Keratan and chondroitin sulfate lysosomal degradation (Morquio / GM2); GALNS/GLB1/HEXA/HEXB

Keratan sulfate and the galactose/N-acetylhexosamine termini of chondroitin, dermatan and keratan sulfate are degraded in the lysosome by an ordered exolytic cascade that complements the heparan-sulfate pathway. Keratan sulfate is a repeating -Gal-GlcNAc- polymer with 6-O- sulfates on both sugars: N-acetylgalactosamine-6-sulfatase / galactose-6-sulfatase (GALNS) removes the 6-O-sulfate from a terminal galactose-6-sulfate (and, in chondroitin-6-sulfate, from GalNAc-6-sulfate), and GlcNAc-6-sulfatase (GNS, in the heparan-sulfate module) removes it from GlcNAc-6-sulfate; lysosomal acid beta-galactosidase (GLB1) then hydrolyses the exposed terminal beta-galactose; and lysosomal beta-hexosaminidase — the alpha-beta heterodimer hexosaminidase A (HEXA+HEXB) and the beta-beta homodimer hexosaminidase B (HEXB) — removes the terminal beta-N-acetylhexosamine, regenerating a substrate for the next round. These enzymes are broadly specific: GLB1 also degrades the GM1 ganglioside, and hexosaminidase A (with the GM2-activator) degrades the GM2 ganglioside, so the same genes underlie both mucopolysaccharide and glycosphingolipid storage diseases. Disorders: MPS IVA (GALNS, Morquio A), GM1 gangliosidosis / MPS IVB (GLB1, Morquio B), Tay-Sachs (HEXA) and Sandhoff (HEXB) GM2 gangliosidoses.

MODULE:keratan_chondroitin_sulfate_lysosomal_degradationDRAFTMetabolic Pathwaymodules/keratan_chondroitin_sulfate_lysosomal_degradation.yaml
keratan sulfate proteoglycan catabolic processGO:0042340 glycosaminoglycan metabolic processGO:0030203
GO:0042340
keratan sulfate proteoglycan catabolic process
The module is grounded in keratan sulfate proteoglycan catabolic process (GO:0042340); it covers the ordered lysosomal exolytic degradation of keratan (and chondroitin) sulfate termini.
GO:0030203
glycosaminoglycan metabolic process
The enzymes also act on chondroitin/dermatan sulfate termini (glycosaminoglycan metabolic process, GO:0030203) and on gangliosides.
Reactome:R-HSA-2024096
HS-GAG degradation
The keratan/chondroitin sulfate exolytic steps follow the human Reactome GAG-degradation reactions R-HSA-1630304 (GALNS), R-HSA-1630306 (GLB1) and R-HSA-1638053 (HEXA/HEXB).
file:human/GALNS/GALNS-ai-review.yaml
GALNS gene review (human)
The galactose-6-sulfate / GalNAc-6-sulfate desulfation step (UniProtKB:P34059, GO:0043890) matches the completed human GALNS review.
file:human/GLB1/GLB1-ai-review.yaml
GLB1 gene review (human)
The beta-galactosidase step (UniProtKB:P16278, GO:0004565) matches the completed human GLB1 review.
file:human/HEXA/HEXA-ai-review.yaml
HEXA gene review (human)
The beta-hexosaminidase alpha-subunit step (UniProtKB:P06865, GO:0004563) matches the completed human HEXA review.
file:human/HEXB/HEXB-ai-review.yaml
HEXB gene review (human)
The beta-hexosaminidase beta-subunit step (UniProtKB:P07686, GO:0004563) matches the completed human HEXB review.
4Nodes
3Parts
0Variant Sets
0Variants
4Annotons
2Connections

Derived QC

Recommended-field compliance

100.0% recommended fields populated

All recommended fields populated.

Module deep research

✗ none found

No MODULE:keratan_chondroitin_sulfate_lysosomal_degradation deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ every leaf node grounds to a representative protein.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Gene-review completeness (4/4 grounded genes reviewed)

4 complete review(s) · 0 with deep research · 0 missing review · 4 reviewed but lacking deep research

Gene Review Complete Deep research
GALNS P34059 ✓ ✓ ✗
GLB1 P16278 ✓ ✓ ✗
HEXA P06865 ✓ ✓ ✗
HEXB P07686 ✓ ✓ ✗

Details

Context
lysosomal lumenGO:0043202
Keratan / chondroitin sulfate lysosomal degradationMetabolic Pathwaykeratan_chondroitin_sulfate_lysosomal_degradation
keratan sulfate proteoglycan catabolic processGO:0042340 glycosaminoglycan metabolic processGO:0030203
Context
lysosomal lumenGO:0043202

