1-40 Exocytosis and Endocytosis

Question 1

T/F: Both N-linked glycosylation and O-linked glycosylation occur post-translationally and surgars are added to proteins one at a time

Answer 1

False: N-linked glycosylation occurs co-translationally, while o-linked glycosylation occurs post-translationally.

In N-linked glycosylation, a preformed oligosaccharide of 14 sugars is added (in ER), while in O-linked glycosylation, sugars are added to proteins one at a time (in golgi).

Question 2

What is a proteoglycans?

Answer 2

Proteoglycans typically contain greater than or equal to 95% of carbohydrate by mass and contain a very small core protein.
If it has a glycosaminoglyan (GAG) chain, then it is a proteoglycan.
Synthesized in the ER

Question 3

What is a glycoprotein?

Answer 3

Mass of carbohydrate in each glycoprotein varies from about 2-80%
Number of carbs varies from one to hundreds.
The structure of each carbohydrate unit in a single protein can vary-this is called microheterogeneity

Question 4

Why are there no carbohydrates extending from proteins (creating glycocalyx) on the cytosolic side?

Answer 4

Pg. 617 in notes

Enzymes responsible for addition of sugars are not located in the cytosol, rather they are within the lumen of the ER and golgi

Question 5

What differentiates the A allele, B allele, and O allele?

Answer 5

The ABO blood group antigens are determiend by a SINGLE genetic locus, the ABO locus, that encodes glgcosyltransferase.
The A allele encodes a GalNAc transferase.
The B allele encodes a Gal transferase.
The O allele does not encode a functional protein

Question 6

T/F: Proteins destined for secretory vesicles are sorted and packaged in the cis-golgi network.

Answer 6

False: They are sorted and packaged in the Trans-golgi network

They have special surface properties that cause them to aggregate in the lumen of the trans Golgi network, which allows them to be packaged into vesicles at very high concentration. In this way, secretory cells can release large quantities of protein very quickly in response to a trigger

Question 7

T/F: Both the constitutive exocytosis pathway (default pathway) and regulated pathway require signals to travel to the plasma membrane for secretion

Answer 7

False: In the default pathway, all proteins passing through the Golgi will go to PM in vesicles unless directed elsewhere by specific signal and this operates continually. In the regulated pathway, secretory vesicles bud off trans Golgi and accumulate near the PM and fuse to release contents ONLY in response to an extracellular signal

Question 8

T/F: Both pinocytosis and phagocytosis involve the upatake of fluid and small molecules to deliver to lysosome in a continuous process

Answer 8

False: Pinocytosis involves the uptake of fluid and small molecules in small vesicles and is a continuous process in all eukaryotic cells. Phagocytosis involves the uptake of large particles in large vesicles and requires receptor activation at cell surface and occurs in specialized cells (macrophages and neutrophils)

Question 9

T/F: The early endosome resembles an “immature” lysosome

Answer 9

False: The early endosome is rpimarily a “sorter” and is located near the PM. The late endosome resembles an “immature” lysosome and may even become a lysosome via uptake of lysosomal proteins

Question 10

What are the 3 “fates” or pathways that endocytosed receptors may follow?

Answer 10

1. Recycling back to the PM
2. Degradation (sometimes used for receptor downregulation)
3. Transcytosis, which occurs in polarized cells with different PM domains

Question 11

What are the 3 major pathways in which materials are delivered to lysosomes?

Answer 11

1. Endocytosis (pinocytosis)
2. Phagocytosis
3. Autophagy

Question 12

What is autophagy

Answer 12

The digestion of obsolete cell parts; a double membrane surrounds an organelle forming an autophagosome which then fuses with a late endosome/lysosome

Question 13

What is the cause of mucopolysaccharidoses?

Answer 13

Defect in enzyme required for degradation of glycosaminoglycans

Hurler Disease

Question 14

What is the cause of oligosaccharidoses?

Answer 14

Defect in the enzyme required for degradation of oligosaccharides

Schindler Disease

Question 15

What is the cause of Sphingolipidoses?

Answer 15

Defect in the enzyme required for degradation of sphingolipids

Gaucher & Tay-Sachs Disease

Question 16

What is the cause of Inclusion-cell disease?

Answer 16

Defect in enzyme required for making M6P tag…which then fails to target to lysosome and the lysosomal proteins will not make it to the lysosome and large accumulations of products will result in the lysosome

Question 17

What is the structural difference between a glycoprotein and a proteoglycan?

Answer 17

Glycoproteins have bonded oligosaccharides that don’t have serial repeat. Proteoglycans have bonded glycosaminoglycans with no serial repeat.
Glycoproteins are mostly protein with little carbohydrate, while proteoglycans are mostly carbohydrate with a small core protein

Question 18

What class of enzyme is responsible for adding sugars to O-linked glycoproteins?

Answer 18

Glycosyltransferases add sugars

Question 19

What is the genetic difference between individuals whose blood types are type A vs B vs O vs AB?

Answer 19

They differ in the enzymes encoded at their ABO locus. Type A have one (AO) or two (AA) A alleles encoding a GalNAc transferase. Type B have one (BO) or two (BB) B alleles encoding a Gal transferase, Type AB have one A and one B allele, and type O has two null alleles encoding non-functional enzymes

Question 20

What is the biochemical difference in blood cells that are type A vs B vs O vs AB?

Answer 20

They differ in the terminal sugars on the glycoproteins and glycolipids on the surface of the blood cells. Type A has a terminal GalNAc, type B has a terminal Gal, type AB has both types, and type O has neither

Question 21

Where are how are glycoproteins degraded in the cell?

Answer 21

Degraded in lysosome by acidic hydrolases. First endoglycosidases remove oligosaccharides from proteins, then glycosidases act sequentially to remove individuals sugars in reverse order to the way they were built. The protein portion is degraded by proteases.