Final answer:
The ubiquitin-proteasome system involves tagging proteins with ubiquitin to signal their degradation by the proteasome complex. This system plays a crucial role in cellular functions and preventing the accumulation of toxic proteins, whose misfolding is associated with diseases like Alzheimer's.
Step-by-step explanation:
Proteasome-Mediated Protein Degradation
The process of degrading proteins that are old, damaged, or misfolded is an essential cellular function. One of the key mechanisms involved in this process is the ubiquitin-proteasome system. Proteins that are earmarked for destruction are tagged with a small molecule known as ubiquitin. This tagging signals the proteasome, a large complex of enzymes, to target and breakdown the indicated proteins into peptide fragments. The linear sequence of amino acids in proteins determines their three-dimensional shape which, in turn, dictates their function. When proteins are incorrectly folded, they lose their function and may become toxic, contributing to disorders like Alzheimer's and Creutzfeld-Jacob disease which are associated with proteopathies.
Within the ubiquitin-proteasomal degradation pathway, three main types of enzymes are involved: ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and ubiquitin ligases (E3). In particular, E3 ubiquitin ligases confer specificity for the proteins to be degraded. Dysfunctions within this pathway are often implicated in diseases due to the buildup of dysfunctional proteins. New therapeutic approaches are being explored to mitigate effects of these protein accumulations, potentially offering insights into the treatment of related diseases.
The ubiquitin-proteasome pathway is distinct from the lysosomal degradation pathway, which also plays a role in protein turnover. Lysosomes contain proteases, like cathepsins, that break down proteins ingested by the cell. While ubiquitin tags proteins for degradation within the cytosol, lysosomes are responsible for decomposing both intracellular and extracellular proteins.