Explain why protein degradation by proteasomes requires atp even though proteolysis is an exergonic process.

Protasomes are protein complexes which degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Proteins are tagged for degradation with a small protein called ubiquitin, and the tagging reaction is catalyzed by enzymes called ubiquitin ligases. ATP is required to activate ubiquitin in the first step of linking it to the target protein. 
The reaction is
Ubiquitin + E1.SH + ATP = Ub.S.E1 + AMP + PP

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