Ubiquitin-conjugating enzymes (, UBC or E2 enzymes) catalyse the covalent attachment of ubiquitin to target proteins. Ubiquitin is conjugated to the target protein through the coordinated action of three enzyme activities designated E1, E2, and E3. The E1 or ubiquitin-activating enzyme forms, in an ATP-dependent manner, a thioester linkage between its active site cysteine and the carboxy terminus of ubiquitin. The activated ubiquitin moiety is then transferred from E1 to the active site cysteine in E2 through a trans-thiol esterification reaction. The UBC enzyme 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. The UBC core is an alpha/beta domain containing one four-stranded antiparallel β-sheet and four α-helices (). Three of these helices flank two opposite edges of the sheet, and one helix lays diagonally across one broad face of the sheet. The other face of the sheet is exposed to solvent. One turn of a 3(10)-helix is located between the fourth strand of the sheet and the second α-helix. The active site cysteine is situated in a segment between the fourth strand of the sheet and the 3(10)-helix []. The signature pattern, of this entry, contains the active-site cysteine and spans the complete catalytic domain.
Ubiquitin-conjugating enzymes (UBC or E2 enzymes) () [, , ]catalyse the covalent attachment of ubiquitin to target proteins. Ubiquitinylation is an ATP-dependent process that involves the action of at least three enzymes: a ubiquitin-activating enzyme (E1, ), a ubiquitin-conjugating enzyme (E2), and a ubiquitin ligase (E3, , ), which work sequentially in a cascade []. The E1 enzyme mediates an ATP-dependent transfer of a thioester-linked ubiquitin molecule to a cysteine residue on the E2 enzyme. The E2 enzyme then either transfers the ubiquitin moiety directly to a substrate, or to an E3 ligase, which can also ubiquitinylate a substrate.There are several different E2 enzymes (over 30 in humans), which are broadly grouped into four classes, all of which have a core catalytic domain (containing the active site cysteine), and some of which have short N- and C-terminal amino acid extensions: class I enzymes consist of just the catalytic core domain (UBC), class II possess a UBC and a C-terminal extension, class III possess a UBC and an N-terminal extension, and class IV possess a UBC and both N- and C-terminal extensions. These extensions appear to be important for some subfamily function, including E2 localisation and protein-protein interactions []. In addition, there are proteins with an E2-like fold that are devoid of catalytic activity (such as protein crossbronx from flies), but which appear to assist in poly-ubiquitin chain formation.
