Lysosome definition
Lysosome refers to the organelle present in eukaryotic cells which has as its main function the degradation of particles.
Lysosome Characteristics
Lysosomes are membrane-bound organelles present in eukaryotic cells, Figure 1. Inside, these organelles contain various hydrolytic enzymes, such as proteases, nucleases, lipases, phosphatases, among others. All hydrolytic enzymes present in lysosomes are acid hydrolases and, therefore, are capable of hydrolyzing DNA, RNA, polysaccharides, proteins and lipids.
Figure 1 – Electron microscopy image of the endocytic pathway. E – inicial endosome; M – late endosome; L – lysosome.
Because there is a high concentration of acid hydrolases within it, lysosome lumen has to be maintained at pH≈5, pH at which the activity of these enzymes is optimal. Maintenance of such a low pH is only possible by the existence of H+ATPase pumps on lysosome membrane, which, as the name implies, require energy (ATP) to pump H+ ions from cell cytosol into lysosome lumen.
In its membrane, lysosomes have specific receptors that allow the resulting material from degraded particles to be brought to where it is needed, inside or outside the cell. Lysosomal membrane has another particularity: it is highly glycosylated to protect its integrity against the acidic hydrolases present in the lumen.
For formation of lysosomes vesicles, different pathways merge: lysosomal proteins originate in vesicles of the secretory pathway (which has a route through endoplasmic reticulum and Golgi complex); and particles that are degraded in lysosomes originate in vesicles from the endocytic pathway or other pathways through which cell destroys its ‘garbage’ (obsolete organelles or cell parts). Macromolecules are absorbed by the endocytic pathway, in which organelles called endosomes mature into lysosomes. In the case of obsolete parts of the cell that need to be destroyed (autophagy), these are delivered to lysosomes through autophagosomes. One last route through which particles are brought to lysosomes for degradation is found in specialized cells (in humans they are macrophages and neutrophils) that form a phagosome which matures in a lysosome.
Substances that are not degraded successfully are retained in lysosomes, forming the so-called ‘residual cups’.
Lysosome anomalies
Mutations may occur in the genes encoding hydrolytic enzymes present in lysosomes and, if they do occur, an accumulation of undegraded material will ensue. These mutations lead to more than 30 human genetic diseases that are commonly referred to as ‘lysosomal storage’.
References:
Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).
