Lysosome A. Characteristics - Digestive organelles of animal cells - Heterogeneous population of lysosomes within a single cell - Mixture of small and large lysosomes - Bags of Hydrolytic Enzymes - Enzymes = Acid Hydrolases - Function at low pH [~pH = 5] - Low pH is maintained by a H+ Pump - Degrades proteins, lipids and other cellular constituents taken in by endocytosis, phagocytosis, etc. B. Three Pathways to Degradation in Lysosomes 1. Autophagy - Destruction of obsolete parts of the cell itself (i.e various organelles) 2. Phagocytosis - Destruction of large particles and microorganisms 3. Endocytosis - Destruction of macromolecules taken up from extracellular fluid C. Lysosomal Hydrolases 1. Lysosomal Proteins - Lysosomal hydrolases are transported via the secretory pathway and are diverted to the lysosome. - *Note: - Lysosomal Hydrolases are soluble proteins, NOT lysosomal membrane proteins - Lysosomal membrane proteins would possess a different signal than soluble lysosomal proteins. 2. Lysosomal Hydrolase Signal - Mannose-6-Phosphate (M6P)- The presence of M6P on the lysosomal hydrolase targets the lysosomal hydrolase to the
lysosome 3. Mannose 6-Phosphate Addition to Lysosomal Hydrolases in cis Golgi - Lysosomal hydrolases possess a “signal patch” that is recognized by the enzyme N-Acetylglucosamine (GlcNAc) phosphotransferase in the cis Golgi - GlcNAc phosphotransferase aligns lysosomal hydrolase within the catalytic site
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