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Ubiquitin is a highly conserved 76 amino acid protein expressed in all eukariotes. Ub is found either in free form or conjugated to proteins as monomer or as chain of ubiquitin molecules. The most well characterized consequence of polyubiquitination is substrate degradation, while mono-ubiquitinated proteins are not degraded. Ubiquitin attachment to protein substrate is a complex process involving a ubiquitin activating enzyme (E1), a ubiquitin conjugating enzyme (E2) and a ubiquitin protein ligase (E3). The first ubiquitin moiety is transferred to the e-NH2 group of a Lys residue of the protein substrate to generate an isopeptide bond. In successive reactions, a poly ubiquitin chain is synthesized by processive transfer of additional activated moieties to Lys48 of the previously conjugated ubiquitin molecule. Ubiquitin K48R prevents the formation of poly ubiquitin chains via Lys48 linkages with mono ubiquitin molecules, avoiding the degradation of protein substrates.

The conserved 76 amino acid protein ubiquitin (Ub) regulates a host of intracellular processes through its enzymatic conjugation to other cellular proteins.
Ubiquitination occurs through sequential steps catalyzed by activating (E1), conjugating (E2), and ligase (E3) enzymes. The final step results in the formation of an isopeptide bond between Ub’s C-terminal glycine residue (G76) and a lysine residue of the target protein, although N-terminal ubiquitination is also known.
Outcomes of this modification include destabilization of the conjugated protein, altered protein trafficking and functional modulation.
After targeting the protein for specific localizations, ubiquitin is released from the substrate by deubiquitinating enzymes.
A mutant ubiquitin, having a Gly to Ala substitution at the C-terminus (G76A ubiquitin) supported several downstream reactions of the proteolytic pathway but inhibits the deubiquitination process.
As consequence, the Ub derivative becomes irreversibly conjugated to protein, shifting the equilibrium between the bound and unbound form in the direction of conjugation, at the expense of the free form.

UBE2D3 enzymes are human homologs of the yeast UBC4/5 family and play many important regulatory roles in inflammation and cancer. UbcH5a mediates the degradation of a myriad of short-lived regulatory proteins (such as p53 in the presence of E6/E6-AP or MDM2, c-Fos, I?B?, p105) and abnormal proteins. UBE2D3 has 88% and 89% sequence identity with UbcH5a and UbcH5b respectively.

Human Ubquitin-conjugating enzyme 7 (UbcH7) is a ubiquitin-conjugating enzyme (E2) mediating c-fos degradation, transcription factor NF-B maturation, and human papilloma virus-mediated p53 and Myc protein degradation, in vitro. The ubiquitin-conjugating enzymes (E2s) are essential components of the post-translational protein ubiquitination pathway, mediating the transfer of activated ubiquitin to substrate proteins. The human UBE2L1-UBE2L4 gene could potentially encode different isoforms of the UbcH7. UBE2L3 gene, located at chromosome 22q11.2, is the only identical family member with introns and encodes a polypeptide sequence identical to that of UbcH7.

Ubiquitin Conjugating Enzyme E2Q2 (UBE2Q2) is a protein coding gene which receives ubiquitin from the E1 complex and catalyzes its covalent attachment to various proteins. UBE2Q2 which is a part of the ubiquitin-conjugating enzyme family is detected in hypopharyngeal head and neck squamous cell carcinoma and in tumor masses.

UBE2T is part of the E2 ubiquitin-conjugating enzyme family that participates in the protein degradation pathway. UBE2T catalyzes the ATP-dependent attachment of ubiquitin to target proteins, thus tagging them for subsequent destruction by the proteasome. UBE2T is an important factor of the Faconi anemia pathway of DNA damage repair and, upon self-inactivation, may negatively regulate the Faconi pathway.

UBE2E3 or Ubiquitin-conjugating enzyme E2 E3 is part of the E2 ubiquitinconjugating enzyme family. UBE2E3 is needed for the destruction of mitotic cyclins and for cell cycle progression. The ubiquitination process covalently attaches to a short protein of 76 amino acids called ubiquitin, to a lysine residue on the target protein. When a protein has been tagged with one ubiquitin molecule, other rounds of ubiquitination form a polyubiquitin chain that is recognized by the proteasome's 19S regulatory particle, triggering the ATP-dependent unfolding of the target protein which grants passage into the proteasome's 20S core particle, where proteases degrade the target into short peptide fragments for recycling by the cell.

