lysosomes

Question 1

what pathway are lysosomes a part of and describe it ?

Answer 1

• Lysosomes are part of the secretory pathway, which is a series of organelles that are dynamically connected with each other through vesicular transport . this includes the lysosomes, er , golgi , secretory granules and the plasma membrane.

Question 2

describe the structure and function of lysosomes in the lumen

Answer 2

• Key function : carry out digestive hydrolytic reactions. They are the recycling plant.
• 100 lysosomes per cell in the liver ( varies )
• Heterogenous contents – ie here will be substrates that are in various stages of degradation . eg lysosome can have another vesicle inside. a
• Acidic (ph 4-5)
• Single membrane
• 40 hydrolytic enzymes ( enzymes that catalyse reactions which break covalent bonds using water)

Question 3

what does the lumen contain in lysosomes

Answer 3

contain hydrolases , which are generally soluable enzymes and have a high proton concentration which gives a low ph.
contain enzymes and transport proteins.

Question 4

lysosomes also have organelle specific membrane proteins. describe them

Answer 4

v-ATPase - pumps protons into the lumen of the lysosome which gives a low ph

• ion channels and transporters.
• The breakdown of certain lipids produces cholesterol is released by the cholesterol transporters. for example the released cholesterol can then be used for membrane biogenisis.
• the breakdown of carbohydrates by glycosidases produces monosaccharides - sugar transporters.
• nucleoside transporters.
• amino acid transporters.
• SNAREs , tethering factors and small GTPases - used for the exchange of material with other organelles within the secretory pathway system through tethering and fusion processing
• Move within cell along microtubules requiring motor adaptors

Surface hold signaling complexes involved in metabolic signing as well as transcription factors for gene regulation which are both part of immune responses

Question 5

how do lysosomes move through the cell?

Answer 5

lysosomes are attached to microtubules by motor adaptors, and the lysosomes move through the cell along the microtubules.

Question 6

what are lysosome related organelles ?

Answer 6

These share many of the characteristics enzymes but additional cell type and organelle specific features and proteins.

Question 7

give an example of lysosome related organelles.

Answer 7

Osteoclasts create an extracellular lysosome like compartment called the ruffle border, in which the cells secrete protons to create an acidic environment and lysosome specific degradative enzymes that carry out bone resorption.

Question 8

describe the properties of melanosomes

Answer 8

• type of lysosome related organelle.
  -Are produced by melanocytes, and other cells in the body including different types of cells in the eye.
• mature melanosomes are characterised by high concentrations of melanin .
• mature melanosomes are formed in a 4 step process :
  Endosomes acquire the specific the proteins which produce melanin and make up the specific protein composition through a series of fusion and fission processes that carry out interactions with other organelles in the secretory pathway.
• Melanosomes are produced in melanocytes, they are then transferred to keratinocytes in the skin.

Question 9

describe how extracellular substrates are delivered to lysosomes

Answer 9

endocytosis
• Fluid-phase endocytosis of molecules and lipoproteins (includes receptor-mediated endocytosis)
• Phagocytosis of particles ≥ 0.5 µm

Question 10

what is endocytosis and give an example

Answer 10

particles bind to cell surface receptors which triggers the invagination and the formation of the vesicle which can mature into early endosomes, late endosomes and finally lysosomes where digestion can be completed. Eg of particles that are taken up by the receptor mediated endocytosis is LDL

Question 11

describe LDL

Answer 11

• LDL role : supplier of building blocks for cellular membranes.
• Core – neutral lipids, cholesterol ester and tryglicerides which are surrounded by lipid monolayer which consists of unesterified cholesterol and phospholipids.
• Associated with the monolayer is the apoprotein B-100 which is important for recognition of these particles by cell surface receptors.

Question 12

what is associated with the monolayer of LDL and why is it important?

Answer 12

apoprotein B-100 which is important for the recognition of these particles by cell surface receptors.

Question 13

how does the LDL get into the lysosome

Answer 13

• The surface receptors are called LDL receptors.
• LDL particles containing cholesterol ester, binds to the LDL receptor, it leads to the formation of the coated pits, which captures the LDL molecules bound to the LDL receptors.
• The coated pits can pinch off through vesicler budding into the inside of the cell which leads to the formation of the early endosomes.
• As the ph drops in the vesicles, this triggers the dissociation of the LDL particle from the LDL receptor. The LDL receptor can then recycle back to the cell surface where they can bind to a new LDL particle.
• The ldl particles are then delivered to the endosomes and lysosomes.
• The cholesteryl ester molecule is hydrolysed to form cholesterol and fatty acid. This is then released for further metabolism outside the lysosomes.

Question 14

what are the 3 stages of phagocytosis

Answer 14

• Engulfment – lysosome fusion – digestion.

Question 15

give 3 examples of componants that could be taken up by phagocytosis.

Answer 15

bacterium , aging blood cells, dead cell particles

Question 16

outline the steps of phagocytosis in detail

Answer 16

1) Particle will bind to the cell surface and is then engulfed. This occurs through the plasma membrane wrapping itself around the particle. This forms a phagosome.
2) Lysosomes will fuse to the phagosome and will form the phagolysosome.
3) The lysosomes deliver the hydrolytic enzymes. The membranes carry vacuolar ATPase molecules which are proton pumps that lead to the acidification of the inside of the phagolysosomes, the acidic PH activates the hydrolytic enzymes which then digests the particle that has been take up . digestion.

Question 17

give an example of an important cell that is involved in phagocytosis

Answer 17

• Macrophages in phagocytosis can take up cells larger than itself.
• Eg They take up aging red blood cells via phagocytosis . one macrophage can take up more than one red blood cell at a time.

