Protein turnover is vital to cellular homeostasis. Many proteins are degraded efficiently only after they have been post-translationally "tagged" with a polyubiquitin chain. Ubiquitylation is a form of Post-Translational Modification (PTM): addition of a ubiquitin to the chain is catalyzed by E3 ligases. and removal of ubiquitin is catalyzed by a De-UBiquitylating enzyme (DUB). https://lecoureures.shop/product-category/ski-de-fond-equipement-fixations-classic/