The lysosome is a membrane-enclosed cellular organelle that possesses distinct enzymes involved in the digestion of unwanted or damaged cellular polymers including proteins, DNA, RNA, carbohydrates, and lipids. Lysosomes also play an integral role in the breakdown of pathogenic molecules especially viral particles and organic toxins to reduce the cause of disease. Another function of lysosomes is digestion of the fragments of unwanted or aged cellular organelle such as mitochondrion.
What does the Lysosome do in a animal cell? (Function of Lysosomes)
- The lysosome in the animal cell plays a pivotal role in maintaining cellular homeostasis.
- Lysosome can digest both intra- and extracellular components.
- Play a major role in autophagy.
- Lysosomes are involved in crinophagy (secretion), in which removal of excess secretory granules takes place.
- Lysosome participates in the dissolution of blood clots.
- The acrosome is a modified lysosome that is located at the tip of the sperm and involved in fertilization.
- The lysosome is also involved in phagocytosis.
- The lysosome plays a crucial role during starvation to provide nutrients to the body.
- Involved in the secretion of thyroid hormones.
- Involved in the development of organism, e.g. degradation of tadpole tail during the metamorphous by lysosomal enzymes is a crucial process.
- Involved in the regression of structures or organs after the periods, e.g. Uterus, Mullerian and Wolffian bags.
- The lysosome is an important organelle in the antigen-presenting cells and proteolytic cells, e.g. macrophages require lysosomes (scavenging cells of the immune system) to digest the pathogenic molecule. Hence, the abnormal macrophages in the immune system are considered a diagnostic marker for Gaucher’s disease.
What are the characteristics of lysosomes?
- Lysosomes are popular as suicidal bags of the cell and they are referred to as recycle bins of the cells as well.
- The lysosomes are single membrane acidic compartments, that contain a bunch of acid hydrolases.
- Lysosomes are present in animal cells but absent in plant cells, where lysosome function is undertaken by vacuoles or peroxisomes.
- The lysosome contains a high acidic pH environment compared to the cytoplasm of the cell. This acidic nature of the lysosomes is due to the presence of V-class proton pumps on the lysosomal membrane. They pump the protons from the cytoplasm to the inside of the lysosomes, the process requires energy.
- They have several important roles in cellular metabolism, e.g. they degrade extracellular proteins which are engulfed by endocytosis and the aged materials of cytoplasm such as organelles and cytosolic proteins.
- The lysosome contains more than 50 types of hydrolytic enzymes.
- Lysosomal enzymes are however inactive at physiological pH (7.0 to 7.8) and they can catalyze the enzymatic reaction only at their optimal acidic pH.
- If the lysosomal membrane breaks down, the enzymes become inactive at cytosolic pH.
- Acid phosphatase is a marker enzyme of lysosomes because lysosomes can be recognized by staining this enzyme.
- In some cases, Cathepsin is also considered as a marker enzyme of lysosomes.
- Lysosomes contain an enzyme labilase that makes the lysosomal membrane labile.
- The lysosomes have also contained the molecule, cortisone (stabilizer), which stabilizes the lysosomal membrane.
- Intracellular compartments, such as aged organelles, are taken into the lysosomes by a process called autophagy. For instance, when a mitochondrion comes to the end of its ten-day life, then it is engulfed by a membrane that is derived from the endoplasmic reticulum. The ER membrane enclosed-mitochondrion vesicle then fuses with a lysosome, resulting in the digestion of old mitochondria.
- Several defects in the gene that encodes specific lysosomal enzymes have been discovered. A certain defect in a functional gene may be resulted in reduced enzymatic production, affected enzymatic activity within the lysosomal environment, inappropriate localization of an enzyme within the lysosome, or faulty post-translational modification of an enzyme.
Lysosome-mediated protein degradation
Delivery of foreign or cellular proteins to lysosomes is very common in the cells. Lysosomes are terminal compartments of the endocytic and autophagic pathways. Newly synthesized lysosomal proteins/enzymes will be delivered to lysosomes from the trans-Golgi network via early endosomes, recycling endosomes, and late endosomes/multivesicular bodies. Following lysosomal fusion with late endosomes, endolysosomes, lysosomes are re-formed by a process called the maturation process.
Defective lysosomes can be noticed in different diseases including Rheumatoid arthritis, Gout, Silicosis, Asbestosis, Black lung disease, Byssinosis (cotton dust), Acidosis, Anoxia, Chronic granulomatous, etc.
- The Cell: A Molecular Approach. 2nd edition.
- Andrea Ballabio. The awesome lysosome. EMBO Mol Med. 2016 Feb; 8(2): 73–76.
- Haoxing Xu and Dejian Ren. Lysosomal Physiology. Annu Rev Physiol. 2015; 77: 57–80.
- Lysosomal membrane dynamics: structure and interorganellar movement of a major lysosomal membrane glycoprotein. J Cell Biol. 1986 May 1; 102(5): 1593–1605.
- Alberts B, Johnson A, Lewis J, et al. Transport from the Trans Golgi Network to Lysosomes. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002.
- Frances M. Platt, Barry Boland, Aarnoud C. van der Spoel. Lysosomal storage disorders: The cellular impact of lysosomal dysfunction. J Cell Biol. 2012 Nov 26; 199(5): 723–734.
- Debjyoti Bandyopadhyay, Austin Cyphersmith, Jairo A. Zapata, Y. Joseph Kim, and Christine K. Payne. Lysosome Transport as a Function of Lysosome Diameter. PLoS One. 2014; 9(1): e86847.
- Paul Luzio, Yvonne Hackmann, Nele M.G. Dieckmann, and Gillian M. Griffiths. The Biogenesis of Lysosomes and Lysosome-Related Organelles. Cold Spring Harb Perspect Biol. 2014 Sep; 6(9): a016840.