Glycoproteins

Question 1

What is a glycoprotein?

Answer 1

a protein with covalently attached carbohydrate

Question 2

Where does glycosylation occur?

Answer 2

RER lumen

Question 3

When does glycosylation occur?

Answer 3

as the polypeptide is being synthesized, aka cotranslationally

Question 4

How does the glycosylation occur?

Answer 4

a preassembled oligosaccharide is transferred by dolichol phosphate to an appropriate Asn in the recognition sequence

Question 5

What is Dolichol phosphate?

Answer 5

an intermembrane protein in the RER membrane that catalyzes the N-glycosylation of proteins as they are translated/synthesized

Question 6

Where on the protein are the N-linked sugars attached?

Answer 6

to the nitrogen group of an appropriate asparagine in the recognition sequence of the protein

Question 7

Where on the protein are the O-linked sugars attached?

Answer 7

to the oxygen of the hydroxylated side chain of amino acids serine, threonine, or hydroxylysine

Question 8

Collagen and fibronectin are both __-linked glycoproteins.

Answer 8

O-linked

Question 9

Will most glycoproteins, if glycosylation is inhibited, be more or less soluble, and why?

Answer 9

less soluble, because the sugars have lots of hydrophilic hydroxyl groups that increase solubility

Question 10

What are the major recognition functions of glycoproteins?

Answer 10

hormone receptor, cell surface receptor, integrins, cell surface adhesion molecules

Question 11

What is the main function of integrins?

Answer 11

to anchor cells to the extracellular matrix (necessary for Viability of most cells)

Question 12

What mediates the aggregation of like cell types?

Answer 12

adhesion molecules on the cell surface (glycoproteins), such as cadherins

Question 13

What is the immune function of glycoproteins?

Answer 13

host defense and pathogen presentation; WBC traveling through endothelium

Question 14

What is the role of selectins in the extravasation of WBCs?

Answer 14

selectins are on the endothelial cells and they adhere to the WBCs, recognize, and then they weakly stick and roll along endothelium with further help of integrins

Question 15

How are sugars linked together to form oligosaccharides?

Answer 15

Linked by enzymes through many of their hydroxyl groups to form oligosaccharides

Question 16

Fibronectin binds both cells and ___________ molecules.

Answer 16

Extracellular matrix

Question 17

Describe the structure of Fibronectin.

Answer 17

A dimer of two similar subunits joined by disulfide bonds at the C terminal.

Question 18

True or false: glycoproteins protect proteins from degradation by proteases.

Answer 18

True

Question 19

What cell surface components are necessary for cell-cell recognition and interaction and aggregation?

Answer 19

Glycoproteins

Question 20

What glycoprotein mediates diapedesis of WBCs along the endothelium of vessels?

Answer 20

Selectin

Question 21

What glycoprotein mediates margination (extravasation) of WBCs through the endothelium of vessels?

Answer 21

Integrin

Question 22

What is unique about albumin?

Answer 22

it’s an extracellular protein that is not glycosylated

Question 23

What are proteoglycans?

Answer 23

a protein core that is heavily glycosylated with several GAG (glycosaminoglycan) residues

Question 24

What sugars are used in the formation of glycoproteins?

Answer 24

mannose, GlcNac/NAG, sialic acid

Question 25

What’s the difference between a glycoprotein and a proteoglycan?

Answer 25

glycoproteins have lower carbohydrate content, whereas proteoglycans have a higher protein content

Question 26

When do N-linked oligosaccharides undergo modifications?

Answer 26

prior to secretion of the glycoprotein from the RER, as well as in the ER–>Golgi processing

Question 27

Dolichol phosphate attaches what to the nascent protein?

Answer 27

a standard, preassembled oligosaccharide

Question 28

Describe the structure and function of fibronectin.

Answer 28

it binds both cells and ECM molecules; dimer joined by disulfide bonds, 3 forms from one gene, and it’s multifunctional with modules

Question 29

Inhibiting a selectin may inhibit what important cell-cell interaction?

Answer 29

WBC extravasation

Question 30

How do glycoproteins come in with regard to lysosomal function?

Answer 30

a phospho-mannose tag is needed to target/keep the protein/enzyme in the lysosome

Question 31

What is I-cell disease?

Answer 31

a lysosomal enzyme disorder where the enzymes are glycosylated and not taken to the lysosome, therefore there’s a build-up of waste in the lysosome and cell death; resulting in mental retardation and skeletal abnormalities

Question 32

How are oligosaccharides involved in protein life spans?

Answer 32

If the sialic acid is lost from the oligosaccharide, the half life is decreased x700 because it is targeted for uptake by liver cells.

Question 33

What is the modular function of fibronectin?

Answer 33

globular polypeptide that functions for a specific binding purpose/anchor