Ubiquitin activating enzyme (E1), repeat 1. E1, a highly conserved small protein present universally in eukaryotic cells, is part of cascade to attach ubiquitin (Ub) covalently to substrate proteins. This cascade consists of activating (E1), conjugating (E2), and/or ligating (E3) enzymes and then targets them for degradation by the 26S proteasome. E1 activates ubiquitin by C-terminal adenylation, and subsequently forms a highly reactive thioester bond between its catalytic cysteine and ubiquitin's C-terminus. E1 also associates with E2 and promotes ubiquitin transfer to the E2's catalytic cysteine. Ubiquitin-E1 is a single-chain protein with a weakly conserved two-fold repeat. This CD represents the first repeat of Ub-E1.
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