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1

What are lysosomes?

Acidic organelles found in every eukaryotic cell that contain hydrolytic enzymes and mediate intracellular digestion.

2

Typically, lysosomal hydrolytic enzymes degraded macromolecules in a ___ fashion.

Sequential

3

What keeps the pH low in lysosomes?

Proton ATPases

4

If a lysosome were to burst, what would happen?

The released enzymes would alarm the cell but would not be lethal, since the optimal pH for enzymatic action is less than 7, which is the pH of the cell.

5

Acid hydrolases work optimally at a pH of ___.

5

6

What are the 7 main acid hydrolases?

1. Nucleases
2. Proteases
3. Glycosidases
4. Lipases
5. Phosphatases
6. Sulfatases
7. Phospholipases

7

___ evolve into lysosomes as they move to the cell interior and acquire ___.

Endosomes; hydrolytic enzymes

8

___ are the end of the endocytic pathway.

Lysosomes

9

Describe the endocytic pathway.

Plasma membrane > early endosome > late endosome > endolysosome > lysosome (note that cargo can enter the early endosome from the TGN as well)

10

As the endocytic pathway proceeds, the pH ___.

Decreases

11

Where do the enzymes needed for degradation in the lysosomes come from? Where do the substrates targeted for degradation in the lysosome come from?

Fed to lysosomes from the TGN; Early endocytic pathways

12

What happens to hydrolytic enzymes in the cis-golgi?

Fully folded but inactive glycoproteins from the ER get phsophorylated on mannose residues, destining them for the lysosome.

13

What happens to hydrolytic enzymes in the trans-golgi?

Phospho-mannose marked proteins get sorted for vesicular transport to endosomes and then to lysosomes.

14

Where do the substrates for lysosomal degradation come from?

Many sources, both extracellular and cytoplasmic

15

What are the three general ways substrates are transported to the lysosome?

1. Bacteria from the extracellular space get phagocytosed (form phagosome)
2. Substrates from outside the cell are brought in via receptor mediated endocytosis (form endosome)
3. Dying/damaged organelles from inside the cell are enclosed by a double cell membrane from the ER via autophagy (form autophagosome)

16

True or false - lysosomes are defined by their appearance.

False - lysosomes are defined by a common set of properties.

17

What are the common set of properties that define a lysosome?

1. Luminal pH of ~4.5
2. Presence of active forms of lysosomal hydrolases
3. Presence of lysosomal membrane proteins (LAMPs)

18

What do LAMPs do?

Form an inner lining to protect the lysosomes from enzymes

19

Lysosomal enzymes are phosphorylated on ___ residues in the ___.

Mannose; CGN

20

Virtually all proteins from the ER have ___ glycans.

N-linked

21

How are lysosomal enzymes phosphorylated on their mannose residues?

Signal patch on the lysosomal hydrolase (in its inactive zymogen form) and UDP-GlcNac bind to the enzyme GlcNac phosphotransferase. This allows for transfer of GlcNac-P to the 6-OH on mannose (release of UMP). Another enzyme removes GlcNac, leaving mannose-6-phosphate.

22

The transport of enzymes to the lysosomes involves ___ receptors.

Mannose-6-phosphate

23

Proteins marked with M-6-P move to the ___, where they bind to receptors.

TGN

24

M6P receptors in the TGN are ___ proteins with cytoplasmic tails that interact with ___. Clathrin mediated budding from here makes transport vesicles that bind to ___. Here, the cargo is released and ___ removed by acid phosphatases. What happens to the empty receptors?

Transmembrance; clathrin adaptors; endosomes; 6-P; they are incorporated into retromer vesicles that bud from the endosomes and return to TGN.

25

The one-way flow of proteins to the lysosome is caused by what?

Slowly decreasing pH levels in various organelles; note that this ultimately comes from the H+ ATPases.

26

Once in the lysosome, digestive enzymes are activated by ___.

Proteolytic cleavage

27

Lysosomal proteases are synthesized as ___. These proteases acquire limited proteolytic activity at a ___ pH. Once activated, the protease can do what?

Inactivate precursors; proenzymes; low; process additional inactive precursors

28

Lysosomal enzymes that are accidentally secreted are recovered by ___. How does this work?

Receptor-mediated endocytosis; Plasma-membrane M6P receptors bind the escaped lysosomal enzyme. This is then returned to the lysosome by clathrin and receptor-mediated endocytosis.

29

Lysosomes degrade plasma membrane proteins via ___.

Multivesicular bodies

30

Describe the process of lysosomal degradation of plasma membrane proteins via multivesicular bodies.

The proteins receive a mark on the cytoplasmic tails (often ubiquitination conjugation). This mark places the proteins into a complex of proteins called ESCRT which stimulate an inward pinching off of a membrane vesicle.