Glycoproteins (N-glycans) Flashcards
O-glycanprotein
linked to a polypeptide chain through an O-glycosidic linkage
Most common: alpha-glycosidic linkage between
N-acetylgalactosamine and serine/threonine on a peptide chain
N-linked glycoprotein
Beta-N-glycosidic linkage between
N-acetylglucosamine and Asn
Two types of N-linked glycosylation
High mannose
- only N-acetylglucosamine and Mannose
Complex type
- Gal and sialic acid
- N-acetylglucosamine and mannose
High Mannose sugar chain characteristics
In order…
Asn residue
GlcNAc (beta linkage)
GlcNAc (“)
Mannose (“)
…several peripheral mannose residues
Complex type (acidic) sugar chain characteristics
N-acetylglucosamine + galactose + sialic acid
At the peripheral positions of the sugar chain
What consensus sequence is the Asn residue always next to?
-X-Ser/Thr-
Where does assembly and processing of N-glycans take place
ER and golgi
3 general steps of making a N-glycan
- Assembly of a high mannose type sugar chain on the lipid carrier DOLICHOL PHOSPHATE
- Transfer of the high mannose chain from DP —> Asn residue of a nascent peptide chain
- Processing of the high mannose —> complex chain
Where is the Dolichol phosphate located
Embedded in the ER membrane
More detail of the first step of N-gylcan synthesis
(Making high mannose type)
Add residues step-wise
Each step needs a glycosyltransferase and a glycosyl donor
The addition of 3 glucose residues at the end of one of the peripheral Mannoses —> signals the chain is complete
Now it can be transferred to the Asn residue
Processing the high mannose —> complex
ER and golgi
***NEED glycosidases to remove the glucose and mannose residues from the peripheral positions of the high mannose sugar
In tandem…need glycosyltransferases with proper sugar donors to add
N-acetylglucoamine, Gal, and Sialic acid