Lysosomes Flashcards
What are lysosomes? What proteins do they contain? What is their pH optimum?
Lysosomes are membrane enclosed compartments filled with hydrolytic enzymes that are used for the controlled intracellular digestion of macromolecules . They contain about 40 types of hydrolytic enzymes , including proteases, nucleases, glycosidases, lipases, phospholipases, phosphatases, and sulfatases . All are acid hydrolases . For optimal activity they require an acid environment , and the lysosome provides this by maintaining a pH of about 5.0 in its interior. The membrane of the lysosome normally keeps the digestive enzymes out of the cytosol, but even if they should leak out, they can do little damage at the cytosolic
pH of about 7.2.
What happens with the digested material?
Transport proteins in this membrane allow the final products of the digestion of macromolecules such as amino acids, sugars, and nucleotides to be transported to the cytosol, from where they can be either excreted or reutilized by the cell.
How do lysosomes maintain the acidic pH?
An H+ pump in the lysosomal membrane uses the energy of ATP hydrolysis to pump H+ into the lysosome , thereby maintaining the lumen at its acidic pH. A similar or identical vacuolar H ATPase is thought to acidify all endocytic and
exocytic organelles, including lysosomes, endosomes, selected compartments of the Golgi apparatus , and many transport and secretory v esicles . Most of the lysosomal membrane proteins are unusually highly glycosylated, which helps to protect them from the lysosomal proteases in the lumen.
What are the three pathways of lysosomal degradation?
Lysosomes = meeting places of different internalizing-digestive processes. Digestive enzymes are delivered to them by a route that leads outward from the ER via the Golgi apparatus.
- phagocytosis -> phagosome -> lysosome
- endocytosis -> early endosome -> late endosome -> lysosome
- autophagy -> autophagosome -> lysosome
What is endocytosis?
Endocytosed molecules are initially delivered in vesicles to
endosomes . Some molecules are selectively retrieved and recycled to the plasma membrane (like some receptors), while others pass on into late endosomes . It is here that endocytosed materials first meet the lysosomal hydrolases , which are delivered to the endosome from the Golgi apparatus.
The interior of the late endosomes is mildly
acidic (pH ~6), and it is the site where the hydrolytic digestion of the endocytosed molecules begins . Mature lysosomes form from the late endosomes, accompanied
by a further decrease in internal pH . Lysosomes are thought to be produced by a gradual maturation process, during which endosomal membrane proteins are selectively retrieved from the developing lysosome by transport vesicles that deliver these proteins back to endosomes or the trans Golgi network.
What is autophagy? Name an example.
Mitochondrion has a lifetime of about 10 days (liver cells). Lysosomes containing (and presumably digesting) mitochondria, as well as other organelles. The process seems to begin with the enclosure of an organelle by membranes of unknown origin, creating an autophagosome which then fuses with a lysosome (or a late endosome).
What is the role of mannose 6-phosphate? Describe how is the M6P added to proteins and how is it recognized and transported into desired compartments.
M6P groups are added exclusively to the N-linked oligosaccharides of these soluble lysosomal enzymes as they pass through the lumen of the cis Golgi networ.
The M6P groups are recognized by transmembrane M6P receptor proteins , which are present in the trans Golgi network.
The receptor proteins bind to lysosomal hydrolases on the lumenal side of the membrane and to adaptins in assembling clathrin coats on the cytosolic side. In this way, they help package the hydrolases into clathrin coated vesicles that bud from the trans Golgi network . The vesicles subsequently deliver their contents to a late endosome.
In the late endosomes, the phosphate is removed from the mannose sugars attached to the hydrolases , further ensuring that the hydrolases do not return to the Golgi apparatus with the receptor.
What is the binding pH of M6P receptor in trans-Golgi and dissociation pH of M6P receptor in late endosomes?
The M6P receptor protein binds its specific oligosaccharide at pH 6.5 6.7 in the trans Golgi network and releases it at pH 6 , which is the pH in the interior of late endosomes .
Can M6P hydrolases and M6P receptors end up on an ‘undesired’ place? How is this fixed?
Not all of the hydrolase molecules that are tagged with M6P for delivery to lysosomes get to their proper destination. Some escape the normal packaging process in the trans Golgi network and are transported “by to the cell surface , where they are secreted into the extracellular fluid.
Some M6P receptors, however, also take a detour to the plasma membrane, where they recapture the escaped lysosomal hydrolases and return them by receptor
mediated endocytosis to lysosomes via early and late endosomes
What enzyme phosphorylates lysosomal hydrolases? How does it work?
The GlcNAc phosphotransferase enzyme that recognizes
lysosomal hydrolases in the Golgi apparatus has separate catalytic and recognition sites. The catalytic site binds both high mannose N-linked oligosaccharides and UDP-GlcNAc. The recognition site binds to a signal patch that is present only on the surface of lysosomal hydrolases . The GlcNAc is cleaved off by a second enzyme, leaving the M6P exposed.
Two enzymes act sequentially to catalyze the addition of M6P groups to lysosomal hydrolases.
-The first is a GlcNAc phosphotransferase that specifically binds the hydrolase and adds GlcNAc phosphate to one or two of the mannose residues on each oligosaccharide chain.
-The second enzyme then cleaves off the GlcNAc residue, leaving behind a newly created M6P marker . Since most lysosomal hydrolases contain multiple oligosaccharides, they acquire many M6P residues, providing a high affinity
signal for the M6P receptor.
Under which conditions do cells secrete lysosomes? Briefly explain.
Lysosomal secretion of their undigested content
enables all cells to eliminate indigestible debris . For most cells, this is used only when cells are stressed.
Some cell types, however, contain specialized lysosomes that have acquired the necessary machinery for fusion with the plasma membrane. Melanocytes in the skin produce and store pigments in their lysosomes (melanosomes) which is then released into the extracellular space. The pigment is then taken up by keratinocytes , leading to normal skin pigmentation.
Albinism = hypopigmentation, genetic disorder, defects in melanosome exocytosis
