ESSENTIALS OF GLYCOBIOLOGY LECTURE 14 DEGRADATION AND TURNOVER OF GLYCOCONJUGATES Hud Freeze

USUAL TURNOVER Most glycans are extracellular or on cell surface Membrane recycling Receptor and non-receptor mediated endocytosis To Endosome Lysosome Lysosomal exoglycosidases degrade glycans at low pH Specific lysosomal transporters carry neutral hexose, acetylatedaminohexose (GlcNAc, GalNAc), and Anionic sugars (GlcA, Sia) to the cytosol.

SUGAR CHAIN DEGRADATION ENZYMES Most Are: Lysosome/Endosome Low pH optimum, Sugar/anomeric specificity Exo-glycosidases Targeted to lysosome through P-lectins and Man-6-P But Some Are: Non-lysosomal Active near neutral pH Endoglycosidases Targeted as membrane bound molecules Not in the lysosome

Special Features for Degradation of Different Glycoconjugates TYPEFEATURE Glycoproteins N-linkedER/Golgi/Cytoplasm/ Lysosome O-linkedUnexpected Products ProteoglycansEndoglycosidases Unique ORDER Non-glycohydrolase enzymes GlycosphingolipidsAssisting 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 mannose From glucose Glc Glc-6-P Fru-6-P Man-6-P Glycans Mannose in plasma comes from Oligosaccharide turnover in cells

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 -GalNAcase deficiency--produce GalNAc terminated Oligosaccharides? Excretion of GalNAc Ser/Thr? No!! Why Not? The oligosaccharides are larger size! How to explain this? Ser/Thr

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

Hyaluronan degradation

HEPARAN SULFATE DEGRADATION

CHONDROITIN SULFATE DEGRADATION

GSL Degradation Needs Assistants

LIFE CYCLE OF G M2 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