Human Ubiquitin

Ubiquitin is a small (76-residue) protein expressed in all eukaryotic cells—it derives its name from its widespread (ubiquitous) distribution.

Ubiquitin is covalently conjugated to specific cellular proteins in an ATP-dependent reaction, which condenses the terminal carboxyl group of ubiquitin with lysine amino groups on target proteins.

In the first reaction, catalyzed by the ubiquitin-activating enzyme E1, a thioester is formed between the C-terminal Gly residue of ubiquitin and an internal Cys residue of E1 in a two-step process.

Formation of this high-energy covalent linkage is driven by ATP hydrolysis, via a ubiquitin-adenylate intermediate.