GLB1 (human) — curation notes

UniProtKB: P16278 (BGAL_HUMAN). HGNC:4298. EC 3.2.1.23. Glycosyl hydrolase family 35 (CAZy GH35).

Core biology (grounded in UniProt + cached publications; falcon deep research unavailable, HTTP 402)

GLB1 encodes lysosomal acid beta-galactosidase (β-Gal), an exoglycosidase that
hydrolyses terminal non-reducing β-D-galactose residues from a broad range of
substrates.

• UniProt CATALYTIC ACTIVITY: "Hydrolysis of terminal non-reducing beta-D-galactose
  residues in beta-D-galactosides.; EC=3.2.1.23" [file:human/GLB1/GLB1-uniprot.txt].
• UniProt FUNCTION [Isoform 1]: "Cleaves beta-linked terminal galactosyl residues from
  gangliosides, glycoproteins, and glycosaminoglycans." [file:human/GLB1/GLB1-uniprot.txt].
• pH optimum 4.5–5.5 (acid hydrolase) [file:human/GLB1/GLB1-uniprot.txt].

Two disease-relevant catabolic roles:
1. Ganglioside/glycosphingolipid catabolism — removes the terminal galactose of GM1
ganglioside (→ GM2) and of its asialo derivative GA1 (→ GA2). Deficiency → lysosomal
accumulation of GM1 and GA1 PMID:31720227. Also acts (with sap-B) on lactosylceramide/GM1 in vitro
PMID:8200356.
2. Keratan sulfate / glycosaminoglycan and glycoprotein catabolism — GM1
gangliosidosis / Morquio B accumulate keratan sulfate and β-galactose-terminated
N-/O-linked glycans. GLB1 deficiency is a "broad oligosaccharidosis" PMID:31720227.

Structure / mechanism

• Crystal structure (homodimer): catalytic TIM barrel + β-domain 1 + β-domain 2; "β-Gal
  is an exoglycosidase that catalyzes the hydrolysis of terminal β-linked galactose
  residues" PMID:22128166. Active site: proton donor Glu188, nucleophile Glu268
  (UniProt ACT_SITE). Binds galactose product; homodimer PMID:22128166.

Lysosomal multienzyme complex

• GLB1 forms a lysosomal complex with protective protein/cathepsin A (PPCA/CTSA),
  α-neuraminidase (NEU1) and GALNS; PPCA stabilises β-Gal multimers
  PMID:15714521.
  Loss of PPCA (galactosialidosis) → combined β-Gal/NEU1 deficiency; PPCA restores
  high-MW β-Gal multimers PMID:3084261.

Localisation

• Lysosome (isoform 1). Synthesised as precursor in RER → Golgi → mature high-MW multimer
  in lysosome PMID:3084261. UniProt SUBCELLULAR LOCATION [Isoform 1]: Lysosome
  [PMID:2511208, PMID:3084261].

Isoform 2 (EBP / elastin-binding protein / S-Gal)

• Alternatively spliced, catalytically inactive (~67 kDa); functions in elastic-fibre
  assembly (elastogenesis) as a recycling chaperone for tropoelastin; localises to
  perinuclear cytoplasm / cell surface, NOT lysosome [UniProt FUNCTION [Isoform 2],
  SUBCELLULAR LOCATION [Isoform 2]; PMID:2511208]. The perinuclear-region (GO:0048471) and
  cytoplasm (GO:0005737) CC annotations pertain to this non-catalytic isoform.

Disease

• Allelic disorders, all autosomal recessive lysosomal storage diseases:
  GM1-gangliosidosis types 1/2/3 (MIM 230500/230600/230650) and Mucopolysaccharidosis
  type IVB / Morquio B (MIM 253010) [UniProt DISEASE].

