Glycosylation Flashcards
What is the glycocalyx?
A carbohydrate rich layer on the surface of (primarily eukaryotic cells)
Where does initial glycosylation take place?
In the ER
Where do the glycocalyx carbohydrates originate from?
Adbsorbed membrane glycoproteins and proteoglycans
What is the difference between glycoproteins and proteoglycans?
- Glycoproteins have relatively few sugars bound whereas proteoglycans have many sugars bound
- Glycoproteins contain fewer shorter branched sugars than proteoglycans
Do glycoproteins or proteoglycans make up more of the glycocalyx?
Glycoproteins
What function do proteoglycans often serve?
- They form huge complexes bound to water and cations
- Form gels
- Shock absorbers
What do proteoglycans contain long unbranched molecules of?
Glucosaminoglycans (GAGs)
Where are proteoglycans found in large numbers?
- Connective tissue
- Extracellular matrix
- Vitreous humour
What is the most common proteoglycan in cartilage?
Aggrecan
What is the structure of aggrecan?
- Made from 2 types of protein and 3 types of GAGs
- Very long backbone contains core proteins
- As big as bacterial cell
Give an example of a GAG
Chondistin-4-sulfate
What is the structure of GAGs?
Unchbranched polysaccharaides made from repeating disaccharide subunits of modified aldose sugars
What enzyme activates their cognate nucleotide sugars?
Glycosyl transferase
What form are monosaccharides almost always in?
Ring form
How are monosaccharides activated?
In O-linked glycoproteins where are the sugars attached?
To the O of serine or threonine
What is the first sugar usually attached in O-linked glycoproteins?
N-acetylgalactosamine
In collagen what are O-linked sugars attached to?
A hydroxylysine
Why does lack of vitamin C cause scurvy?
- Vitamin C is a cofactor for lysyl hydroxylase
- No vit C means a lack of glycosylation on lysines
- This results in less stable collagen
In N-linked glyoproteins how are sugars attached?
Via the nitrogen of an asparagine
What is the initial sugar attached in N-linked glycoproteins?
N-acetylglucosamine
Where does glycosylation of proteins begin?
in the ER
What directs a protein to the ER translocator for glycosylation?
N-terminal signal sequences
How are N-linked oligosaccharides transferred onto the protein?
‘en bloc’
How is the 14-mer oligosaccharide transferred onto a growing polypeptide?
By an oligosaccharyl transferase associated with the translocator
What phospholipid is essential for N-linked glycosylation?
dolichol phosphate
Why is dolichol phosphate essential for N-linked glycosylation?
- Anchors the oligosaccharide to the inner leaflet of the ER membrane
- Cleavage of the phosphate bond by oligosaccharyl transferase provides energy to transfer 14-mer to the protein
What are the 2 types of N-linked oligosaccharide?
- Complex
- High mannose
Where does processing of N-linked oligosaccharides occur?
In the ER and Golgi
Why do N-linked oligosaccharides have a common core of sugars?
Because they all start as the same 14 mer trasnferred from dolichol
Are high mannose or complex oligosaccharides bigger?
Complex
In the ER are many resident proteins soluble or membrane bound?
Soluble
In the golgi are resident proteins membrane bound or soluble?
Membrane bound
What happens in oligosaccharide processing?
- Removal of glucoses
- Removal of mannoses
- build structure up again and activated sugars added on
What are the stuctures of all glycosidases and glycosyltransferases in the golgi?
Transmembrane enzymes arranged into large complexes
what are problems with studying the glycocalyx?
- Secondary gene products
- Difficult to distinguish similar monosaccharides
- Can only be seen with EM
What are the roles in glycocalyx?
- Cell morphology (used in microvilli for example)
- Regulation of membrane protein diffusion
- Cancer development
