Substrate proteins destined for elimination are initially attached to polymers of the highly conserved ubiquitin (Ub) protein (see ). This covalent modification of the substrate targets the conjugated protein to a multicatalytic protease complex, the 26S proteasome . The ubiquitin attachment site(s) in substrate proteins is commonly a lysine (K) side chain. A well-defined series of enzymes orchestrates polyubiquitin attachment to proteins. Ubiquitin is first activated in an ATP-consuming reaction by an E1 ubiquitin-activating enzyme, to which it becomes attached by a high-energy thioester bond. Subsequently, the activated ubiquitin is transferred to the active site cysteine (C) of a second protein, an E2 ubiquitin-conjugating enzyme. With the aid of a third enzyme, called E3 or ubiquitin-protein ligase, the E2 catalyzes transfer of (poly)ubiquitin onto the protein destined for degradation.