5.16:
The Proteasome
In eukaryotes, incorrectly folded proteins are degraded through pathways including the ubiquitin-proteasome pathway.
A ubiquitin ligase recognizes signals that show the difference between normal proteins those targeted for degradation. Several ubiquitin molecules are transferred to a specific amino acid on the target protein to mark them for degradation.
An ATP-dependent protease complex called the proteasome degrades these polyubiquitinated proteins in the proteolytic core surrounded by caps.
Deubiquitinase, an enzyme in the cap, cleaves ubiquitin from the substrate permitting the ubiquitin to be recycled.
The cap uses energy from ATP hydrolysis to unfold the target protein. With each round of ATP hydrolysis, the unfolded protein is fed into the core.
Here, it is digested by the proteases to form short peptides that are released into the cytosol, where peptidases further degrade them into their constituent amino acids for reuse.
5.16:
The Proteasome
Eukaryotic cells can degrade proteins through several pathways. One of the most important among these is the ubiquitin-proteasome pathway. It helps the cell eliminate the misfolded, damaged, or unwarranted cytoplasmic proteins in a highly specific manner.
In this pathway, the target proteins are first tagged with small proteins called ubiquitin. This involves participation of a series of enzymes including— E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin ligase). The synergy of all these three enzymes helps to attach ubiquitin molecules to the target proteins covalently. These ubiquitinated proteins are then recruited to the proteasome for further processing and degradation.
Targeted destruction of proteins is critical to the well-being of the cell, and any alterations in the ubiquitin-proteasome pathway can lead to disease. For example, if misfolded proteins are not degraded, they form protein aggregates in the cytoplasm. Such protein aggregates can lead to prominent neurodegenerative disorders, such as Parkinson, Huntington, and Alzheimer's.
On the contrary, excess quality control by the ubiquitin-proteasome pathway may also lead to disease. For example, the destruction of misfolded but partially functional chloride ion channels leads to cystic fibrosis, a life-threatening disorder in humans.