E1activating

E1 activating refers to the initial step in the ubiquitin-proteasome system and related pathways in which a ubiquitin or ubiquitin-like protein is prepared for conjugation by an E1 activating enzyme. In this process, the E1 enzyme forms a high-energy thioester bond with the modifier protein, enabling its transfer to downstream enzymes.

Mechanism in brief: the ubiquitin or ubiquitin-like protein is first activated in an ATP-dependent reaction that converts its C-terminus into an adenosine monophosphate (ubiquitin-AMP). The catalytic cysteine of the E1 then attacks this intermediate to form an E1–modifier thioester, releasing AMP. The modifier is subsequently transferred to an E2 conjugating enzyme via transthioesterification. E3 ligases often facilitate the final transfer of the modifier to a substrate lysine, completing the conjugation as an isopeptide bond. The cycle can then repeat to modify additional substrates.

Components and diversity: the ubiquitin system centers on E1 activating enzymes, E2 conjugating enzymes, and E3 ligases. In humans, the main ubiquitin E1 is UBA1 (also called UBE1), with another E1, UBA6, providing alternative activation. There are also E1 enzymes for ubiquitin-like proteins (UBLs) such as SUMO and NEDD8, exemplified by SUMO E1 enzymes SAE1/UBA2. Different organisms possess varying sets of E1s that recognize their respective modifiers.

Biological significance: E1 activation controls the initiation of ubiquitination and UBL conjugation, influencing protein stability, localization, and activity. This affects proteostasis, cell cycle progression, DNA repair, and signaling pathways. Regulation of E1 activity is thus central to cellular homeostasis and responses to stress.

Clinical relevance: defects or dysregulation of E1 activating enzymes can contribute to diseases such as cancer and neurodegenerative disorders. E1 inhibitors are investigated as potential therapeutics to modulate ubiquitin-like modification pathways.