Among ubiquitin-conjugating enzymes， the mammalian ubiquitin conjugating enzyme UbcH1， also known as HIP2， is unique in its ability to catalyze the in vitro synthesis of unanchored Lys48-linked poly-ubiquitin chains from mono- or poly-ubiquitin， E1， and ATP. In addition， UbcH1 can catalyse the cyclization of longer poly-ubiquitin chains， including tetra- and penta-ubiquitin. Recombinant UbcH1 charges and supports ubiquitinylation in vitro. Typical enzyme concentration to support conjugation in vitro is 100nM to 1?M. Recently， HIP2（or UbcH1) has been shown to bind to the N terminus of Huntington and may play a role in Huntington disease.