Annotation-review judgement calls

• PMID:11927518 (atherosclerosis / endothelial senescence): measures
  senescence-associated β-gal (SA-β-gal) histochemical staining, a phenotypic readout
  of lysosomal β-Gal activity at pH 6, not a biochemical characterisation of the GLB1
  enzyme's substrate specificity. The IDA "beta-galactosidase activity", "carbohydrate
  metabolic process", and "cytoplasm" annotations from this paper are over-annotations /
  peripheral (SA-β-gal signal is lysosomal, not cytoplasmic). Kept but marked
  over-annotated; not core.
• protein binding (GO:0005515) IPIs: PMID:32296183 (HuRI binary interactome — GOLM1,
  SLC10A6, SLC30A2, SLC7A1; membrane transporters, likely non-physiological Y2H hits);
  PMID:15498789 (NGX6 paper — GLB1 not the subject). Bare protein binding, uninformative.
  Per policy: MARK_AS_OVER_ANNOTATED, not REMOVE.
• GO:0042803 protein homodimerization activity IPI PMID:22128166: real — the crystal
  structure shows a homodimer. ACCEPT (non-core).
• Exosome / granule / extracellular-region CC terms (HDA proteomics + Reactome
  neutrophil-degranulation): GLB1 is a soluble lysosomal enzyme detected in these
  proteomic/secretory contexts but its site of action is the lysosome. Kept as non-core.
• IEA response to cortisone (GO:0051413) and response to Thyroglobulin triiodothyronine
  (GO:1904016): rat-ortholog electronic transfers (GO_REF:0000107), no human evidence,
  not molecular function of the enzyme. Marked over-annotated.

2026-10-09 completion pass (alignment with dismech GM1 gangliosidosis entries)

• The three GO:0005515 protein binding IPI rows were re-actioned: PMID:32296183 (binary interactome) and PMID:15498789 (GOA interactor is ezrin, UniProtKB:P15311; the cached abstract describes NGX6 associating with ezrin, not GLB1) are REMOVE as uninformative; PMID:22128166 is MODIFY to GO:0042803 protein homodimerization activity, which GOA also carries from that paper.
• The SA-beta-gal paper (PMID:11927518) beta-galactosidase activity IDA is now ACCEPT for consistency with every other row on that term (SA-beta-gal is lysosomal beta-galactosidase activity); carbohydrate metabolic process IDA is KEEP_AS_NON_CORE, matching the IEA row.
• Core function 1 (GO:0004565 beta-galactosidase activity, directly involved in GO:0006689 ganglioside catabolic process, lysosomal lumen) matches the dismech GM1_Gangliosidosis_Type_1 and _Type_3 molecular nodes, which mark the same MF and BP DECREASED.
• GLB1 added as the GM1/lactosylceramide entry step of modules/glycosphingolipid_lysosomal_degradation.yaml (Reactome R-HSA-1605624, R-HSA-1606312).

2026-11-06: product-specific functional review

The long GLB1 product supplies lysosomal beta-galactosidase activity. The shorter S-Gal product has a distinct matrix-binding role; the same historical elastin-binding abbreviation also occurs in literature about the unrelated sterol-isomerase locus, so construct identity matters. PMID:2511208(https://pubmed.ncbi.nlm.nih.gov/2511208/), PMID:8383699(https://pubmed.ncbi.nlm.nih.gov/8383699/).

The review now separates recombinant self-association, cellular staining, granule cargo and urinary-vesicle detection from the principal catalytic function. The 647-residue alternative product remains represented without assigning its localization or activity from sequence length alone. The primary-evidence details and remaining access limits are in the manual synthesis.

All 62 incoming annotation contexts are retained, including the two source-distinct cDNA rows and four AND extension groups. One isoform-specific matrix-binding MF is proposed; no new biological process is inferred from necessity or cargo membership. The prior notes above, historical sessions and quote associations remain preserved.

2026-11-06: review follow-up

The follow-up harmonizes the two repeated-term decisions while preserving all incoming contexts. Direct publication support replaces the duplicated guide quotations. Cache availability remains distinct from separately read original material. The earlier notes and initial validation/status record above remain preserved; current status is recorded in the review and source guide.