Lecture 14 - Lysosomes Flashcards
Structure and function (14)
100 lysosomes per cell. Dense. Single membrane. Heterogenous contents (lots of vesicles inside). Low, acidic pH 4.5-5. Low pH denatures the proteins e.g. of bacteria. 40 Hydrolytic enzymes (acid hydrolases), activated by proteolytic cleavage, which needs an acidic environment. Enzymes optimum between pH 4-5. Proteases Lipase Glycosidases Nucleases Phosphatases Sulfatases
Substrate delivery- Extracellular (5)
Endocytosis.
The taking in of matter by a living cell by invagination of its membrane to form a vacuole.
Fluid-phase endocytosis of molecules and lipoproteins (includes receptor-mediated endocytosis) ‘cell drinking’
Phagocytosis of particles ≥ 0.5 µm (uptake and digestion).
Substrate delivery- Intracellular (8)
• 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.”.
• 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.
Substrate delivery to lysosomes (6)
Autophagy supplies materials to cells.
Used to degrade obsolete (out-dated) parts.
Enclosure of organelle with a double membrane, forming a autophagosome.
This fuses with a lysosome.
Autophagy of organelles/cytosolic proteins increases when eukaryotic cells are starved/changing (development).
AA generated are recycled to allow continued protein synthesis.
Receptor mediated endocytosis (5)
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.
Selectively taken to lysosome for digestion.
Complimentary recptors to extracellular substrate.
Membrane invaginates forming a coated pit and a vesicle around substrate.
Receptors are recycled and put back on plasma membrane.
Pinocytosis (1)
Type of endocytosis in which soluble materials are taken up from the environment and incorporated into vesicle for digestion.
Receptor mediated endocytosis - LDL (7)
Low density lipoproteins.
LDL binds to LDL receptor on plasma membrane.
Internalised in a clathrin coated vesicle, pH drops.
Vesicles lose their coat and fuse with endosomes (acidic environment).
LDL dissociates from its receptors and ends up in the lysosome.
It is degraded producing cholesterol.
Receptors are returned to the plasma membrane va transport vesicles.
Why does the pH drop during receptor mediated endocytosis?
As the molecule being transported in (e.g. LDL) is actively transported against the concentration gradient.
Phagocytosis of RBCs (6)
A form of endocytosis.
Phagocytic cells including macrophages and neutrophils.
Particles bind to phagocytic cell surface and activate one of a variety of surface receptors.
Receptors recognise antibodies by binding to surface of microorganism.
Binding of antibody-coated bacteria to the receptors induces the phagocytic cell to extend sheet like projections - PSEUDOPODS.
These engulf the bacterium and fuse their tips to form a phagosome.
Phagosome binds with lysosome, and the microbe is destroyed.
Targeting of lysosomal hydrolases to lysosomes (6)
- Lysosomal enzyme (made in the ER) undergo glycosylation in the ER, oligosaccharides are added to the protein and then undergo modification the golgi apparatus.
- Delivered to endosomes in vesicles that bud from the TGN (Trans Golgi Network).
- Lysosomal hydrolases are recognised in the TGN due to marker: mannose-6-phosphate which are added to N-linked-oligosaccharides as the lysosomal enzyme pass through Cis-Golgi-Network.
- Without the M6P they will be taken along the default pathway.
- Transmembrane M6P receptor proteins are present in TGN binds to M6P (at pH 5.6-6.7).
- The hydrolases are packaged in Clathrin-coats (in the Clathrin coated pits of the cytosolic side) and bud off the TGN (in a coat called retromer).
I-cell disease (8)
Due to a single gene defect: recessive.
All hydrolases missing from lysosomes are found in the blood.
Secreted rather than transported to lysosomes due to no phosphorylation and glycosylation of enzyme.
Defective/missing GlcNAc phosphotransferase.
Mutation of enzyme changes its shape - problems with attachment of M6P.
GlcNAc -phosphotransferase (the components whicha re coded for by GNTAB and GNPTG).
GNPTG cause mucolipidosis type 111 (Pseudo-Hurler polydystrophy).
Mutation in GNPTAB cause mucolipidosis type 11.
I-cell disease - Defective GlcNAc (3)
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 cell surface and secreted.
I-cell disease - Symptoms (7)
o Skeletal abnormalities.
o Developmental delay.
o Enlarged liver and spleen.
o Impaired hearing.
o Growth problems
o Death from pneumonia or congestive heart failure usually occurs within the first decade of life.
o Autosomal-recessive disorder caused by a deficiency of the enzyme UDP-N -acetylglucosamine: N-acetylglucosaminyl-1-phosphotransferase (“GlcNAc phospho- transferase”).
Lysosomal storage diseases (4)
I-cell disease.
Pesudo-Hurler polydystrophy.
Sandhoff disease GM2 Gangliosidosis.
Tay-Sachs disease.
Mutations in lysosomal hydrolyses causes substrate accumulation (2)
Multilamellar bodies in Sandhoff Disease (GM2 Gangliosidosis).
Lipids can’t be degraded, these form membranes which accumulate in the lysosomes.
