Digestion Flashcards
Intracellular Digestion
Catabolism. Processes of breaking down biological structures into their component macromolecules within cytoplasmic organelles
Where does most intracellular digestion occur?
Within a membrane bound organelle - compounds that can break down macromolecules can potentially damage the cell
What are the 2 types of intracellular digestion?
- Heterophagy
- Autophagy
Heterophagy
Digestion of endocytosed materials.
- materials that entered via pinocytosis in a pinocytotic vesicle
- materials that entered via phagocytosis in a phagosome
Autophagy
Digestion of cell’s own components. Cells forms a vesicle called an autosome
What are the organelles that specialize in intracellular digestion?
- Endosomes
- Lysosomes
- Peroxisomes
- Proteasomes
What are the two endosomes and what do they do?
- Early endosome - sort endocytosed materials
- Late endosome - lysosome precursors (enzymes not activated)
Lysosomes
Digest numerous macromolecules via active acid hydrolases. Phagosomes and autosomes tend to fuse directly with lysosomes
Peroxisomes
Digest very long fatty acids via oxidative enzymes
Proteasomes
Digest misfolded proteins via proteolysis
What is the maturation to a lysosome?
Early endosome -> Late endosome -> Lysosome (decreasing in pH)
What do early endosomes, late endosomes, and lysosomes all do? What are their characteristics?
Ingest, sequester, and break down materials that enter via endocytosis
Membrane bound and have acidic lumen contents
Early Endosomes
Sorting centers
Uncouple receptors and ligands
Identify contents
Send ligands to late endosomes (usually) or lysosomes
Send receptors back to cell membrane (recycle)
Where are early endosomes located?
Close to cell membrane
Late Endosomes
Pre-Lysosomes
More acidic than early endosomes
Have same shipping label as lysosomes - mannose-6-phosphate
Where are late endosomes located? Where do they get there enzymes?
Located further from cell membrane.
Golgi send lysosomal proteoenzymes tagged with mannose-6-phosphate (inactive acid hydrolases)
Lysosomes
Digestion and destruction
Degrade macromolecules using hydrolytic enzymes (acid hydrolases)
Late endosomes becomes lysosomes when pH drops
Abundant in white blood cells
What are examples of macromolecules that can be broken down by lysosomes and into what?
Proteins -> Amino acids
Polysaccharides/Oligosaccharides -> Monosaccharides
DNA & RNA -> Nucleosides and inorganic phosphate
Tracylglyerol -> Fatty acids and glycerol
Phospholipases -> Inorganic phosphate and head group
What is the structure of lysosomes and what are some characteristics?
Vary in size
Membrane that stands up to hydrolytic enzymes
Lumen has low pH that contains 40-50+ hydrolytic enzymes
Can lysosomes be seen in LM?
Yes via immunohistochemistry or special stain for acid phosphatase
Don’t stain well in H&E
What makes the lysosome membrane able to tolerate the enzymes? What else does it have?
Extra cholesterol and heavily glycosylated on lumen side
Membrane has H+ pumps to lower pH and highly specific transport proteins
Mannose-6-Phosphate receptors
What are the paths of heterophagy? What do they take up?
Take up extracellular macromolecules
- Small stuff endocytosed via pinocytosis -> sorted by early endosomes -> sent to late endosomes -> turn into lysosomes
- Larger stuff enodcytosed via phagocytosis -> phagosomes merge with lysosomes -> phagolysosomes
What is the path of autophagy? What is taken up?
Takes up in house macromolecules and organelles
Autophagosomes merge with lysosomes -> autophagolysosomes
What happens after lysosomal digestion?
Most materials digested and sent where needed in the cell, but lysosomes can’t digest everything
Anything not digested remains in lysosome lumen
Residual bodies - spent lysosomes form
What are residual bodies?
Spent lysosomes that are elimanated via exocytosis or accumulate.
Accumulate –> lipofuscin
What occurs with lysosomal storage diseases? What is an example?
Inherited lysosomal enzyme deficiencies which leads to dysfunctional lysosomes that can’t break down everything it should and undigested macromolecules accumulate, leading to cell death
Diagnostic biopsies would look for accumulations of substance that would normally be digested by lysosomes.
