Question

Three classes of proteins are responsible for tagging proteins destined for degradation by the proteasome complex. Name the enzymes and briefly describe the function of each enzyme involved.

Answer 1

Tagging of small peptide to the protein for degradation is called ubiquitination.

Three enzymes are needed for this process i.e.,

E1 ( ubiquitin activating enzyme) : this enzyme activates ubiquitin protein by covalently attacking to active site cysteine residue. It is energy requiring process hence, ATP is also involved. This results in thio-ester linkage between C-terminal COOH of ubiquitin and cysteine sulfhydryl group.

E2 (Ubiquitin conjugating enzyme): thisenzyme help in transfer of Ubiquitin from E1 to active site of cysteine of E2 via trans-estrification. E2 enzyme carries Ubiquitin to lysine residue in the target enzyme.

E3 (Ubiquitin ligases): this enzyme selectively recognises the target protein/condemned protein and transfers Ubiquitin from E2 to lysine residue of the target protein. Ubiquitinylated cascade creates isopeptide bond between lysine residue of target protein and C terminal glycine of E3.