 sepiapterin reductase (NADP+) activity | IBA GO_REF:0000033 | ACCEPT | Summary: This is the core molecular function of SPR. The enzyme catalyzes the NADP+-dependent reduction of sepiapterin and related pteridine substrates. PMID:1883349 cloned human SPR and established it as "an enzyme involved in tetrahydrobiopterin biosynthesis." UniProt confirms EC 1.1.1.153 with experimental evidence and kinetic characterization. Reason: This annotation precisely describes the core enzymatic function of SPR. The IBA annotation is well-supported by phylogenetic analysis and confirmed by extensive experimental evidence including kinetic characterization. Supporting Evidence: PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin reductase--an enzyme involved in tetrahydrobiopterin biosynthesis. UniProt:P35270 file:human/SPR/SPR-deep-research-falcon.md model: Edison Scientific Literature PMID:31244106 We report the crystal structures of six chemically diverse inhibitors complexed with SPR, identifying relevant interactions and binding modes in the sepiapterin pocket. |
| GO:0006729 tetrahydrobiopterin biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: SPR catalyzes the terminal reduction steps in BH4 biosynthesis, making this the core biological process. The deep research confirms SPR operates downstream of GCH1 and PTS in de novo BH4 biosynthesis and participates in BH4 salvage from sepiapterin. SPR deficiency causes BH4 deficiency leading to dopamine and serotonin deficiencies (PMID:11443547). Reason: This is the primary biological process in which SPR functions. As the terminal reductase in BH4 biosynthesis, this annotation is accurate and represents the core function. Well supported by IBA phylogenetic evidence and extensive literature. Supporting Evidence: PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin reductase--an enzyme involved in tetrahydrobiopterin biosynthesis. PMID:9792819 Genomic organization and chromosomal localization of the human sepiapterin reductase gene. |
| GO:0004757 sepiapterin reductase (NADP+) activity | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation based on InterPro domain (IPR006393 Sepiapterin_red) and Rhea reaction mappings. Consistent with the IBA annotation and experimental evidence. Reason: This IEA annotation is accurate and consistent with the experimentally verified IBA annotation. The InterPro domain IPR006393 specifically identifies sepiapterin reductase family members. Supporting Evidence: UniProt:P35270 |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation based on UniProt subcellular location vocabulary. UniProt entry states "SUBCELLULAR LOCATION: Cytoplasm." This is consistent with the cytosolic IDA annotation and the soluble nature of the enzyme. Reason: While cytosol (GO:0005829) is more specific and experimentally verified, cytoplasm is not incorrect. The IEA is a reasonable inference from UniProt annotation. Both cytoplasm and cytosol annotations can coexist since cytosol is_a cytoplasm. Supporting Evidence: UniProt:P35270 |
| GO:0006729 tetrahydrobiopterin biosynthetic process | IEA GO_REF:0000002 | ACCEPT | Summary: IEA annotation based on InterPro domain IPR006393 (Sepiapterin_red). This is consistent with the IBA annotation and well-established function of SPR in BH4 biosynthesis. Reason: This IEA is accurate and consistent with the experimentally supported IBA annotation. The InterPro domain is specific to sepiapterin reductase family members that function in BH4 biosynthesis. Supporting Evidence: PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin reductase--an enzyme involved in tetrahydrobiopterin biosynthesis. |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000043 | ACCEPT | Summary: IEA based on UniProt keyword KW-0560 (Oxidoreductase). SPR is indeed an oxidoreductase (EC 1.1.1.153), catalyzing NADPH-dependent reductions. However, this is a very general term compared to the specific GO:0004757 (sepiapterin reductase activity). Reason: While this term is much less specific than GO:0004757, it is not incorrect. SPR is an oxidoreductase. The more specific term (sepiapterin reductase activity) is also annotated, so this general term can be retained as a broader classification. Supporting Evidence: UniProt:P35270 |