Example. Tay-Sachs Disease
Tay-Sachs Disease
Lysosomal Storage Disease. Lysosomal enzyme that breaks down gangliosides is malformed. Gangliosides accumulate in CNS
What are the functions of peroxisomes? (x10 overall) What do they mainly do?
Mainly do enzymatic oxidation
- Digestion
- Enzymatic oxidation of EtOH, phenol, and other toxins
- Beta oxidation of very long chain fatty acids and some long chain fatty acids
- Beta oxidation of polyunsaturated fatty acids
- alpha oxidation of some branch chain fatty acids - Maintain cellular redox balance and ROS homeostasis
- Regulate H2O2 levels in cell
- Prevents oxidative stress and imbalances between ROS generation and antioxidant defenses - Synthesize plasmaolgens - common in myelin sheath
- Help synthesize bile acids (liver cells only)
What occurs if peroxisomes defects affect plasmalogen synthesis?
Abnormalities in myelin sheath covering nerve cell axons
How do peroxisomes maintain redox balance and ROS homeostasis? Why?
Have enzymes in lumen that generate H2O2 (oxidases) and enzymes that destroy H2O2 (peroxidases).
Maintaining balance help keep oxidative stress in check and regulating H2O2 levels keeps cytosol pH close to 7.
What is the most important peroxidase?
Catalase - only in peroxisomes
What is the structure of peroxisomes? Where are they located?
Spherical membrane bound organelles (1 bilayer). Resemble late endosomes
Located throughout cytoplasm but often associated with SER and mitochondria.
What is one way you could visualize peroxisomes?
LM. Use immunohistochemistry for catalase or a special stain for catalase
What synthesizes peroxisome membrane proteins and hwo do they get to the peroxisome? What else do they synthesize
Free Polyribosomes in the cytosol and chaperonins in cytosol attach peroxisome targeting signal so proteins are shipped to peroxisome.
Free polyribosomes in cytosol also syntehsize oxidative enzymes for peroxisome lumens
Zellweger Syndrome
Some forms result from mutation in gene encoding receptor for peroxisomal targeting signal.
Shipping label is correct but receptor in peroxisomes membrane cannot read it so nothing gets into the peroxisome, including any peroxisomal enzymes
Leads to severe neurological dysfunction, craniofacia abnormalities, and liver dysfunction
No cure
What are the functions of proteasomes?
Main digesters/degraders of intracellular proteins (made in house)
Important for degrading malformed or unfolded proteins
Proteolysis
Digestion of proteins into small peptide chains
What is the structure of a proteasome? Can it be seen in LM or TEM?
Has two gates and between the two gates is the middle chamber. Hollow
Too small to resolve, even in TEM
Where does proteolysis occur on the proteasome?
Proteolysis via proteases occurs in middle chamber
What are the 3 types of proteins degraded by proteasomes?
- Newly synthesized but malformed (or denatured) proteins
- Regulator/Signaling proteins
- Antigenic proteins - epitopes cleaved and moved to cell surface
What happens when epitopes are cleaved and moved to cell surface?
Initiates immune response because white blood cells (T-lymphocytes) can interact with the epitopes
How are malformed proteins sent to proteasome? What happens next?
They are tagged with ubiquitin by chaperonins and sent to the proteasome
At the proteasome, ubiquitin molecules are removed and protein is unfolded and proteases in middle chamber break it down to small peptides
What happens if malformed proteins accumulate quickly and targeting with ubiquitin does not quite work?
The cell attempts the unfolded protein response (UPR), which slows down protein synthesis cell-wide and cell increase chaperonin production to try to refold the misfolded proteins
If this doe not work, cell becomes so stressed it enter apoptosis
What is the clinical relevance of the ubiquitin-proteasome system?
Pathogenesis of many neurodegenerative diseases. Aggregations of malformed proteins form in or near neurons.
Compound issue in older patients since proteasome activity decrease with age
Chaperone-Mediated Autophagy
Chaperonins recognize a degradation signal in a misfolded protein and bring it to lysosome. Degradation signal is a molecular side chain that is only visible if protein improperly folded
Pathway by which oxidatively damaged proteins are destroyed
Macroautophagy
A large accumulation of misfolded proteins triggers the cell to form an autophagosome around the protein aggregation
Autophagosomes has double membrane (2 bilayers) and merges with 1 or more lysosomes to becomes autophagolysosome
Cells use macroautophagy to destroy damaged or worn out organelles
