Lysosomes Flashcards
Describe the structure and function of lysosomes.
- Cells contain over 100 individual lysosomes
- Each lysosome is surrounded by a single membrane
- Lysosomal interior is acidic (pH 4-5)
- Lysosomes contain over 40 hydrolytic enzymes capable of degrading: lipids (lipases), carbohydrates (glycosidases), proteins (proteases)
- Carry out the controlled intracellular digestion of both extracellular materials & worn-out organelles
How does the lysosome not degrade the contents of the cell uncontrollably?
All lysosomal enzymes are optimally active in the acidic conditions maintained within the lysosomes. The membrane of the lysosome normally keeps these destructive enzymes out of the cytosol, but the enzymes acid dependence protects the contents of the cytosol against damage even if some of them should escape.
What is also important about the membrane of the lysosomes that maintains pH of the lysosome?
The membrane also contains a H+ pump, which pumps H+ ions into the lysosome, thereby maintaining its contents at an acidic pH.
The lysosomal membrane contains transporters that allow the final products of the digestion of macromolecules, such as amino acids, sugars & nucleotides, to be transferred to the cytosol. Here, they can be excreted or utilized by the cell.
Describe substrate delivery to lysosomes.
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.
What is the mannose 6-phosphate (M6P) pathway?
It is the major route for targeting lysosomal enzymes to lysosomes.
Describe the mannose 6-phosphate (M6P) pathway.
The specialized 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.
The tagging permits the enzymes to be sorted & packaged into transport vesicles, which bud off & deliver their contents to the lysosome via late endosomes.
Describe receptor-mediated endocytosis.
LDLs bind to a receptor on the plasma membrane. This causes the membrane to invaginate (such receptors often aggregate together).
Once inside the cell, the pH within the vesicle will drop, which will lead to the dissociation of the receptor & LDL molecule.
The receptor is then transported back to the membrane & the LDL molecules remain in the vesicle.
The vesicles are delivered to the lysosomes , where LDLs are broken down into cholesterol and fatty acids.
Describe I-Cell disease.
It results from a defective phosphotransferase (GlcNAC).
Some of the symptoms are:
- skeletal abnormalities
- development delay
- enlarged liver and spleen
- impaired hearing
- death from pneumonia or congestive heart failure, usually within the first 10 years.
How is the lysosmal membrane not degraded?
The membrane is heavily associated with glycoproteins that prevent the lumen enzymes from reaching and degrading the membrane.
How are cells (e.g. RBCs) turned over?
The cells turnover by uptake of macrophages.
Steps of phagocytosis
1) Engulfment
2) Lysosome fusion- lysosomes fuse with the phagosome to form a phagolysosome
3) Digestion
Describe receptor-mediated endocytosis
LDL’s are particles that carry cholesterol/ triglycerides in the blood
1) LDL receptors bind to the LDL’s and the receptors cluster in coated pits, with the coat formed from clathrin proteins and it clustering on the cystolic membrane.
2) Vesicle pinches off into the cell
3) As the pH drops, the LDL complex dissociates
4) Receptors go to membrane to repeat cycle
5) LDL transported to lysosomes via early and late endosomes
6) Component of LDL is cholesteryl ester; cholesterol covalently linked to fatty acids
7) Cholesteryl ester hydrolyses by lysosomal enzymes to produce cholesterol and fatty acids
8) Cholesterol exported, becomes ‘esterfied’ by Lecithin-Acytransferase (LCAT) and becomes available for membrane biogenesis.
Examples of mutations in glycosylation and what they causes.
- Mutations in GNPTG cause mucolipidosis type III (Pseudo-Hurler polydystrophy)
- Mutations in GNPTAB cause mucolipidosis type II (I-cell disease) or type III
- Mutations in GNPTAB, GNPTG or NAGPA can cause persistent stuttering
What is the enzyme involved in the generation of the M6P tag? And what is this encoded by?
GluNAc-phosphotransferase
(N-acetylglucosamine)
GNPTAB and GNPTG
Effect of I cell disease on the M6P pathway.
No generation of mannose 6-phosphate tag, so the lysosomal enzymes will follow the default pathway, rather than the M6P pathway.