Question 18

why is an acidic PH required

Answer 18

to activate the hydrolytic enzymes which then digests the particles that has been taken up by digestion.

Question 19

outline the steps in macroautophagy

Answer 19

1) Initially, a structure is formed that produces a double membrane that can then wrap around intracellular particles (organelles or pieces of cytosol) . this forms an autophagosome.
2) The autophagosome fuses with a lysosome to form an autophagolysosome.
3) This will deliver hydrolytic enzymes and proton pumps, the PH decreases and the hydrolases digest the componants of the material that’s been delivered.

Question 20

what is microphagy

Answer 20

macroautophagy of mitochondria

Question 21

what is xenophagy

Answer 21

macroautophagy of infectious particles.

Question 22

do autophagosomes have a single or double layer membrane

Answer 22

a double layer membrane

Question 23

where can viruses emerge from ?

Answer 23

endosomes.

Question 24

describe how viruses can emerge from endosomes.

Answer 24

• HIV viruses binds to the cell surface and fuses with the plasma membrane, delivers its contents leading to infection
• Many other viruses, eg influenzas, are taken up through endocytosis. The virus end up in endosomes, early and late, where the drop in PH activates the viral proteins that mediate the fusion of the viral membrane with the endosomal membrane, which then leads to the release of the viral content.
• Influenza virus is an enveloped virus, polio has a capsid which is taken up through endocytosis and once it reaches endosomes, the low ph activates mechanism for injection of the viral genome into the cytoplasm.
• Adenovirus is also a capsid virus taken up by endocytosis. the entire viral particle is delivered to the cytoplasm and will dock on the nuclear pore complexes in order to deliver its genome.

Question 25

what is the major route for targeting lysosomal enzymes to lysosomes.

Answer 25

mannose 6-phosphate pathway

Question 26

outline the steps in M6P pathway

Answer 26

• Lysosomal hydrolases are post translationally modified by protein glycosylation in the er.
• As the protein is still being synthesised and enters the lumen of the er, a large carbohydrate structure is transferred onto the protein, specifically on an aspergine residue (N).
• All lysosomal hydrolases receive the oligosaccharides which are then subsequently modified in a series of steps.
• What distinguishes lysosomal hydrolases from other n glycosylated proteins in the secretory pathway is that one of the mannose sugars in the oligosaccharides structure is phosphorylated to produce mannose 6-phosphate (M6P) .
• The mannose 6-phosphate residue is added in the cis golgi as the proteins are following the export from the er
• Once the protein reaches the trans golgi , the mannose 6-phosphate residue is recognised by a specific receptor called the M6P receptor.
• The complex of the receptor with the mannose 6-phosphate modified ligand clusters in the membrane in a way that leads to the formation of budding vesicles using a specific protein called the clathrin-coat.
• The clathrin-coated vesicles then carry the complex of m6p receptor and m6p modified lysosmal hydrolases toward the late endosomes from whether they can be transported onwards to lysosomes.

Question 27

what can cause lysosomal diseases

Answer 27

• The products of hydrolysis must be released from the lysosomes, if any enzyme is missing, their substrates accumulate in the lysosomes. This leads to lysosomal storage disorders.
• Mutations in lysosomal hydrolyses causes substrate accumulation

Question 28

How does I-cell disease arise?

Answer 28

no M6P tag is created and as a consequence, the lysosomal hydrolases behave as if they were secreted proteins.

Since they cannot be recognised by the M6P receptor, they follow the path of secreted proteins and are secreted out of the cell.

Rather than being found in lysosomes, all the lysosomal hydrolases are found in the cells supernatant in the blood stream.

No hydrolases reach the lysosomes therefore digestion cannot occur leading to accumulation.

Question 29

If no M6P tag is created, how do the lysosomal hydrolases behave ?

Answer 29

secreted proteins

Question 30

why is a defect in the enzymes responsible for the M6P tag serious ?

Answer 30

as its the process required for the proper targeting of all lysosomal hydrolases.

Question 31

what causes sandhoff disease?

Answer 31

a defect in the lysosome enzyme required for the breakdown of gangliosides.

Question 32

in sandhoff accumulation of entire membrane structures inside the lysosomes are referred to as ?

Answer 32

multilamellar bodies.

Question 33

what can cause persistent stuttering ?

Answer 33

• Certain point mutations in genes required for the generation of the mannose 6-phosphate tag cause persistent stuttering

Question 34

what are the 3 genes involved in coding proteins to make the m6p tag ?

Answer 34

GNPTAB, GNPTG, or NAGPA

Question 35

• Mutations in GNPTG cause

Answer 35

mucolipidosis type III

Question 36

• Mutations in GNPTAB cause

Answer 36

mucolipidosis type II

Question 37

how does the lysosome maintain the acidic pH ?

Answer 37

→hydrogen ion pump which requires ATP

→active transport of H+ ions from the cytoplasm

Question 38

what is LDL hydrolyzed into?

Answer 38

cholesteryl ester and fatty acid by lipase enzymes.

Question 39

What modification do lysosomal enzymes have that others don’t?

Answer 39

mannose-6-phosphate

Question 40

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

Answer 40

phosphoryl transferase

Question 41

describe how intracellular substrates are delivered to the lysosome

Answer 41

• Microautophagy (invagination of the lysosomal membrane)
• Macroautophagy (cytosol or organelles wrapped in ER membrane, which then fuses with lysosomes)
• Selective transport of proteins across the lysosomal membrane