Lysosome Formation and Function

Lysosome Development

• Lysosomes contain about 47 different enzymes.

• If an enzyme is nonfunctional, it can lead to genetic diseases (e.g., Tay-Sachs disease).

Mannose-6-Phosphate Tagging

• Proteins destined for lysosomes are tagged with mannose-6-phosphate (M6P).

• In the trans-Golgi network (TGN), these tagged proteins bind to M6P receptors.

• This binding initiates receptor-mediated vesicle formation or budding.

• These vesicles contain the enzymes needed within the lysosome.

Vesicle Fusion and Endosome Formation

• Vesicles bud off from the TGN and fuse with early endosomes.

• Early endosomes are formed by vesicles coming in from outside the cell fusing together.

• The fusion of M6P-tagged vesicles with early endosomes results in the formation of late endosomes.

Enzyme Inactivation and Activation

• Lysosomal enzymes are initially inactive because they are dangerous if activated prematurely.

• Late endosomes have a proton pump (V-type ATPase) that is ATP-dependent.

• These proton pumps start pumping hydrogen ions (H+) into the endosome.

pH Regulation

• The pH inside the late endosome drops to about 5.5.

• At this pH, no further fusion occurs, and the late endosome becomes a lysosome.

• The V-type pumps continue to lower the pH even further.

• When the pH reaches between 4 and 5, the lysosomal enzymes are activated.

Acid Hydrolases

• Lysosomal enzymes are called acid hydrolases because they are activated by an acidic pH.

• These enzymes catalyze hydrolysis reactions, breaking down macromolecules.

• Each enzyme breaks down specific components (e.g., gangliosides, cerebrosides).

Breakdown Products and Transport

• Once macromolecules are digested into smaller products, they pass through the lysosomal membrane via transporters.

• These products enter the cytoplasm of the cell and are directed into various pathways based on cellular needs.

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