Keratan/chondroitin sulfate lysosomal degradation, grounded to the human enzymes GALNS (UniProtKB:P34059, GO:0043890, EC 3.1.6.4), GLB1 (P16278, GO:0004565, EC 3.2.1.23) and the beta-hexosaminidase subunits HEXA (P06865) and HEXB (P07686) (both GO:0004563, EC 3.2.1.52; complex GO:1905379). GO molecular-function/BP/location terms were taken from the completed human gene reviews and verified against the local go.db; Reactome reaction ids/titles were verified against the local reactome cache. Each step uses a PANTHER family selector; the beta-hexosaminidase step is a PROTEIN_COMPLEX node (HEXA + HEXB, forming HexA alpha-beta and, for HEXB, HexB beta-beta). This is the sibling module to heparan-sulfate degradation: keratan sulfate is a -Gal-GlcNAc- polymer degraded by alternating desulfation (GALNS for Gal/GalNAc-6- sulfate, GNS for GlcNAc-6-sulfate — GNS reviewed in the heparan-sulfate module), beta- galactosidase (GLB1) and beta-hexosaminidase (HEXA/HEXB). The enzymes are broadly specific and also degrade gangliosides — GLB1 the GM1 ganglioside, hexosaminidase A the GM2 ganglioside — so the same genes cause both mucopolysaccharide (Morquio) and glycosphingolipid (GM1/GM2 gangliosidosis) storage diseases; GLB1/NEU1 act in a lysosomal multienzyme complex with CTSA. Disorders: MPS IVA (GALNS, Morquio A), GM1 gangliosidosis / MPS IVB (GLB1, Morquio B), Tay-Sachs (HEXA) and Sandhoff (HEXB). Chondroitin/dermatan sulfate degradation additionally uses ARSB (reviewed) and GUSB/IDS/IDUA (reviewed in the heparan-sulfate module).

Connections

galns_step -> glb1_step Provides Input For
GALNS desulfation of terminal galactose-6-sulfate exposes the beta-galactose for GLB1.
glb1_step -> hex_step Provides Input For
GLB1 removal of terminal galactose exposes the beta-N-acetylhexosamine for beta-hexosaminidase.
Part 1: galactose/GalNAc 6-desulfation
removal of 6-O-sulfate from terminal galactose-6-sulfate / GalNAc-6-sulfateReactiongalns_step

Annotons

GALNS: N-acetylgalactosamine-6-sulfatase / galactose-6-sulfatase
galns_activity
Participant: Family: Sulfatase family (GALNS)
Family:
Sulfatase family (GALNS)PANTHER:PTHR42693
Representative Members: GALNS (human)UniProtKB:P34059

Function

N-acetylgalactosamine-6-sulfatase activityGO:0043890
Substrates: keratan sulfate (terminal galactose-6-sulfate) chondroitin-6-sulfate (terminal GalNAc-6-sulfate)
Products: keratan/chondroitin sulfate (terminal galactose / GalNAc) sulfate

Locations

lysosomal lumenGO:0043202

Deficiency = MPS IVA (Morquio A syndrome).

Part 2: terminal galactose hydrolysis
hydrolysis of terminal beta-D-galactoseReactionglb1_step

Annotons

GLB1: lysosomal acid beta-galactosidase
glb1_activity
Participant: Family: Glycosyl hydrolase 35 family (GLB1)
Family:
Glycosyl hydrolase 35 family (GLB1)PANTHER:PTHR23421
Representative Members: GLB1 (human)UniProtKB:P16278

Function

beta-galactosidase activityGO:0004565
Substrates: keratan sulfate (terminal beta-galactose) GM1 ganglioside (terminal galactose)
Products: D-galactose shortened keratan sulfate chain / GM2 ganglioside

Locations

lysosomal lumenGO:0043202

Deficiency = GM1 gangliosidosis / MPS IVB (Morquio B).

Part 3: terminal N-acetylhexosamine hydrolysis (beta-hexosaminidase)
hydrolysis of terminal beta-N-acetylhexosamineProtein Complexhex_step

Annotons

HEXA: beta-hexosaminidase alpha subunit
hexa_activity
Participant: Family: Glycosyl hydrolase 20 / beta-hexosaminidase family (HEX)
Family:
Glycosyl hydrolase 20 / beta-hexosaminidase family (HEX)PANTHER:PTHR22600
Representative Members: HEXA (human, alpha subunit)UniProtKB:P06865

Function

beta-N-acetylhexosaminidase activityGO:0004563
Substrates: keratan sulfate (terminal beta-GlcNAc) GM2 ganglioside (terminal GalNAc, with GM2-activator)
Products: N-acetylhexosamine shortened chain / GM3 ganglioside

Locations

lysosomal lumenGO:0043202

Deficiency = Tay-Sachs disease (GM2 gangliosidosis type I).

HEXB: beta-hexosaminidase beta subunit
hexb_activity
Participant: Family: Glycosyl hydrolase 20 / beta-hexosaminidase family (HEX)
Family:
Glycosyl hydrolase 20 / beta-hexosaminidase family (HEX)PANTHER:PTHR22600
Representative Members: HEXB (human, beta subunit)UniProtKB:P07686

Function

beta-N-acetylhexosaminidase activityGO:0004563
Substrates: keratan/dermatan sulfate (terminal beta-N-acetylhexosamine) GM2/GA2 ganglioside
Products: N-acetylhexosamine shortened chain

Locations

lysosomal lumenGO:0043202

Deficiency = Sandhoff disease (GM2 gangliosidosis type II; affects both HexA and HexB).