| GO:0005829 cytosol | IDA GO_REF:0000052 | ACCEPT | Summary: IDA annotation based on immunofluorescence data (HPA). SPR is a soluble cytosolic enzyme that functions in the cytoplasm where BH4 biosynthesis occurs. UniProt confirms cytoplasmic localization. Reason: Cytosol is the appropriate specific cellular component for SPR. The enzyme functions in the soluble cytoplasmic compartment where it participates in BH4 biosynthesis alongside GCH1 and PTS. IDA evidence from immunofluorescence is reliable. Supporting Evidence: UniProt:P35270 |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | MARK AS OVER ANNOTATED | Summary: HTP annotation from a high-throughput mitochondrial proteome study. SPR is primarily a cytosolic enzyme and mitochondrial localization is not the primary site of function. This may represent detection in a comprehensive proteomics survey rather than functional mitochondrial localization. Reason: SPR is well-established as a cytosolic enzyme (UniProt, IDA evidence). The mitochondrial detection in this HTP study may be due to contamination or minor dual localization but does not represent the primary functional compartment. BH4 biosynthesis occurs in the cytosol. This annotation could be misleading about primary localization. Supporting Evidence: UniProt:P35270 PMID:34800366 Epub 2021 Nov 19. Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context. |
| GO:0070062 extracellular exosome | HDA PMID:23533145 In-depth proteomic analyses of exosomes isolated from expres... | KEEP AS NON CORE | Summary: HDA annotation from proteomic analysis of exosomes isolated from expressed prostatic secretions in urine. The study identified ~900 proteins in exosome preparations using shotgun proteomics. Reason: Detection of SPR in exosomes represents a finding from high-throughput proteomics but does not indicate a primary functional localization. Many cytosolic proteins are detected in exosomes. This is not core to SPR function but may be retained as a non-core observation. Supporting Evidence: PMID:23533145 2013 Apr 23. In-depth proteomic analyses of exosomes isolated from expressed prostatic secretions in urine. |
| GO:0070062 extracellular exosome | HDA PMID:19056867 Large-scale proteomics and phosphoproteomics of urinary exos... | KEEP AS NON CORE | Summary: HDA annotation from large-scale proteomics of urinary exosomes. The study identified 1132 proteins in human urinary exosomes using LC-MS/MS. Reason: Similar to the other exosome annotation - detection in a proteomics survey does not indicate primary functional localization. Can be retained as non-core observation. Supporting Evidence: PMID:19056867 2008 Dec 3. Large-scale proteomics and phosphoproteomics of urinary exosomes. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-1497853 | ACCEPT | Summary: TAS annotation from Reactome pathway annotation for SPR phosphorylation by CaMK2. The reaction occurs in the cytosol. However, note that UniProt indicates the Ser-213 phosphorylation by CaMK2 does not change kinetic parameters. Reason: Cytosol localization is accurate for SPR. Reactome pathway curation places SPR in cytosol consistent with its role in BH4 biosynthesis. Supporting Evidence: Reactome:R-HSA-1497853 |
| GO:0005829 cytosol | TAS Reactome:R-HSA-1475414 | ACCEPT | Summary: TAS annotation from Reactome for the de novo BH4 synthesis reaction where SPR reduces DHNTP (6-pyruvoyltetrahydropterin precursor) to BH4. Reason: Cytosol localization is appropriate for the de novo BH4 synthesis pathway. Supporting Evidence: Reactome:R-HSA-1475414 |
| GO:0005829 cytosol | TAS Reactome:R-HSA-1497869 | ACCEPT | Summary: TAS annotation from Reactome for the BH4 salvage pathway where SPR reduces sepiapterin to BH2. Reason: Cytosol localization is appropriate for the BH4 salvage pathway reaction. Supporting Evidence: Reactome:R-HSA-1497869 |
| GO:0004757 sepiapterin reductase (NADP+) activity | ISS GO_REF:0000024 | ACCEPT | Summary: ISS annotation based on sequence similarity to experimentally verified orthologs (with reference to mouse SPR UniProtKB:P18297). Mouse SPR is well characterized. Reason: The ISS annotation is valid and consistent with the direct experimental evidence for human SPR (EC 1.1.1.153). Human and mouse SPR share 74% sequence identity (PMID:1883349). Supporting Evidence: PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin reductase--an enzyme involved in tetrahydrobiopterin biosynthesis. |
| GO:0006729 tetrahydrobiopterin biosynthetic process | TAS PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin re... | ACCEPT | Summary: TAS annotation based on the original cloning paper by Ichinose et al. (1991). The paper cloned human SPR cDNA and established its role in BH4 biosynthesis. Reason: This is the foundational paper establishing SPR function in BH4 biosynthesis. TAS evidence is appropriate for this well-established role. Supporting Evidence: PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin reductase--an enzyme involved in tetrahydrobiopterin biosynthesis. |
