ESSENTIALS OF GLYCOBIOLOGY

LECTURE 14DEGRADATION AND TURNOVER OF GLYCOCONJUGATES

Hud Freeze

USUAL TURNOVER

• Most glycans are extracellular or on cell surface

• Membrane recycling

• Receptor and non-receptor mediated endocytosis

ToEndosomeLysosome

Lysosomal exoglycosidases degrade glycans at low pHSpecific lysosomal transporters carry neutral hexose, acetylatedaminohexose (GlcNAc, GalNAc), and Anionic sugars (GlcA, Sia) to the “cytosol”.

SUGAR CHAIN DEGRADATION ENZYMES

Most Are:Lysosome/EndosomeLow pH optimum, Sugar/anomeric specificityExo-glycosidasesTargeted to lysosome through P-lectins and Man-6-P

But Some Are:Non-lysosomalActive near neutral pHEndoglycosidasesTargeted as membrane bound moleculesNot in the lysosome

Special Features for Degradation of

Different Glycoconjugates

TYPE FEATURE

GlycoproteinsN-linked ER/Golgi/Cytoplasm/ LysosomeO-linked Unexpected Products

Proteoglycans EndoglycosidasesUnique ORDERNon-glycohydrolase enzymes

Glycosphingolipids Assisting proteins

Special Problems for N-Linked Sugar Chains

• N-glycosylation occurs in ER-Topology for lysosomal degradation is wrong

• ~50% of ER proteins misfold and are degraded - what happens to the sugar chain? To glycopeptides?

• Protein synthetic rate and glycosylation rate must be coordinated

• Competition for lectin-based chaperones

lots of Man( )Released

What happens to the released mannose?

OLIGOSACCHARIDE HOUSE-KEEPING CENTRAL

ER

Golgi

Lysosomecytosol

MANNOSE METABOLISM IN CELLS AND MORE

Cells also produce mannoseFrom glucoseGlc Glc-6-P Fru-6-P Man-6-P

Glycans

Mannose in plasma comes fromOligosaccharide turnover in cells

Lysosomal degradation of N-linked oligosaccharides

Lysosomal degradation of N-linked oligosaccharides

Enzymatic defects are usually found by accumulation of Partially degraded oligosaccharides in urine

O-LINKED OLIGOSACCHARIDE DEGRADATION

Same enzymes used for N-linked oligosaccharide degradation

-GalNAc’ase deficiency--produce GalNAc terminatedOligosaccharides?

Excretion of GalNAcSer/Thr?

No!!Why Not?

The oligosaccharides are larger size!

How to explain this?

Ser/Thr

Ser/Thr

Partially degraded polysaccharides accumulate in tissuesand urine.Structural analysis of glycans used to work out pathway

Hyaluronan degradation

HEPARAN SULFATE DEGRADATION

HEPARAN SULFATE DEGRADATION

CHONDROITIN SULFATE DEGRADATION

GSL DegradationNeeds Assistants

LIFE CYCLE OF GM2 ACTIVATOR PROTEIN

Model for the degradation of membrane-bound GlcCer by glucocerebrosidase and SAP-C and Cer by acid ceramidase and SAP-D, respectively. Besides the interaction of lysosomal enzyme and activator protein, the model emphasizes binding of activator protein and lysosomal enzymes to the vesicular surface containing BMP.

REMEMBER THAT

•Different types of glycans have unique degradation pathways•Mutations in different degradative enzymes lead to rare diseases •Limiting glycan synthesis in genetic disorders reduces pathology•Tissue specific differences in salvage/de novo biosynthesis may

be important for health and lead to pathology