Lysosomes

Question 1

What is the lysosome key function and describe its structure

Answer 1

Single membrane, digestive, hydrolytic reactions. 40 hydrolytic enzymes, heterogenous contents

Question 2

What is the optimum pH for most hydrolytic enzymes in the lysosome

Answer 2

pH 4-5 (acidic)

Question 3

Name some membrane proteins in the lysosome

Answer 3

v-ATPase - protein complex in the membrane which pumps protons into the lysosome lumen to maintain a low pH

Ion channels and transporters

Transporters for products of hydrolysis reactions e.g. polysaccharides into sugars

Cholesterol transporters for when cholesterol is broken down

SNARES, tethering factors for organelle fusion

Motor adaptors for motility

Signalling complexes - Metabolic signalling, immune responses

Transcription factors - Gene regulation, immune responses

Question 4

List some LROs

Answer 4

Melanocyte
Mast cell
Basophil
Platelet
Neutrophil
T cell
Osteoclast
Endothelial cell
Neuron

Question 5

Describe melanosomes

Answer 5

High concentration of melanin, which protects us from sunlight by absorbing UV light

Question 6

How are melanosomes formed

Answer 6

1. Endosomes acquire specific proteins to produce melanin, with fusion fission processes with other organelles such as golgi

Question 7

What are melanosomes produced by and where is their destination

Answer 7

Produced by melanocytes, transported to keratinocyte

Question 8

How are extracellular substrates taken up and describe

Answer 8

Endocytosis - Fluid-phase endocytsosis of molecules and lipoproteins

Phagocytosis of particles Less than 0.5 um

Question 9

How are intracellular substrates taken up

Answer 9

Microautophagy - Invagination of the lysosomal membrane (self-eating - inward budding of the lysosomal membrane)

Macroautophagy - Cytosol or organelles wrapped in ER membrane, which then fuses with lysosomes for degradation

Selective transport of proteins across the lysosomal membrane

Question 10

State and describe an example of endocytsosis of lipoproteins

Answer 10

LDL (low-density lipoprotein) which in high conc can cause cardiovascular disease. Helps build membranes.

When LDL which contain cholesteryl ester bind to the receptor of the membrane, a coated pit is formed which captures the LDL molecules bound to the receptors. They can pinch off through vesicular budding, in the inside of the cell, forming an endosome. As the pH drops, the LDL particles dissociated from the receptors, and the receptors are recycled to be used again in the membrane.

The LDL particles are ultimately delivered to endo/lysosomes, and the ester is hydrolysed to cholesterol and fatty acids.

Question 11

Describe phagocytosis of dead cells or pathogens, ageing cells

Answer 11

1. The particle binds to the cell surface and is then engulfed, through the membrane wrapping itself around the molecule. This forms a phagosome
2. The lysosomes will fuse with the phagosome, forming a phagolysosome
3. The lysosomes deliver hydrolytic enzymes which carry vacular ATPase molecules, proton pumps to maintain low acidic pH in lysosome so enzymes can digest the particle.

Question 12

What happens to the shape of red blood cells as they age

Answer 12

From flat to round and more spherical

Question 13

Because macrophages are so good at phagocytosis they can

Answer 13

Take up particles larger than themselves, even simultaneously taking up two cells at once

Question 14

Macroautophagy - Self-eating

Answer 14

Organelle forms double membrane which wraps around cytosol/organelles, forming an autophagosome. This fuses with a lysosome to form an autophagolysosome. The lysosome delivers the digestive enzymes and proton pumps to lower pH.

Question 15

How does endocytosis allow viruses to emerge

Answer 15

In endocytosis, a viral molecule may be taken up in the endosome. The drop in pH causes the activation of viral proteins that mediate the fusion of the viral membrane with the endosomal membrane, leading to the release of viral contents and infection.

Question 16

Describe protein glycosylation in the ER

Answer 16

The protein while being synthesised enters the lumen of the ER. A large carbohydrate is transferred onto the protein on an Asparagine residue (N) in N-glycosylation.

Question 17

Why are lysosomal hydrolases different to other glycosylated proteins in the secretory pathway

Answer 17

One of the mannose sugars in the oligosaccharide structure is phosphorylated to form mannose 6-phosphate.

Question 18

When the protein reaches the transgolgi what happens

Answer 18

The mannose 6-phosphate residue is recognised by a specific receptor called the mannose 6-phosphate receptor. The receptor with its m6p modified ligand clusters in the membrane, leading to the formation of budding vesicles called the Clathrin-coat. The vesicles then carry the receptor-mannose modified lysosomal hydrolyses towards the late endosomes, where they are transported to lysosomes.

Question 19

What is the consequence of missing hydrolytic enzymes in lysosomes

Answer 19

A lysosomal storage disorder would emerge, and the substrate would accumulate as it cannot be digested due to the missing enzyme.

Question 20

Name an example of a lysosomal storage disorder

Answer 20

Sandhoff disease - Defects in an enzyme required for the breakdown of gangliosides. Membrane structures accumulate (multilamellar bodies) inside the lysosome,

Question 21

What happens if there are defects in the enzymes responsible for creating M6P tag.

Answer 21

This is required for the targeting of ALL hydrolytic enzymes in the lysosome. E.g. I cell disease, no M6P tag, cannot be recognised by M6P receptor, secreted out of the cell and then found in the supernatant and blood rather than the lysosomes. The lysosomes cannot digest anything, although autophagy will continue. Substrates will accumulate in the lysosome.

Question 22

What does I cell disease look like on an electromicrograph of a cell

Answer 22

Dense granular inclusions in fibroblasts from a patient with I cell disease

Question 23

Symptoms/effects of I-Cell disease

Answer 23

Skeletal abnormalities, developmental delay, enlarged liver and spleen, impaired hearing, death from pneumonia or congestive heart failure within first decade of life, autosomal-recessive disorder.

Question 24

Point mutations required in the generation for M6P tag causes stuttering in patients

Answer 24

Subtle mutations can lead to different phenotypes rather than completely destroying