E1 enzymes are responsible for the activation of ubiquitin, a small regulatory protein found in almost all tissues. This activation involves the ATP-dependent adenylation of the C-terminal glycine of ubiquitin, followed by the formation of a high-energy thioester bond between the ubiquitin and a cysteine residue on the E1 enzyme. This activated ubiquitin is then transferred to an E2 enzyme, setting the stage for subsequent steps in the ubiquitination pathway.
