Lysosomes

Question 1

Give an introduction into lysosomes

Answer 1

• 100 lysosomes per cell
• Dense
• They have a single membrane
• Heterogenous contents (lots of vesicles inside lysosome)
• Individual lysosomes look different
• Acidic (optimum pH between 4.5-5.0.) A low pH denatures proteins. Hydrolysis reactions are more effective at a lower pH.
• 40 Hydrolytic enzymes (acid hydrolases), activated by proteolytic cleavage which needs acidic environment:
o Protease
o Lipase
o Glycosidases – break down sugars
o Nucleases – break down nucleic acids (DNA and RNA)
o Phosphatases – take off phosphate groups
o Sulfatases – take off sulphate groups

Question 2

Describe substrate delivery in lysosomes

Answer 2

Extracellular substrates: endocytosis – molecules that are taken in
• Fluid-phase endocytosis of molecules and lipoproteins (includes receptor-mediated endocytosis) ‘cell drinking’
• Phagocytosis of particles ≥ 0.5 µm (uptake and digestion). Particles are greater than this value.
Intracellular substrates
• Autophagy - self eating.
• Microautophagy (invagination of the lysosomal membrane, membrane moves inwards then pinches off inside the lysosome).
• Macroautophagy (Cytosol or organelles wrapped in ER membrane, which then fuses with lysosomes)- also known as autophagy or “self-eating.”.
• It helps restructure differentiating cells, but also in adaptive responses to stresses such as:
o Starvation
o Infection.
• Autophagy can remove large objects: macromolecules, large protein aggregates, and even whole organelles.
• Selective transport of proteins across the lysosomal membrane.

Question 3

Describe substrate delivery to lysosomes

Answer 3

Cells have an additional pathway that supplies materials to lysosomes called autophagy. This is used to degrade obsolete (out-dated) parts of the cell- the cell literally eats itself. The process begins with the enclosure of the organelle by a double membrane, creating an autophagosome, which then fuses with a lysosome. Autophagy of organelles and cytosolic proteins increases when eukaryotic cells are starved or when they remodel themselves extensively during development. The amino acids generated can be recycled to allow continued protein synthesis.

Question 4

Describe receptor-mediated endocytosis

Answer 4

• Mechanism of selective uptake of material by animal cells in which a macromolecule binds to a receptor in the plasma membrane and enters the cell in a clarthin-coated vesicle. These are patches of plasma membrane.
• Particles are selectively taken to be delivered to the lysosome for digestion.
• There are receptors on the membrane that are complimentary to the extracellular substrate that needs to be digested.
• Substrate binds to receptor.
• Membrane invaginates to form a coated pit and then a vesicle around the substrate.
• Receptors are recycled and put back on the plasma membrane.
• Pinocytosis – Type of endocytosis in which soluble materials are taken up from the environment and incorporated into vesicle for digestion.
• LDL (low density lipoprotein) binds to LDL receptors on the cell surface and Is internalised in clathrin-coated vesicles, the pH drops, this means there is a higher H+ concentration which is required to produce ATP. The vesicles lose their coat and then fuse with endosomes. In the acidic environment of the endosome, LDL dissociates from its receptors. The LDL ends up in lysosomes, where it is degraded to release free cholesterol (red dots), but the LDL receptors are returned to the plasma membrane via transport vesicles to be used again.

Question 5

Describe phagocytosis of RBCs

Answer 5

• A form of endocytosis.
• Phagocytic cells including macrophages and neutrophils (Defends us against infection by ingesting invading microorganisms).
• To be Taken up by macrophages or neutrophils, particles must first bind to the phagocytic cell surface and activate one of a variety of surface receptors. Some of these receptors recognise antibodies, the proteins that help protect us against infection by binding to the surface of microorganism, Binding of antibody-coated bacteria to these receptors induces the phagocytic cell to extend sheet like projections of the plasma membrane, called pseudopods, that engulf the bacterium and fuse at their tips to form a phagosome. The phagosome then fuses with a lysosome, and the microbe is destroyed.
• Image 1 macrophage over RBC, 2.3 show macrophage engulfing red blood cell.

Question 6

Describe I-cell disease

Answer 6

• I-cell disease is due to a single gene defect: recessive.
• All the hydrolases missing from lysosomes are found in the blood: because they fail to sort properly in the Golgi apparatus.
• They are secreted rather than transported to lysosomes.
• Due to defective or missing GlcNAc phosphotransferase.
• Lysosomal enzymes are not phosphorylated in the cis Golgi network.
• The M6P receptors do not segregate them into the appropriate transport vesicles in the TGN.
• Instead, the lysosomal hydrolases are carried to the cell surface and secreted.
• Mutation of an enzyme will change its shape, causing problems with attachment of the M6P.
• GNPTG cause mucolipidosis type 111 (pseudo-hurler polydystrophy).
• Mutation in GNPTAB cause mucolipidosis type 11.
• No phosphorylation and glycosylation of enzyme will mean that the enzymes are secreted.

Question 7

Describe the symptoms of I-cell disease

Answer 7

• Skeletal abnormalities.
• Developmental delay.
• Enlarged liver and spleen.
• Impaired hearing.
• Growth problems
• Death from pneumonia or congestive heart failure usually occurs within the first decade of life.
• Autosomal-recessive disorder caused by a deficiency of the enzyme UDP-N -acetylglucosamine: N-acetylglucosaminyl-1-phosphotransferase (“GlcNAc phospho- transferase”).