| GO:0006809 nitric oxide biosynthetic process | IDA PMID:15197144 Functional tetrahydrobiopterin synthesis in human platelets | MODIFY | Summary: IDA annotation based on Franscini et al. (2004) study of BH4 synthesis in human platelets. The study demonstrated functional BH4 synthesis in platelets and discussed its importance for NO synthase activity. BH4 is an essential cofactor for NOS enzymes. However, SPR does not directly synthesize NO - it synthesizes BH4 which is required as a cofactor by NOS. Reason: This annotation represents an indirect relationship. SPR synthesizes BH4, which is required as a cofactor for NOS. SPR does not directly participate in NO biosynthesis. The annotation is somewhat over-extended. The core function is BH4 biosynthesis. If the connection to NO synthesis is to be retained, it should be as a more general regulatory term or kept as non-core. Proposed replacements: tetrahydrobiopterin biosynthetic process Supporting Evidence: PMID:15197144 Functional tetrahydrobiopterin synthesis in human platelets. PMID:9792819 Genomic organization and chromosomal localization of the human sepiapterin reductase gene. |
| GO:0008106 alcohol dehydrogenase (NADP+) activity | TAS PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin re... | REMOVE | Summary: TAS annotation based on the cloning paper. The paper noted homology between SPR and other short-chain dehydrogenases. However, GO:0008106 refers to a different enzymatic activity (alcohol dehydrogenase converting ethanol to acetaldehyde), not sepiapterin reductase activity. This appears to be an incorrect mapping. Reason: This is an incorrect annotation. SPR does not have alcohol dehydrogenase activity as defined by GO:0008106 (primary alcohols to aldehydes). SPR is specifically a sepiapterin reductase (EC 1.1.1.153), not an alcohol dehydrogenase (EC 1.1.1.2). The original paper mentions sequence homology to SDR family members but does not demonstrate alcohol dehydrogenase activity. The correct MF term is GO:0004757. Supporting Evidence: UniProt:P35270 PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin reductase--an enzyme involved in tetrahydrobiopterin biosynthesis. |
| GO:0050661 NADP binding | TAS PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin re... | ACCEPT | Summary: TAS annotation based on the cloning paper. SPR uses NADPH as cofactor. UniProt confirms NADP(+) binding sites at multiple residues. Crystal structure (PDB 1Z6Z) shows NADP bound. Reason: NADP binding is a valid molecular function of SPR. The enzyme uses NADPH as the reducing cofactor for its reductase activity. Crystal structure confirms NADP binding. This represents a true, experimentally verified function. Supporting Evidence: UniProt:P35270 KM=14.3 uM for sepiapterin UniProt:P35270 KM=10 uM for NADPH {ECO:0000269|PubMed:10350607, ECO:0000269|PubMed:11825621} PMID:1883349 Cloning and sequencing of cDNA encoding human sepiapterin reductase--an enzyme involved in tetrahydrobiopterin biosynthesis. |
| GO:0004757 sepiapterin reductase (NADP+) activity | TAS PMID:9792819 Genomic organization and chromosomal localization of the hum... | ACCEPT | Summary: TAS annotation based on Ohye et al. (1998) genomic organization paper. The paper states SPR "catalyzes the final step of the biosynthetic pathway of tetrahydrobiopterin." Reason: The annotation is accurate. The paper explicitly describes SPR's enzymatic function in BH4 biosynthesis. Supporting Evidence: PMID:9792819 Genomic organization and chromosomal localization of the human sepiapterin reductase gene. |
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Download this section (compressed HTML)Q: What is the relative contribution of SPR vs. alternative aldo-keto reductases (AKR1A1, AKR1B1) in BH4 biosynthesis in different tissues? The deep research mentions that alternative reductases can form 1'/2'-oxo intermediates but the terminal steps are uniquely dependent on SPR.
Experiment: Measure SPR activity and BH4 levels in patient cells with novel SPR variants to establish genotype-phenotype correlations for SRD. Currently ~60 SRD cases described. Understanding variant effects on enzyme function could improve diagnosis and treatment.
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