UBE2E1 belongs to the E2 ubiquitin-conjugating enzyme family. UBE2E1 accepts ubiquitin from the E1 complex and catalyzes its covalent attachment to other proteins. UBE2E1 catalyzes the covalent attachment of ISG15 to other proteins. UBE2E1 mediates the selective degradation of short-lived and abnormal proteins. In vitro, UBE2E1 also catalyzes 'Lys-48'-linked polyubiquitination.

UBE2D1 is a member of the ubiquitin-conjugating enzyme family. Ubiquitination involves at least 3 classes of enzymes: ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s) and ubiquitin-protein ligases (E3s). UBE2D1 is strongly related to a stimulator of iron transport (SFT), and is up-regulated in hereditary hemochromatosis. In addition, UBE2D1 functions in the ubiquitination of the tumor-suppressor protein p53 and the hypoxia-inducible transcription factor HIF1alpha by interacting with the E1 ubiquitin-activating enzyme and the E3 ubiquitin-protein ligases.

UBE2D2 belongs to the E2 ubiquitin-conjugating enzyme family. UBE2D2 takes part in the ubiquitination of the tumor-suppressor protein p53, which is induced by an E3 ubiquitin-protein ligase. UBE2D2 catalyzes ubiquitination of IkB-alpha in a SCFB-TRCP and phosphorylation dependent method.

UBE2D2 belongs to the E2 ubiquitin-conjugating enzyme family. UBE2D2 takes part in the ubiquitination of the tumor-suppressor protein p53, which is induced by an E3 ubiquitin-protein ligase. UBE2D2 catalyzes ubiquitination of IkB-alpha in a SCFB-TRCP and phosphorylation dependent method.

Human Ubquitin Conjugating Enzyme 7 (UbcH7) is a class I enzyme which functions in the stress response and the control of transcription factors. The enzyme is ubiquitously expressed with high levels of expression seen in adult muscle. UbcH7 mediates the selective degradation of short-lived and abnormal proteins and is highly homologous to UbcH5. It has been demonstrated to participate in the ubiquitinylation of p53, c-Fos and NF-?B. UbcH7 is one of two E2s (UbcH5 being the other) with which HECT domain proteins interact with UbcH7 being able to efficiently substitute for UbcH5 in E6-AP-dependent ubiquitinylation.

UBE2V2 comprises a distinct subfamily within the E2 protein family. They have sequence similarity to other ubiquitin-conjugating enzymes however require the conserved cysteine residue that is vital for the catalytic activity of E2s. UBE2V2 shares homology with ubiquitin-conjugating enzyme E2 variant 1 and yeast MMS2 gene product. UBE2V2 participates in the differentiation of monocytes and enterocytes.

UFM1 is a ubiquitin-like protein which is conjugated to target proteins by E1-like activating enzyme UBA5 and E2-like conjugating enzyme UFC1 in a manner equivalent to ubiquitylation. UFM1 localizes principally to the nucleus, but is also present in diffuse amounts in the cytoplasm. UFM1 is expressed in a variety of tissues, including kidney, brain, heart, liver and lung.

UFC1 is a member of the ubiquitin-conjugating enzyme family. UFC1 is an E2-like conjugating enzyme for ubiquitin-fold modifier-1. UFM1 is activated by UBA5 (a novel E1-like enzyme) by forming a high-energy thioester bond. Activated UFM1 is subsequently transferred to its cognate E2-like enzyme, UFC1, in a similar thioester linkage.

