PYCR2

PYCR2 is a mitochondrial pyrroline-5-carboxylate (P5C) reductase that catalyzes the NAD(P)H-dependent reduction of L-Δ1-pyrroline-5-carboxylate to L-proline, functioning as an obligate oligomer in the final step of proline biosynthesis (PMID:33771508, PMID:33734376). The enzyme follows a sequential ordered kinetic mechanism in which L-P5C binds before NAD(P)H and NAD(P)+ is released before L-proline; disease-associated mutations (R119C, R251C) severely impair catalytic efficiency, protein stability, and multimerization (PMID:33771508, PMID:27130255). Loss of PYCR2 reduces mitochondrial membrane potential, increases apoptotic susceptibility, disrupts mitochondrial dynamics in oligodendroglia, and causes compensatory upregulation of SHMT2-driven glycine synthesis that underlies neurodegeneration in a mouse model of hypomyelinating leukodystrophy 10 (HLD10) (PMID:25865492, PMID:32330411, PMID:36548190). PYCR2 protein turnover is regulated by E4B-mediated K48-linked polyubiquitination targeting it for proteasomal degradation, counterbalanced by PFDN2-mediated stabilization (PMID:35669517, PMID:41656306).

1. 2015 High

   Establishing that PYCR2 is a mitochondrial enzyme whose loss-of-function causes human hypomyelinating leukodystrophy answered whether PYCR2 has a non-redundant role in the nervous system despite overlap with PYCR1.

   Evidence CRISPR-Cas9 knockout in HEK293FT cells, mitochondrial membrane potential and apoptosis assays, zebrafish morpholino rescue with patient variant cDNAs

   PMID:25865492

   Open questions at the time

   • Mechanism linking proline metabolism to myelination was unknown
   • Whether PYCR2 disease variants retained any residual enzymatic activity was not measured
2. 2016 Medium

   Demonstrating that PYCR1/PYCR2 physically associate with RRM2B and are required for its antioxidant function linked proline metabolism to the oxidative stress response beyond simple amino acid biosynthesis.

   Evidence Flag-tag purification of RRM2B complexes with mass spectrometry identification, shRNA knockdown with oxidative stress assays

   PMID:26733354

   Open questions at the time

   • Direct biochemical mechanism by which PYCR2 supports RRM2B antioxidant activity was not defined
   • Whether the interaction is direct or bridged through a complex was not resolved
3. 2016 Medium

   Showing that disease mutations disrupt PYCR2 multimerization established that oligomeric assembly is essential for function, not merely for stability.

   Evidence Multimerization assays on patient-derived nonsense and missense PYCR2 variants

   PMID:27130255

   Open questions at the time

   • Oligomeric state (dimer, decamer) and stoichiometry were not structurally resolved at this stage
   • Whether impaired multimerization affects substrate binding, cofactor binding, or both was unknown
4. 2020 High

   The crystal structure of PYCR2 revealed the dimer interface where the G249V disease mutation resides, and genetic epistasis with SHMT2 identified excess glycine as the neurotoxic effector downstream of PYCR2 loss, resolving how a proline biosynthetic enzyme causes neurodegeneration.

   Evidence X-ray crystallography of apo-PYCR2, Pycr2 knockout mouse with in situ neurotransmitter quantification, SHMT2 knockdown rescue of neuronal morphology

   PMID:32330411

   Open questions at the time

   • Whether glycine excess acts through NMDA receptor excitotoxicity or another mechanism was not resolved
   • Structural basis for cofactor and substrate binding was not captured (apo structure only)
5. 2021 High

   Quantitative kinetic characterization of PYCR2 defined its sequential ordered mechanism and showed that disease mutations impair catalysis by 7- to 366-fold, providing the first enzymological framework for genotype–phenotype correlation.

   Evidence In vitro steady-state kinetics with wild-type and R119C/R251C variants, thermostability assays, circular dichroism spectroscopy

   PMID:33771508

   Open questions at the time

   • Whether cofactor preference (NADH vs NADPH) differs in vivo by tissue type was not addressed
   • No structure of the holo enzyme with substrate/cofactor bound
6. 2021 High

   Yeast complementation and mouse double-mutant analysis confirmed that PYCR2 is a bona fide P5C reductase largely redundant with PYCR1 for systemic proline synthesis, while double mutants are sub-viable, clarifying why single knockouts produce tissue-specific rather than lethal phenotypes.

   Evidence Complementation of yeast Pro3 deletion, Pycr1/Pycr2 double-mutant mice, proline-free diet experiments

   PMID:33734376

   Open questions at the time

   • Tissue-specific expression differences that explain distinct disease phenotypes of PYCR1 vs PYCR2 loss were not fully resolved
   • Whether PYCR3 (PYCRL) provides residual cytosolic activity in double mutants was not tested
7. 2022 Medium

   Identifying E4B as the E3 ubiquitin ligase that targets PYCR2 for K48-linked proteasomal degradation established the first post-translational regulatory mechanism controlling PYCR2 protein levels.

   Evidence In vitro ubiquitination assay, co-immunoprecipitation in HEK293 cells, K48-linkage-specific ubiquitin analysis

   PMID:35669517

   Open questions at the time

   • Physiological signals triggering E4B-mediated PYCR2 degradation were not identified
   • Whether ubiquitination occurs on mitochondrial or cytosolic PYCR2 pools was not determined
8. 2022 Medium

   Disease mutations in PYCR2 cause mitochondrial hyperfusion in oligodendroglial cells and block differentiation, linking the enzyme's loss to a cell-type-specific organellar defect relevant to hypomyelination.

   Evidence Expression of R119C and R251C mutants in FBD-102b oligodendroglial cells, mitochondrial morphology and fusion/fission assays, differentiation markers

   PMID:36548190

   Open questions at the time

   • Whether mitochondrial fusion defect is a direct consequence of reduced proline or of impaired redox balance was not distinguished
   • Findings from a single oligodendroglial cell line; primary oligodendrocyte validation lacking
9. 2026 Medium

   PFDN2 was identified as a stabilizer of PYCR2 protein by protecting it from proteasomal degradation, providing a counterbalance to E4B-mediated ubiquitination and connecting PYCR2 stability to the prefoldin chaperone system.

   Evidence Reciprocal co-immunoprecipitation, immunofluorescence colocalization, cycloheximide chase with MG132 rescue in colorectal cancer cells and xenografts

   PMID:41656306

   Open questions at the time

   • Whether PFDN2 competes with E4B for PYCR2 binding or acts on distinct protein pools is unknown
   • Relevance of PFDN2-PYCR2 axis outside cancer contexts not tested

• A holo crystal structure of PYCR2 with substrate and cofactor bound, the precise mechanism linking proline/glycine imbalance to oligodendrocyte dysfunction, and the physiological signals regulating PYCR2 protein turnover remain unresolved.
• No holo structure with substrate and NAD(P)H cofactor
• Mechanism by which glycine excess or proline deficit causes oligodendrocyte-specific vulnerability is not established
• Upstream signals controlling E4B and PFDN2 regulation of PYCR2 are undefined