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Ubiquitylation Products

The enzymes of the ubiquitylation pathway play a pivotal role in a number of cellular processes including, but not exclusively, the targeted proteasomal degradation of substrate proteins. Three classes of enzymes are involved in the process of substrate ubiquitylation; activating enzymes (E1s), conjugating enzymes (E2s) and protein ligases (E3s). Ubiquitylation of substrate proteins depends on the sequential action of these three enzymes. In an ATP-dependent first step, an E1 enzyme forms a thioester linkage with ubiquitin which is then transferred to the sulphydryl group of the active-site cysteine on an E2 enzyme forming a ubiquitin-thioester intermediate. An E3 then acts as an adaptor to bind both substrate protein and E2 'loaded' with ubiquitin. The E3 facilitates isopeptide bond formation between ubiquitin and the substrate protein.
Although still a post-translational modification, albeit involving a functional protein rather than a function group, ubiquitylation is a much more complex process than phosphorylation mainly due to the ability of ubiquitin to form polyubiquitin chains of a variety of different linkage types and complexity, but also because there are further related ubiquitin-like (Ubl; including SUMO, NEDD8, ISG15, and FAT10) proteins that may each follow a similar specific enzymatic cascade but resulting in different outcomes for the Ubl modified target substrate.  At a further level of system complexity modifications of either the E3 ligase or the substrate may alter their ability to modify substrates or themselves be modified; for example by NEDDylation (NEDD8 being a further Ubl) in the case of E3 ligases or phosphorylation in the case of both E3 ligases and substrates.

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