HPR / HPR1 (At1g68010, Q9C9W5) curation notes

Deep research could not be run in this environment (no provider API keys). These notes are a
literature summary built only from cached publications (publications/PMID_*.md) and the UniProt
record (HPR-uniprot.txt). No *-deep-research-*.md file exists for this gene.

Identity and family

• HPR1 (UniProt gene name HPR) is the peroxisomal NADH-dependent hydroxypyruvate reductase (glycerate
  dehydrogenase, EC 1.1.1.29), a member of the D-isomer specific 2-hydroxyacid dehydrogenase family
  (PANTHER PTHR10996:SF257; Pfam PF00389/PF02826). It has a C-terminal microbody targeting signal
  PMID:9177031. Encoded by a single gene that, unlike in pumpkin,
  is not alternatively spliced into a cytosolic form PMID:9177031. UniProt nevertheless
  lists a second isoform differing by one residue (K264 -> KV), which does not affect the PTS1.

Activity

• Catalyses NADH-dependent reduction of hydroxypyruvate to glycerate PMID:22303256.
• Recombinant HPR1 was assayed alongside HPR2 with several substrate/cofactor combinations; both show
  relaxed specificity PMID:18952776. The cached text lacks the
  Table 1 values, so the glyoxylate and NADPH rates of HPR1 cannot be checked here.
• hpr1 knockouts retain only ~5% of leaf NADH-HPR activity PMID:18952776.

Localization

• Immuno-EM: exclusively peroxisomal in green leaves PMID:9177031;
  leaf peroxisome proteomics (PMID:12154131, PMID:17951448).
• Proteomic hits in chloroplast, mitochondrion, apoplast and crude cytosol fractions treated as
  contamination (projects/ARATH/organelle-audit.md rule 3).

Physiological role

• Photorespiration: HPR1 performs the main peroxisomal reduction of hydroxypyruvate; hpr1 single mutants
  are only mildly air-sensitive because cytosolic HPR2 (At1g79870) and plastid HPR3 (At1g12550) provide
  bypasses; hpr1 hpr2 doubles are air-sensitive [PMID:18952776 "The combined deletion of both HPR1 and
  HPR2, however, results in distinct air-sensitivity and a dramatic reduction in photosynthetic
  performance."; "showing that HPR1 plays the dominant role in photorespiration"]
  PMID:21205613.
• hpr1 increases the CO2 compensation point and non-respiratory CO2 release per oxygenation (PMID:21567290,
  abstract only).
• Seed germination: HPR1 reoxidizes peroxisomal NADH for beta-oxidation only when peroxisomal MDH is
  absent PMID:19812894. Conditional, so not added as
  a new annotation.
• Expression: shoot-specific, induced by light and mildly by drought PMID:19529821.

Annotation-specific notes

• glyoxylate reductase (NADPH) IBA (PTN000107947; donors HPR2, HPR3, E. coli GhrA/GhrB, human GRHPR):
  HPR1 is NADH-preferring; no cached data on HPR1 NADPH-glyoxylate activity -> UNDECIDED.
• cytosol: HDA (crude cytosolic fraction) and IBA (bacterial/cytosolic donors) on a PTS1 protein that
  immuno-EM places exclusively in peroxisomes. One action for the term group (organelle-audit rule 6):
  MARK_AS_OVER_ANNOTATED for both.
• poly(U) RNA binding IDA (PMID:17511964, abstract-only): poly(U)-affinity enrichment explicitly aimed at
  nucleotide-binding motifs; an NAD-binding Rossmann-fold dehydrogenase is expected to be retained.
  Marked over-annotated.

Module comparison (modules/photorespiration_c2_cycle.yaml, peroxisomal_hpr1_variant)

Module asserts GO:0008465 / GO:0009853 / GO:0005777 for HPR1. This review agrees on all three. The
review also accepts the more specific GO:0009854 (oxidative photosynthetic carbon pathway, part_of
photorespiration), which is supported by IMP.