Lysosomes are spherical membrane-bound organelles that are generated by the Golgi apparatus. They contain hydrolytic enzymes, so they function as part of the cell’s recycling system. In this article, we will look at the structure, synthesis, and function of lysosomes, and consider their relevance to clinical practice.
Lysosomes are acidic membrane-bound organelles found inside cells, usually about 1 micrometre in length. Lysosomes contain numerous hydrolytic enzymes that catalyze hydrolysis reactions. The membrane surrounding the lysosome is vital in ensuring that these enzymes do not leak into the cytoplasm and damage the cell from within. To maintain the acidic pH of the lysosome, protons are actively transported to the organelle through the lysosomal membrane.
The lysosome and the enzymes it contains are synthesized separately. Lysosomal proteins are formed in the same way as any other protein. The first step is the initiation of the production of mRNA strands from the relevant DNA segments. The mRNA strands proceed to the rough endoplasmic reticulum, where ribosomes build hydrolytic enzymes.
Importantly, these are tagged with mannose-6-phosphate within the Golgi apparatus to target them to the lysosome. As a result, vesicles containing these enzymes bud off from the Golgi apparatus. Two enzymes are responsible for the binding of the mannose-6-phosphate tag: N-acetylglucosamine phosphotransferase and N-acetylglucosamine phosphoglucomutase.
This vesicle, now in the cytoplasm, then joins with a late endosome which is another acidic membrane-bound organelle. The late endosome has proton pumps within its membrane that keep its internal environment acidic. Low pH causes dissociation of the mannose-6-phosphate receptor protein. This receptor can then be recycled back to the Golgi apparatus. The phosphate group is also removed from the mannose-6-phosphate tag, to prevent the entire protein from returning to the Golgi apparatus. The late endosome may eventually mature into a lysosome, having received enzymes from the Golgi apparatus.
Hydrolytic enzymes contained within the lysosome allow foreign particles to be destroyed. Lysosomes play an important role in phagocytosis. When macrophages phagocytize foreign particles, they contain them inside a phagosome. The phagosome will then join with a lysosome to form a phagolysosome. These enzymes are critical in oxygen-independent killing mechanisms. Lysosomes also help defend against pathogen entry through endocytosis by degrading pathogens before they reach the cytoplasm.