Photocatalytic Diselenide Contraction as a Tool for Site-Selective Isosteric Ubiquitylation.

Ubiquitylation is a highly conserved post-translational modification (PTM) in eukaryotes, which serves as a critical regulatory mechanism for protein homeostasis, cellular transport, signal transduction pathways, and numerous other functions. The biological function of ubiquitylation is dictated predominantly by the topology of its linkage. Deciphering ubiquitin's complex biochemistry necessitates novel synthetic methods that deliver well-defined, biosimilar ubiquitylation. To this end, a semisynthetic strategy relying on the recombinant expression of ubiquitin combined with chemoselective photocatalytic diselenide contraction (PDC) was established to enable site-selective biomimetic selenalysine-linked ubiquitylation. The modification of ubiquitin with a C-terminal selenol was fine-tuned to avoid hydrolysis. The conditions of the PDC reaction, such as solvent composition, phosphine concentration, and irradiation, were optimized for efficient ubiquitylation of a Tau F derived peptide. Furthermore, it was demonstrated that the selenalysine linkage undergoes efficient cleavage by deubiquitylating enzymes, comparable to the native isopeptide linkage. The presented method expands the toolbox of site-selective ubiquitylation techniques. It is tolerant to many functional groups and will help to further elucidate the complexities of ubiquitylation.