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MCB > Glycosylation > Flashcards

Glycosylation Flashcards

1
Q

What is the glycocalyx?

A

A carbohydrate rich layer on the surface of (primarily eukaryotic cells)

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2
Q

Where does initial glycosylation take place?

A

In the ER

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3
Q

Where do the glycocalyx carbohydrates originate from?

A

Adbsorbed membrane glycoproteins and proteoglycans

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4
Q

What is the difference between glycoproteins and proteoglycans?

A
  • Glycoproteins have relatively few sugars bound whereas proteoglycans have many sugars bound
  • Glycoproteins contain fewer shorter branched sugars than proteoglycans
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5
Q

Do glycoproteins or proteoglycans make up more of the glycocalyx?

A

Glycoproteins

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6
Q

What function do proteoglycans often serve?

A
  • They form huge complexes bound to water and cations
  • Form gels
  • Shock absorbers
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7
Q

What do proteoglycans contain long unbranched molecules of?

A

Glucosaminoglycans (GAGs)

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8
Q

Where are proteoglycans found in large numbers?

A
  • Connective tissue
  • Extracellular matrix
  • Vitreous humour
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9
Q

What is the most common proteoglycan in cartilage?

A

Aggrecan

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10
Q

What is the structure of aggrecan?

A
  • Made from 2 types of protein and 3 types of GAGs
  • Very long backbone contains core proteins
  • As big as bacterial cell
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11
Q

Give an example of a GAG

A

Chondistin-4-sulfate

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12
Q

What is the structure of GAGs?

A

Unchbranched polysaccharaides made from repeating disaccharide subunits of modified aldose sugars

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13
Q

What enzyme activates their cognate nucleotide sugars?

A

Glycosyl transferase

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14
Q

What form are monosaccharides almost always in?

A

Ring form

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15
Q

How are monosaccharides activated?

A
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16
Q

In O-linked glycoproteins where are the sugars attached?

A

To the O of serine or threonine

17
Q

What is the first sugar usually attached in O-linked glycoproteins?

A

N-acetylgalactosamine

18
Q

In collagen what are O-linked sugars attached to?

A

A hydroxylysine

19
Q

Why does lack of vitamin C cause scurvy?

A
  • Vitamin C is a cofactor for lysyl hydroxylase
  • No vit C means a lack of glycosylation on lysines
  • This results in less stable collagen
20
Q

In N-linked glyoproteins how are sugars attached?

A

Via the nitrogen of an asparagine

21
Q

What is the initial sugar attached in N-linked glycoproteins?

A

N-acetylglucosamine

22
Q

Where does glycosylation of proteins begin?

A

in the ER

23
Q

What directs a protein to the ER translocator for glycosylation?

A

N-terminal signal sequences

24
Q

How are N-linked oligosaccharides transferred onto the protein?

A

‘en bloc’

25
Q

How is the 14-mer oligosaccharide transferred onto a growing polypeptide?

A

By an oligosaccharyl transferase associated with the translocator

26
Q

What phospholipid is essential for N-linked glycosylation?

A

dolichol phosphate

27
Q

Why is dolichol phosphate essential for N-linked glycosylation?

A
  • 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
28
Q

What are the 2 types of N-linked oligosaccharide?

A
  • Complex
  • High mannose
29
Q

Where does processing of N-linked oligosaccharides occur?

A

In the ER and Golgi

30
Q

Why do N-linked oligosaccharides have a common core of sugars?

A

Because they all start as the same 14 mer trasnferred from dolichol

31
Q

Are high mannose or complex oligosaccharides bigger?

A

Complex

32
Q

In the ER are many resident proteins soluble or membrane bound?

A

Soluble

33
Q

In the golgi are resident proteins membrane bound or soluble?

A

Membrane bound

34
Q

What happens in oligosaccharide processing?

A
  • Removal of glucoses
  • Removal of mannoses
  • build structure up again and activated sugars added on
35
Q

What are the stuctures of all glycosidases and glycosyltransferases in the golgi?

A

Transmembrane enzymes arranged into large complexes

36
Q

what are problems with studying the glycocalyx?

A
  • Secondary gene products
  • Difficult to distinguish similar monosaccharides
  • Can only be seen with EM
37
Q

What are the roles in glycocalyx?

A
  • Cell morphology (used in microvilli for example)
  • Regulation of membrane protein diffusion
  • Cancer development

MCB (28 decks)

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