Topic 5, Lecture 2

Question 1

Describe the new box next to the lysosome.

Answer 1

It is a nonspecific engulfing

Question 2

What are the three main modifications that happen to carbohydrates in the ER?

Answer 2

Addition and processing of carbohydrates, formation of disulfide bonds, and folding and assembly of multisubunit proteins

Question 3

Define glycosylation

Answer 3

Addition of carbohydrates

Question 4

Define Oligo

Answer 4

Chain of sugars

Question 5

What are the six main benefits of carbohydrates?

Answer 5

1. Aids in the folding of proteins, 2. Establishes a “glycol code”, 3. Participates in protein sorting the Golgi apparatus, 4. Protects the cell from pathogens, 5. Cell adhesion and recognition, 6. Plays a role in some signaling pathways

Question 6

What is the benefit of the carbohydrate layer on the outside of the cell?

Answer 6

Protects the cell surface from degradation

Question 7

Describe how carbohydrates aid in protein folding.

Answer 7

Carbohydrates are polar, which make the protein soluble in the cytosol and prevent aggregation

Question 8

Describe the glycol code

Answer 8

Marks the protein’s movement in the golgi

Question 9

What are the two types of carbohydrates?

Answer 9

N-Linked and O-Linked

Question 10

Describe O-Linked carbohydrates

Answer 10

Small and simple, linked to the -OH groups of serine and threonine

Question 11

Describe N-linked carbohydrates

Answer 11

Most common (and most important), large and complex, branched, linked to asparagine

Question 12

What is the core region of an oligosaccharide composed of?

Answer 12

Two N-acetylglucosamines and three mannoses

Question 13

How are carbohydrates added to proteins?

Answer 13

Via a preformed precursor

Question 14

What is the preformed precursor composed of?

Answer 14

3 glucoses, 9 mannoses, and 2 N-acetylglucosamines

Question 15

What is the non-variable region of a carbohydrate?

Answer 15

Core Region

Question 16

What is the variable region of a carbohydrate?

Answer 16

Any additional sugars not apart of the core region

Question 17

Describe the synthesis of the preformed precursor up to the “flip”.

Answer 17

Dolichol is synthesized as a byproduct of the cholesterol biosynthetic pathway in the smooth endoplasmic reticulum.

The fully formed dolichol molecule migrates through the membrane of the endoplasmic reticulum until it reaches an area that has a high concentration of CTP (cytidine triphosphate), UDP-N-acetylglucosamine (uridine diphosphate), and GDP-mannose (guanosine diphosphate) on the cytosolic side of the ER membrane.

The dolichol removes a phosphate group from the CTP to create a CDP (cytidine diphosphate), and the phosphate group is added to the dolichol molecule.

The dolichol molecule (that still has a phosphate group attached) attacks two UDP-N-acetylglucosamine by removing the N-acetylglucosamine from both UDP molecules and removing a phosphate group from one UDP molecule to form UDP, UMP (uridine monophosphate), and a dolichol molecule with two phosphate groups and two N-acetylglucosamines is formed.

The dolichol molecule (with both phosphates and N-acetylglucosamines) attacks five molecules of GDP-mannose to remove all five mannose sugars and bind them to the growing dolichol molecule in cytosol.

A transporter known as M5-DLO Scramblase “flips” the molecule to the ER lumen side of the membrane by an unknown mechanism.

Question 18

Describe the synthesis of the preformed precursor after the “flip”

Answer 18

The growing dolichol molecule (plus all of its substituents) will attack four non-central dolichol phosphate + GDP-mannose molecules to remove all four mannoses and add them onto our central dolichol molecule in the ER lumen.

The growing dolichol molecule (plus all of its substituents) will attack three non-central dolichol phosphate + UDP-glucose molecules to remove all three glucoses and add them onto the central dolichol molecule to create the final form of the preformed precursor.

Question 19

Describe how the preformed precursor is added to the protein.

Answer 19

The preformed precursor will be attached to the binding site of the oligosaccharyl transferase that is located directly next to the Sec61 translocon.

As the protein is being translated through the ER membrane, certain asparagines (how the cell determines which asparagines is unknown) will bind to the adjacent binding site of the oligosaccharyl transferase.

The oligosaccharyl transferase will bind the preformed precursor to the asparagine to form an N-linked glycoprotein.