Ubiquitin activating enzyme

Ubiquitin activating enzyme (Uba) or E1 enzyme catalyzes the first step in the ubiquitination of a protein tagged to be degraded by the proteasome. Ubiquitin activating enzyme (E1) and ATP produce a C-terminal acyl adenylated ubiquitin molecule which binds to ubiquitin conjugating enzyme (E2) (Ubc) cysteine^[1]. The Ubc then binds to ubiquitin ligase (E3) via a conserved binding region. E3 catalyzes the transfer of ubiquitin from the Ubc-ubiquitin complex to a lysine residue of the target protein. Uba3 is the catalytic subunit of NEDD8 activating enzyme and APPB1 is its regulatory subunit. Similar to ubiquitin and SUMO, NEDD8 binds to proteins after processing of its C-terminal.
For more details on Uba1 see Uba1.
For ubiquitin-like modifier-activating enzyme Atg7 see Autophagy-related protein

(PDB entry 1y8x). .

3D structures of Ubiquitin activating enzyme