Lysosomes

Question 1

Describe the structure of lysosomes

Answer 1

· Part of the secretory pathway

· Heterogenous content- substrates at various stages of degradation

· ~100 lysosomes per cell

· Single membrane

· ~ 40 hydrolytic enzymes

• Transmembrane proteins are heavily glycosylated, protecting bilayer from being digested

Question 2

Summarise some of the functions of lysosomes

Answer 2

· Carry digestive hydrolytic functions.

· Hydrolytic enzymes break down covalent bonds by using water

Question 3

Why do enzymes work at low pH and how is the pH maintained?

Answer 3

a protective mechanism because if the lysosome were to rupture the hydrolytic enzymes would not work because the cytosol is neutral, it maintains the pH by using a hydrogen ion pump which requires ATP (active transport of hydrogen ions from the cytoplasm)

Question 4

Describe some lysosome related organelles(3)

Answer 4

· Vacuole ATPase- large protein complex in the membrane and pumps protons into the lumen
· Lysosomes have proteins- small GTPase, tethering factories
Surface of lysosomes have signals like metabolic, and transcription signals
· Osteoclasts-extra cellular surface

Question 5

How do lysosomes travel?

Answer 5

· by microtubules that require certain adaptors

Question 6

How do lysosomes exhange materials with other organelles?

Answer 6

· in the secretory pathway through vesicular binding and fusion

Question 7

What are melanosomes?(5)

Answer 7

· Produced by melanocytes but also in other cell types incl ones in the eye

· Mature melanosomes are characterised by high conc of melanin

· Melanin absorbs UV light

· Formed in 4 step process that starts with endosomes.

· After production they are transported to keratinocytes by process not yet clear

Question 8

Describe lysosomal substrates(3)

Answer 8

· Extracellular particles brought in by phagocytosis that fuse with the lysosomes

· Molecules brought in by way of endosomes

· Move out of organelles and become wrapped in a portion of ER and fuse with lysosomes

Question 9

Describe receptor mediated endocytosis

Answer 9

• The surface receptors are LDL receptor molecules. • When LDL particles containing cholesteryl, ester bind to LDL receptor and aggregate they form coated pits. which capture the LDLs and pinch off through vesicular budding inside the cells forming endosomes.

· As the pH drop in the vesicles there is dissociation of the LDL particle from the receptor and they are then recycled with the membrane.

· The LDL particles are then delivered to the lysosomes via endosomes.

· An early endosome is an organelle, a sorting compartment that helps separate the molecule from its receptor. Chemical changes occur within the endosome to form a late endosome. The late endosome splits into two, in which one endosome contains the molecule, while the other contains the receptor.

· And they get hydrolysed from a cholesteryl ester to cholesterol and fatty acids by acidic lipase enzymes.

Question 10

What are low density lipoproteins?(3)

Answer 10

At the core is neutral lipids- cholesteryl ester and triglycerides surrounded by lipid monolayer that consists of esterified cholesterol phospholipids.

Cholesteryl ester is cholesterol esterified with a fatty acid
Associated is by apoprotein B-100- important for recognition of cell receptors

Question 11

Describe phagocytosis(4)

Answer 11

Particle binds to the surface and is engulfed- plasma membrane wraps itself around the particle
• Phagosome forms around the particles.
• Lysosomes fuse with the phagosome and forms a phagolysosome

• The membrane carry ATPase and activates the hydrolytic enzymes and digest the content

Question 12

Give example of molecules that undergo phagocytosis

Answer 12

bacteria RBCs as RBCs age they lose their shape

Question 13

Describe difference between microautophagy and macroautophagy

Answer 13

• Microautophagy (invagination of the lysosomal membrane)
• Macroautophagy (cytosol or organelles wrapped in ER membrane, which then fuses with lysosomes). Substrates from these organelles are delivered for degradation

Question 14

Describe how HIV enters a cell

Answer 14

HIV virus binds to the cell surface and fuses with the membrane and delivers its content

Question 15

Describe how influenza enters the cell

Answer 15

Influenza is taken by endocytosis ending up in early and late endosomes

Drop of pH activates of viral proteins that mediate the fusion of viral membrane and endosomal membrane leading to release of viral content

Question 16

How does poliovirus enter the cell?

Answer 16

Polio virus has a capsid taken up by endocytosis.

Once there the pH activates mechanism for the injection of their content”

Question 17

How does adenovirus enter the cell?

