Lecture 9 Glycoproteins/Carbs Flashcards
Glycoproteins
Proteins that contain short glycan chains Usually 3 to 15 sugars (up to 20) Oligosaccharides are highly branched Do not have a repeating unit Contain much smaller amounts of sugar than protein
- usually 10 – 15% carbohydrate by weight
Most:
cell surface proteins secreted proteins ER proteins golgi proteins
are glycoproteins
carbohydrates on glycoproteins function
Assist in protein folding to correct conformation Enhance protein solubility Stabilize the protein against denaturation Protect the protein from proteolytic degradation Target the protein to specific subcellular locations Serve as recognition signals for carbohydrate binding proteins (lectins)
Oligosaccharides on glycoproteins usually contain
Amino sugars
N-acetylglucosamine N-acetlygalactosamine
Neutral sugars galactose mannose fucose
Acidic sugar
sialic acid (N-acetylneuraminic acid)
Proteoglycans
Contain as much as 50 – 60% sugars Sugar chains are long, unbranched polymers Chains may contain hundreds of monosaccharides Usually have a repeating disaccharide unit.
All N-linked glycoproteins have
branched oligosaccharide chains with a common core of 3 mannose and 2 GlcNAc residues
The high-mannose type is a precursor for other types
N-linked
A glycoprotein may have a single N-linked oligosaccharide or it may have several. These oligosaccarides may have all identical structure or different. Multiple different structures may be found at the same site on glycoprotein due to processing.
O-linked
Number of O-linked structure can vary. Some proteins have both N-linked and O-linked.
N-linked vs O-linked w/regards to LDL
N-linked oligos are not involved in LDL binding
N-linked oligos help protein fold properly in the ER O-linked oligos help keep protein in its native state
All sugar interconversions involve
sugar phosphates or sugar nucleotides…
Normal cells can use glucose to make all other sugars needed
Mucins
Secreted by epithelial cells lining the respiratory, gastrointestinal and genitourinary tracts
Very large molecules – mole wt over 1,000,000 As much as 80% of weight is carbohydrate About 1/3 of aa’s are serine or theonine – most are substituted with O-linked oligosaccharides
Mucins charge
Negative charges on sialic acid repel each other and prevent protein from folding
Mucins surface
The protein solution is very viscous –
forming a protective barrier on the epithelial surface providing lubrication between surfaces facilitating transport processes
Some oligosaccharides participate in interaction and binding to various bacterial cell surfaces
Endothelial cell surface
Lectins on surface recognize carbohydrate signals on leukocytes.
N-linked oligosaccharide assembly
begins in the endoplasmic reticulum
N-linked oligo processing continues
in the golgi
Processing of O-linked oligosaccharides also occurs in the golgi
Oligosaccharides
Made directly on polypeptide
Functions of oligosaccharides
N-linked oligos have an important role in protein folding High-mannose oligos target some proteins to specific sites in the cell Increase the solubility and stability of proteins Involved in chemical recognition interactions
Bacterial capsule
Blocks phagocytosis Aid in bacterial adhesion to host tissues Prevent drying out of bacteria.
Different bacteria have different repeating units
- often 4 monosaccharide units
Many, but not all, oral streptococci synthesise extracellular polysaccharides (EPS) by
polymerising the fructose or glucose moiety of sucrose.
Fructose polymers are either
β-2,1 linked like the storage compound inulin which is found in some vegetables or β-2,6 linked.
Some β-2,1 polymers are
cross linked with β-2,6 bonds and some β-2,6 polymers are cross-linked with β-2,6 bonds.
Some β-2,6 polymers are
highly branched with short chains.
Fructose polymers function as
energy storage and turn over rapidly in plaque.
