Ubiquitin-conjugating enzymes (EC 2.3.2.23) (UBC or E2 enzymes) [1,2,3] catalyze the covalent attachment of ubiquitin to target proteins. An activated ubiquitin moiety is transferred from an ubiquitin-activating enzyme (E1) to E2 which later ligates ubiquitin directly to substrate proteins with or without the assistance of 'N-end' recognizing proteins (E3).

In most species there are many forms of UBC (at least 9 in yeast) which are implicated in diverse cellular functions.

A cysteine residue is required for ubiquitin-thiolester formation. There is a single conserved cysteine in UBC's and the region around that residue is conserved in the sequence of known UBC isozymes. We have used that region as a signature pattern. We also developed a profile that spans the complete catalytical domain.