Block B Lecture 4 - Protein Glycosylation Flashcards
What is glycosylation?
The process by which sugar molecules (glycans) are covalently attached to proteins or lipids.
(Slide 3)
What are “glycans”?
sugar chains that are attached to proteins or lipids
(Slide 3)
What does a protein plus a glycan result in?
A glycoprotein
(Slide 3)
What proteins does glycosylation mainly occur?
On membrane-associated or secreted proteins
(Slide 3)
What are 4 functions of glycosylation?
Answers Include:
Helps to maintain the structure and stability of proteins
Helps traffic the protein to the correct part of the cell
Forms part of the extracellular matrix
Helps proteins bind to the extracellular matrix
Aids in receptor signalling
Prevents proteins from self-binding
(Slide 6)
How is it determined if N-linked or O-linked glycosylation occurs?
It is based on which amino acid in the polypeptide is the anchor point for the sugar, asparagine for N-linked and either a serine or a threonine for O-linked
(Slide 9)
What are the amino acids motifs for both N-linked and O-linked glycosylation?
For N-linked it is N-X-S/T, where N is asparagine, X is any amino acid and then this is followed by either a serine (S) or threonine (T), the sugar is actually attached to the asparagine.
For O-linked it is just S/T, so a serine (S) or a threonine (T)
(Slide 9)
Where and when does N-linked glycosylation occur?
In the endoplasmic reticulum, as proteins are produced
(Slide 10)
What are the 5 steps of N-linked glycosylation?
- Synthesis of basic block of sugars on the lipid carrier (dolichol phosphate) on the cytoplasmic side of the ER membrane
- This is flipped to the lumen side of the ER membrane
- Further sugar modification occurs
- Transfer to polypeptide chain
- Further sugar modification occurs
(Slide 13)
What is the difference between the lumen and cytoplasmic side of the ER membrane?
The lumen side faces inwards, towards the ER, whereas the cytoplasmic side faces towards the cytoplasm
(Slide 13)
What is dolichol phosphate?
A lipid carrier molecule found in the endoplasmic reticulum (ER) membrane. It plays a crucial role in N-linked glycosylation, helping to assemble and transfer glycans onto newly synthesized proteins.
(Slide 14)
Where are sugars attached to dolichol phosphate?
To the phosphate group
(Slide 14)
What is the first sugar which is added in an N-linked glcyan?
N-acetyl glucosamine (GlcNAc)
(Slide 15)
How is N-acetyl glucosamine (GlcNAc) added to dolichol phosphate?
UDP-GlcNAc is transferred to dolichol phosphate by a GlcNAc transferase enzyme
(Slide 15)
Why is UDP involved in the addition of GlcNAc to dolichol phosphate?
As it is an activated carrier of GlcNAc
(Slide 15)
What sugar residues are added after GlcNAc is added to dolichol phosphate?
Another GlcNAc, then mannose molecules followed by glucose molecules
(Slide 16)
At what point is the dolichol-glycan flipped from the cytoplasmic side of the ER membrane to the lumen side?
After the second mannose molecules are added
(Slide 16)
What is responsible for transferring sugar units from the dolichol phosphate to the target protein?
Oligosaccharyltransferase (OST) complex
(Slide 17)
What occurs after the sugar units are transferred from the dolichol phosphate carrier to the protein? Give some examples.
Further sugar processing occurs. This can involve the removal of some monosaccharides which have previously been added, and / or the addition of further, different monosaccharides
(Slide 17)
What does congenital mean?
Present from birth
(Slide 20)
What are congenital disorders of glycosylation caused by?
Mutations in the genes which code for enzymes involved in the glycosylation process
(Slide 20)
What kind of disease are most congenital glycosylation disorders?
Autosomal recessive
(Slide 20)
What is an autosomal disease?
A genetic disorder that’s caused by a mutation on one of the body’s 22 non-sex chromosomes.
(Slide 20)
What are the 2 types of congenital glycosylation disorders?
CDG type I - mutations in genes acting BEFORE the glycan is transferred to the protein
CDG type II - mutations in genes acting AFTER the glycan is transferred to the protein
(Slide 20)
What is the most commonly diagnosed CDG?
A phosphomannomutase 2 (PMM2) deficiency
(Slide 21)
What are 3 examples of symptoms of a phosphomannomutase 2 (PMM2) deficiency?
Answers include:
Seizures
Abnormal fat distribution
Crossed-eyes
Liver problems
Scoliosis
Low muscle tone
idk if this counts but 20% of patients won’t survive past age 4
(Slide 21)
Where does O-linked glycosylation occur?
In the Golgi apparatus
(Slide 22)
What is the first step of O-linked glycosylation?
Same as N-linked glycosylation - Addition of N-acetylgalactosamine (GalNAc)
(Slide 22)
What is a property of O-glycans?
They hydrate very easily
(Slide 22)
What is a hydration reaction?
A chemical reaction that occurs when a substance combines with water
(Slide 22)
Where are O-glycans present?
On lubricating glycoproteins (such as mucins)
(Slide 22)
What is the typical “core” of an O-glycan?
Gal(galactose)-GalNAc(N-acetylgalactosamine)-Ser/Thr
(Slide 24)
What are proteoglycans?
They are a subclass of O-linked glycoproteins which exist in the extracellular matrix (ECM)
(Slide 26)
What are proteoglycans composed of?
A core protein and 1 or more glycosaminoglycan (GAG) chains
(Slide 26)
What are glycosaminoglycan (GAG) chains?
Long, negatively charged polysaccharide chains which are composed of repeating disaccharide units
(Slide 26)
Where are proteoglycans found?
In joints (as they form interstitial “gel”), and also in the brain and connective tissue
(Slide 26)
What function do proteoglycans have?
They have roles in inflammation, differentiation and wound healing
(Slide 26)
What is the first sugar attached in O-glycosylation, and where is it attached?
Xylose is the first sugar attached, and it is attached to the Ser/Thr residue of the core protein
(Slide 26)