Ubiquitin-conjugating enzyme E2S (UBE2S) belongs to the ubiquitin-conjugating enzyme family. UBE2S is able to form a thiol ester linkage with ubiquitin in a ubiquitin activating enzyme-dependent manner, a typical property of ubiquitin carrier proteins. UBE2S catalyzes the covalent attachment of ubiquitin to other proteins. UBE2S acts as a crucial factor of the anaphase promoting complex/cyclosome (APC/C), which is a cell cycle-regulated ubiquitin ligase that controls progression through mitosis. UBE2S acts by purposely elongating 'Lys-11'-linked polyubiquitin chains initiated by the E2 enzyme UBE2C/UBCH10 on APC/C substrates, augmenting the degradation of APC/C substrates by the proteasome and promoting mitotic exit. UBE2S also acts by elongating ubiquitin chains initiated by the E2 enzyme UBE2D1/UBCH5 in vitro; it is nevertheless uncertain whether UBE2D1/UBCH5 acts as an E2 enzyme for the APC/C in vivo. UBE2S is also involved in ubiquitination and consequent degradation of VHL, resulting in an accumulation of HIF1A.

UBL5 is believed to be a reversible modulator of protein function unlike ubiquitin which is a protein degrader. UBL5 influences a nuclear phase obligatory for increasing a reaction against the risk of mitochondrial protein misfolding.

USP15 belongs to the ubiquitin specific protease (USP) family of deubiquitinating enzymes which has a vital role in ubiquitin-dependent processes through polyubiquitin chain disassembly and hydrolysis of ubiquitin-substrate bonds. USP15 connects with the COP9 signalosome, and takes part in transforming growth factor beta signalling through deubiquitination of receptor-activated SMAD transcription factors. Alternatively spliced transcript variants encoding multiple isoforms of this gene are known, and a pseudo gene of USP15 is sited on the long arm of chromosome 2.

Ubiquitin-conjugating enzyme E2 F (UBE2F) is a member of the ubiquitin-conjugating enzyme family and UBE2F subfamily. Ubiquitin-conjugating enzymes or sometimes as ubiquitin-carrier enzymes, run the second step in the ubiquitination reaction which targets a protein for degradation using the proteasome. The UBE2F receives the ubiquitin-like protein NEDD8 from the UBA3-NAE1 E1 complex and catalyzes its covalent attachment to other proteins.
RBX2-UBE2F complex neddylates specific target proteins, for example CUL5 as can be seen from the specific interaction with the E3 ubiquitin ligase RBX2, but not RBX1.

UbcH10 is an essential mediator of mitotic destruction events and cell cycle progression. It catalyzes the destruction of cyclins A and B in conjunction with the anaphase-promoting complex, and therefore, plays an important role in the control of the cell exit from mitosis This activity is essential at then end of mitosis for the inactivation of their partner kinase Cdc2 and exit from mitosis into G1 of the next cell cycle. In addition, UbcH10 bears homology to yeast PAS2, a gene that is essential for biogenesis of peroxisomes. UbcH10 is useful for in vitro ubiquitinylation reactions.

Ubiquitin D (UBD) is an ubiquitin-like modifier (UBL) of the ubiquitin protein family. UBD is an ubiquitin-like protein which can form covalent conjugates, thus targeting proteins for degradation by the 26S proteasome. It is assumed that, UBD has roles in regulation of cell cycle, innate immunity, and apoptosis. UBD is a TNF-a inducible ubiquitin-like protein with a presumed role in the immune response.

UBE2L6 belongs to the E2 ubiquitin-conjugating enzyme family. UBE2L6 enzyme is very similar in its primary structure to the enzyme encoded by UBE2L3 gene. UBE2L6 catalyzes the covalent attachment of ubiquitin to other proteins. UBE2L6 functions in the e6/e6-ap-induced ubiquitination of p53/tp53.

Ubiquitin thioesterase OTUB2 (OTUB2) is a member of the peptidase C65 family. OTUB2 functions as a hydrolase which can remove conjugated ubiquitin from proteins in vitro and may thus play a key regulatory role at the level of protein turnover by preventing degradation.

UBE2N belongs to the E2 ubiquitin-conjugating enzyme family. UBE2N catalyzes the ATP-dependent synthesis of non-canonical polyubiquitin chains, a process which doesn’t set in motion proteasomal degradation. UBE2N mediates the transcription of some target genes and is believed to have a role in cell cycle progression; cellular differentiation and DNA repair mechanisms which ensure cell survival after DNA damage.