Answer 17

Adenovirus is a capsid virus taken up by endocytosis.

Particle is delivered and hooks to the nuclear pore and delivers its content

Question 18

How does coran virus enter the cell?

Answer 18

Corona relies on endocytosis. On the surface is spike protein that binds to AC… gaining entry and end up in endosomes and interacts with the membrane and leads to the release of the viral content
Assembly of new virus occurs at the trans Golgi

Question 19

Describe autophagy

Answer 19

· the process of cell eating

· A double membrane forms that wraps around particles- autophagosome

· Fuses with lysosome that forms phagolysosomes

· pH drops and the hydrolytic enzymes break down the particles

· There are different types of autophagy

Question 20

How are proteins targeted to lysosome?

Answer 20

· Protein glycosylation in the ER
· Lysosomal hydrolases- post translationally modified as the protein is still being synthesized as it enter the lumen of the Er

· Large carbohydrate structure is transferred onto the protein specifically on the asparagine residue- one letter amino acid code- N hence the term N glycosylation.

· All lysosomal hydrolases receive these oligosaccharides And are subsequently modified

Question 21

Describe the process of mannose 6-phosphate

Answer 21

The M6P pathway is the main route in which lysosomal enzymes are targeted to lysosomes

digestive enzymes & membrane proteins of the lysosome are synthesised in the ER & transported through the Golgi apparatus to the trans-Golgi network.

→While in the ER & cis-Golgi network, the enzymes are tagged with a specific phosphorylated sugar group (mannose 6-phosphate).

→When they arrive in the trans-Golgi network they can be recognized by an appropriate receptor, the mannose 6-phosphate receptor.

· Complex of receptor with mannose 6 phosphate clusters in the membrane leading to budding vesicles- clathrin coated pits. These pits carry the complex toward late endosomes where they are carried to lysosomes receptor.

· Proteins that do not have the mannose-6-phosphate (M6P) modification are transported to the plasma membrane, some lysosomal enzymes can accidentally get transported to the membrane but there are M6P receptors there to send them back.

Question 22

Describe the route of the enzymes made in the ER

Answer 22

· transported in vesicles through the golgi apparatus, they enter at the cis side pass through the medial cisternae and at the trans side they are packed into other vesicles and transported into the lysosomes.

Question 23

What distinguishes lysosomal hydrolases from other N-glycosylated proteins?

Answer 23

· that one of the mannose sugars is phosphorylated – mannose 6 phosphate and added to the Golgi.

Question 24

Why are lysosomal diseases a problem?

Answer 24

Mutations have been found in all these enzymes

· Products from hydrolysis must be released from the lysosomes

· If enzymes are missing, they accumulate in the lysosome

Question 25

How does sandhoff disease arise?

Answer 25

Results in the mutation in an enzyme involved in the breakdown of membrane lipids called gangliocytes.

Since they are membrane lipids.It leads to the accumulation of entire membrane structures called multilamellar bodies.

Defects in the enzymes that create the mannose-6-phosphate”

Question 26

How does I cell-disease arise?

Answer 26

1. Due to defective or missing GlcNAc phosphotransferase.
2. Lysosomal enzymes are not phosphorylated in the cis Golgi network.
3. The M6P receptors do not segregate them into the appropriate transport vesicles in the TGN.
4. Instead, the lysosomal hydrolases are carried to the cell surface and secreted.
5. No hydrolases in lysosomes = accumulation of undigested substrates in lysosomes.

Question 27

Describe the symptoms of I-cell disease

Answer 27

Symptoms include skeletal abnormalities, developmental delay, enlarged liver and spleen, impaired hearing

Question 28

Where are lysosomes made?

Answer 28

→in the ER

Question 29

what is LDL hydrolyzed into?

Answer 29

cholesteryl ester and fatty acid by lipase enzymes.

Question 30

what is the name of the enzyme that adds the M6P modification?

Answer 30

phosphoryl transferase

Question 31

Describe substrate delivery to lysosomes

Answer 31

EXTRACELLULAR SUBSTRATES:
→ Fluid-phase endocytosis of molecules & lipoproteins

→Phagocytosis of particles ≥0.5 μm

INTRACELLULAR SUBSTRATES;
→Microautophagy (invagination of the lysosomal membrane)
→Macroautophagy (where the cytosol or organelles are wrapped in ER membrane, then fused with the lysosomes)
→Selective transport of proteins across the lysosomal membrane.