PTR1 (Leishmania major, Q01782) - curation notes

Note: Automated deep research (Falcon/OpenAI) was unavailable in this environment.
These notes are a manual literature review based on the UniProt record, the cached
publications, and targeted PubMed/web searches. All PMIDs below were verified against
PubMed (esummary titles) before caching.

Identity

• Pteridine reductase 1, EC 1.5.1.33; synonyms HMTXR (H-region methotrexate resistance
  protein), LmjF.23.0270. 288 aa, short-chain dehydrogenase/reductase (SDR) family,
  homotetramer (UniProt).
• Originally identified as the amplified H-region gene conferring methotrexate (MTX)
  resistance PMID:1339441.
  That paper placed it in the aldo-keto reductase grouping; later kinetic/stereochemical
  work placed it firmly in the SDR family PMID:9521731.

Molecular function

• NADPH-dependent reduction of unconjugated pterins (biopterin -> H2B -> H4B) and folates:
  PMID:7972081
  PMID:7972081
• Two-step reduction mechanism, structurally resolved:
  PMID:11373620
  PMID:11373620
• Folate -> DHF -> THF (DHFR-like reaction, same stereochemistry):
  PMID:9521731
  PMID:9521731
  -> supports NEW GO:0004146 dihydrofolate reductase activity (GO definition: THF + NADP+ = DHF + NADPH).
  This is not in GOA for Q01782. PTR1 is the catalyst, so no participation concern.
• Tetramer: PMID:9153248.
• MTX insensitivity relative to DHFR-TS: PMID:9153248.

Quinonoid dihydropteridine reductase (QDPR) activity - conflicting evidence

• Bello 1994 reported no activity with qH2B (quote above, PMID:7972081; abstract only).
• Ong 2011 (full text) used an HPLC/spectrophotometric assay and found NADPH-dependent
  QDPR activity in both TbPTR1 and LmPTR1:
  PMID:21239486
  PMID:21239486
• So GO:0004155 6,7-dihydropteridine reductase activity (IDA, PMID:21239486) is supported, but in
  L. major the dedicated LmQDPR is ~20-fold more efficient, so this is a secondary activity
  for LmPTR1 (the dual role is more important in T. brucei, which lacks QDPR). Keep as non-core.
• The UniProt FUNCTION comment still carries the 1994 negative statement; newer data supersede it.

Biological process

• Pterin salvage: Leishmania are pterin auxotrophs; PTR1 is the primary salvage enzyme:
  PMID:7972081
  PMID:9153248
  -> GO:0006729 tetrahydrobiopterin biosynthetic process (IEA, UniPathway) is appropriate: the
  enzyme catalyses the formation of H4B from biopterin (salvage rather than de novo GTP route,
  which kinetoplastids lack PMID:21239486).
• Antifolate resistance: PTR1 amplification confers MTX resistance (bypass of DHFR-TS and/or drug
  sequestration). This is a pharmacological phenotype, not a normal biological process; I did not
  propose "response to methotrexate".
• Downstream phenotypes (metacyclogenesis, oxidative stress sensitivity of ptr1- mutants; glycosome
  and cytokinesis defects in T. brucei RNAi) are consequences of H4B depletion, not processes PTR1
  executes; no process terms proposed for these.
  PMID:20545846

Localization

• T. brucei PTR1 (ortholog, ISO donor Tb927.8.2210) fractionates with cytosol:
  PMID:20545846
• L. donovani PTR1-GFP is cytoplasmic:
  PMID:17275814
• No evidence for glycosomal localization (no PTS1; despite a database-level glycosome mention in
  LeishmaniaDB, which lacked protein-level evidence). Cytosol ISO accepted.
• L. donovani PTR1 is degraded in stationary phase, proposed proteasome-mediated (PMID:17275814),
  linking lower H4B with metacyclogenesis.

Annotation decisions summary

Protist-specific checks

• No animal/plant-only terms transferred. No organelle mismatches (cytosol is consistent with
  data). Not part of trypanothione or RNA editing pathways. No host-process terms. SDR family
  is huge; IBA/ISM rows only assert generic oxidoreductase, so no paralog over-annotation, just
  under-specificity.