GAG, Proteoglycans, Glycoproteins

Question 1

Which GAG is not sulphated?

Answer 1

• Hyaluronic acid:
• • lubricant & shock absorder
    
    • loose CT & cartilage
    • synovial fluid
    • vitreous humour of eye
    • umbilical cord
    • not synthesized on Golgi
    • made on surface of PM
    • NOT sulphate
    • not covalently linked to protein
    • units of UDP-Glucoronate & UDP-Glucosamine

Question 2

How are GAG attached to proteins

Answer 2

via trisaccharide link (Xyl-Gal-Gal) to Ser & Thr except for hyaluronic acid highly -ve charged

Question 3

N-linked Glycoprotein linked to?

Answer 3

Asn

Question 4

N-linked oligosaccharides

Answer 4

• linked to Asn
• classified as:
  
  • complex oligosaccharides
  • high-mannose oligosaccharides
  • hybrid oligosaccharides
• constituents of proteins:
  
  • IgG & IgM
  • Ribonuclease B
  • Ovalnumin B
  • Peptide hormones
• Cleavage of terminal monosaccharide units from N-linked glycoproteins in blood targets them for degradation by liver

Question 5

O-linked glycoprotein

Answer 5

• Ser & Thr
• component of cell surface glycoproteins: ABO blood group determinants
• component of mucins: biological lubricants of GI & resp tracts

Question 6

Proteoglycan vs. Glycoprotein

Answer 6

Proteoglycan

• GAG boud to protein chains with extremely high CHO contents
• linked to protein chian by trisaccharide unit: Xyl-Gal-Gal
  
  • glycosidic bond formed between -OH Ser group and Xyl
• Keratan sulphate linked via Asn
• N-terminus of core protein associates non-covalently with hyaluronic acid molecule
• Depending on types of GAGs present, gels form with varying pore size and charge density which are involved in regulation of the passage of molecules through the matrix
• Hyaluronic acid involved directly in controlling cell migration through the jelly substance of the matrix, particularly during development and wound repair
• Low Proteoglycan = matrix is dense & rigid
• Mucopolysaccharodises: Hurlers & Hunters

Glycoproteins

• Mainly protein
• 8 sugar
• Glycoproteinoses: I-Cell Disease

Question 7

Which GAG is sulphated?

Answer 7

• Heparin:
  
  • sulphated
  • highest -ve charge
  • binds strongly to antithrombin III
  • unlike other GAGs that are extracellular compounds, heparin is an INTRACELL component of mast cells that line arteries, especially in liver, lungs, and skin
  • serves as an anticoagulant

Question 8

Chondroitin 4- and 6-sulfates

Answer 8

• disaccharide unit: N-acetylgalactosamine with sulfate on either C-4 or C-6, and glucoronic acid
• most abundant GAG in the body
• found in cartilage, tendons, ligaments, and aorta
• form proteoglycan aggregates, often aggregating non-covalently with hylauronic acid
• in cartilage, they bind collagen and hold fibers in a tight, strong network.

Question 9

Keratan sulfates I & II

Answer 9

• Disaccharide unit: N-acetylglucosamine and galactose (no uronic acid). Sulfate content is variable and may be present on C-5 of either sugar
• Most heterogenous glycosaminoglycans because they contain additonal monosaccharides such as L-fucose, N-acetylneuraminic acid, and mannose
• KS II is found in loose connective tissue proteoglycan aggregates with chondroitin sulfate, KS I is found in cornea

Question 10

Dermatan sulfate

Answer 10

• Disaccharide unit: N-acetylgalactosamine & L-Iduronic acid (with variable amounts of glucoronic acid)
• Found in skin, blood vessels, and heart valves

Question 11

Heparan sulfate

Answer 11

• disaccharide unit: same as haparin except some glucosamine are acetylated and there are fewer sulfate groups
• EXTRACELL GAG, found in BM and as a ubiquitous component of cell surfaces

Question 12

Glycoprotein structure & fx

Answer 12

• short oligosaccharide (glycan) chains (1-20 sugars) long covalently linked to their polypeptide backbone chains
  
  • carb chains often short & branched
• glycoproteins fx:
  
  • structural proteins
  • enzymes
  • transport proteins
  • Ig
  • membrane receptors
• Newly synthesized glycoprotein can go:
  
  • Extracell secretion
  • Plasma membranes
  • Lysosomes

Question 13

O-linked Glycoprotein synthesis

Answer 13

• synthesis at rER
• Glycosylation occurs post-translationally, utilising sugar nucleotides as precursors w/in ER lumen such as UDP-GalNAc & UDP-Gal
• involves Glycosyltransferases, a membrane-bound microsomal enzyme, which sequentially transfer sugar groups to growing chain
• Chain extension: Golgi apparatus mainly

Question 14

N-linked glycoprotein synthesis

Answer 14

• occurs in 2 stages:
  
  1. assembly of dolichol-linked oligosaccharide
  2. N-linked glycoprotein assembly
• synthesis of N-linked at rER & Golgi apparatus
• Polymerisation of sugar chain involves Dol-P (membrane-bound polyprenol lipid molecule), a substrate for Glycosyltransferase
• 1st added sugar group is phospho-GlcNAc from UDP-GlcNac forming Dol-P-P-GlcNAc.
• Dol-P-P oligo complex is then internalised into lumen
• Oligosaccharide is transferred from dolichol to an Asn resude of growing polypeptide chain
• Trimming of the carb chain begins as the protein moves thru the ER
• In Golgi, further trimming and/or addition of monosaccharides occurs