Sepiapterin Reductase (SPR) is the final enzyme in the de novo biosynthesis of tetrahydrobiopterin (BH4), the essential cofactor for aromatic amino acid hydroxylases and nitric oxide synthases [1]. SPR catalyzes the two-step NADPH-dependent reduction of 6-pyruvoyl tetrahydropterin (PTPS product) through intermediate 7,8-dihydropterin to BH4. SPR deficiency (SRD) is a rare autosomal recessive neurometabolic disorder that presents with dopa-responsive dystonia, and common variants in the SPR gene have been associated with altered Parkinson's disease risk. SPR also plays a role in the regulation of monoamine neurotransmitter synthesis, making it relevant to understanding dopaminergic and serotonergic pathways in neurodegeneration.
| Sepiapterin Reductase Protein | |
|---|---|
| Protein Name | Sepiapterin Reductase (SPR) |
| Gene | SPR |
| UniProt ID | [P35270](https://www.uniprot.org/uniprot/P35270) |
| PDB IDs | 1QPD, 2NBQ, 1ZTR |
| Molecular Weight | 27.8 kDa |
| Subcellular Localization | Cytosol |
| Protein Family | Short-chain dehydrogenase/reductase (SDR) family |
| EC Number | 1.5.1.34 |
SPR is a 261-amino acid enzyme belonging to the short-chain dehydrogenase/reductase (SDR) superfamily [2]:
SPR catalyzes two sequential NADPH-dependent reductions:
The two reductions use one NADPH per step, consuming a total of 2 NADPH per BH4 molecule synthesized. The reaction proceeds via a ping-pong bi-bi mechanism.
SPR catalyzes the final step in the BH4 de novo synthesis pathway [3]:
GTP → GCH1 (GTP cyclohydrolase I) → 7,8-dihydroneopterin triphosphate
→ PTS (6-pyruvoyltetrahydropterin synthase) → 6-pyruvoyl tetrahydropterin
→ PCBD1 (pterin-4-alpha-carbinolamine dehydratase) → 6,7,8-trihydropterin
→ SPR → BH4 (tetrahydrobiopterin)
SPR is the rate-limiting step only under certain metabolic conditions, as the pathway can be regulated at the GCH1 step.
BH4 serves as an essential cofactor for:
Tyrosine hydroxylase (TH): Rate-limiting enzyme in dopamine and norepinephrine synthesis. BH4 is absolutely required for TH activity — without BH4, dopamine synthesis is severely impaired.
Tryptophan hydroxylase (TPH1, TPH2): Rate-limiting enzyme in serotonin (5-HT) synthesis in gut enterochromaffin cells (TPH1) and neurons (TPH2).
Phenylalanine hydroxylase (PAH): Catalyzes phenylalanine to tyrosine conversion in liver. BH4 deficiency leads to phenylketonuria (PKU) if PAH is also affected.
Nitric oxide synthases (NOS1, NOS2, NOS3): BH4 is an essential cofactor for all three NOS isoforms. BH4 availability modulates NO production in neurons (NOS1/nNOS) and endothelium (NOS3/eNOS).
SRD (OMIM #612716) is a rare autosomal recessive neurometabolic disorder caused by biallelic mutations in SPR [@thny2002; @friedman2011]:
Clinical presentation:
Biochemical diagnosis:
Genetics:
Treatment:
| Treatment | Typical Dose | Purpose |
|---|---|---|
| L-DOPA/Carbidopa | 100-500 mg/day | Replace dopamine deficiency |
| 5-Hydroxytryptophan (5-HTP) | 50-300 mg/day | Restore serotonin synthesis |
| BH4 supplementation | 1-5 mg/kg/day | Cofactor replacement (limited CNS penetration) |
| Folate supplementation | Variable | Support remethylation pathways |
L-DOPA is highly effective in SRD — dramatic and sustained response is a diagnostic hallmark [4].
SPR variants influence PD risk through BH4-dependent pathways [@nsengimana2022]:
| Partner | Interaction Type | Functional Consequence |
|---|---|---|
| GCH1 | Sequential pathway | Upstream enzyme providing substrate |
| PTS | Sequential pathway | Produces PTPS, upstream of SPR |
| PCBD1 | Sequential pathway | Produces 6,7,8-trihydropterin substrate |
| PTPS | Sequential pathway | Alternative pathway for BH4 salvage |
| NADPH | Cofactor binding | Provides reducing equivalents |
| TH (tyrosine hydroxylase) | Downstream enzyme | BH4 cofactor for dopamine synthesis |
| TPH2 (tryptophan hydroxylase) | Downstream enzyme | BH4 cofactor for serotonin synthesis |
| NOS1 (nNOS) | Downstream enzyme | BH4 cofactor for NO synthesis |
Blau N, et al. Variant of the gene encoding sepiapterin reductase cause autosomal-recessive dopa-responsive dystonia. Nat Genet. 2003. ↩︎
Tachida Y, et al. Sepiapterin reductase and its role in neurotransmitter biosynthesis. Neurosci Res. 2020. ↩︎
Clarke PA, et al. BH4 pathway and neurotransmitter synthesis in neurodegeneration. Curr Opin Chem Biol. 2019. ↩︎
Zürger N, et al. Sepiapterin reductase deficiency: long-term outcomes and treatment response. Orphanet J Rare Dis. 2022. ↩︎