Final answer:
The UPS involves ubiquitin, ubiquitin-activating (E1), -conjugating (E2), and -ligating (E3) enzymes, and the proteasome. The system targets, unfolds, and degrades unwanted proteins, using ATP hydrolysis, polyubiquitination, and proteolysis, ensuring cellular protein quality control.
Step-by-step explanation:
The ubiquitin-proteasome system (UPS) is essential for protein degradation and turnover in cells. The main components of this system include ubiquitin, ubiquitin-activating enzymes (E1), ubiquitin-conjugating enzymes (E2), ubiquitin ligases (E3), and the 26S proteasome. These players work together in a multi-step process to target and degrade unwanted, old, or damaged proteins within the cell.
The process begins with the activation of ubiquitin by ATP hydrolysis, which attaches to E1. Ubiquitin is then transferred to E2, and the target protein is recognized by E3, which facilitates the transfer of ubiquitin from E2 to the target protein. This tagging with multiple ubiquitin molecules is known as polyubiquitination. The polyubiquitinated protein is then recognized by the proteasome, primarily via its 19S regulatory 'CAP' structures.
Upon recognition, ATP hydrolysis unfolds and translocates the target protein into the core of the proteasome, where it is digested by proteolytic enzymes into short peptide fragments. These are then released into the cytoplasm and further degraded into free amino acids. The variety of proteins and ubiquitins in the cell highlights the sophistication and specificity of the UPS in protein turnover.