The catalytic mechanism of E1 involves several key steps: 1. Adenylation: The E1 enzyme binds ATP and ubiquitin, facilitating the transfer of AMP to the C-terminal glycine of ubiquitin, forming ubiquitin-AMP and releasing pyrophosphate. 2. Thioester Bond Formation: The catalytic cysteine of E1 attacks the carbonyl carbon of ubiquitin-AMP, displacing AMP and forming a thioester bond between the ubiquitin and the E1 enzyme. 3. Ubiquitin Transfer: The activated ubiquitin is transferred from the E1 enzyme to a cysteine residue on an E2 conjugating enzyme.
