Final answer:
ATP is used in the ubiquitin-proteasome system to activate ubiquitin and to unfold target proteins for entry into the proteasome, where they are degraded into peptide fragments.
Step-by-step explanation:
The ubiquitin-proteasome system (UPS) is critical for the degradation of target proteins within a cell, and several steps in this process use ATP. The initial activation of ubiquitin, an essential step in directing proteins towards degradation, is driven by ATP hydrolysis. This occurs when ubiquitin binds to a ubiquitin-activating enzyme (E1), using energy from ATP. Subsequently, ubiquitin-conjugating enzymes (E2) and ubiquitin ligases (E3) work together to transfer ubiquitin to the target protein, forming a poly-ubiquitinated complex.
When the poly-ubiquitinated protein binds to the proteasome, ATP is again crucial as it fuels the unfolding of the target protein. This unfolding step is necessary for the protein to enter the 20S core of the proteasome for degradation. Inside the proteasome, proteolytic enzymes break down the protein into short peptide fragments without the need for ATP. The ubiquitins that were previously attached to the protein are then released and recycled.