<i>C</i>-Ribosylating Enzymes in the (Bio)Synthesis of <i>C</i>-Nucleosides and <i>C</i>-Glycosylated Natural Products
Martin Pfeiffer, Bernd Nidetzky
Abstract
Small-molecule glycosides that exhibit a β-d-ribosyl residue linked to an aglycone via a noncanonical C-glycosidic linkage are of great biological importance. They occur naturally in the C-nucleoside pseudouridine (Ψ), as well as in a diverse class of natural product C-ribosides that involve broad variation in the structure of the aglycone and show an interesting range of biological functions. Many of the natural C-ribosides exhibit strong antimicrobial activity, so they are relevant for the development of antibiotics. Ψ and the N1-methylated derivative thereof are used to replace uridine in synthetic RNAs for therapeutic applications, including the mRNA vaccines against SARS-CoV-2 infections. The natural C-ribosides also serve as inspirational lead structures for medicinal chemistry to develop new drug candidates. Recent evidence on C-β-riboside biosynthetic enzymes [pseudouridine synthases and 4-(d-ribofuranosyl) aminobenzene synthases] reveals a fascinating diversity of active-site structures and catalytic mechanisms used to install the crucial β-d-ribosyl residue on the aglycone. We here review the enzymes in context of their biological pathways with particular focus on the catalytic reactions and possible applications in synthesis. A family of enzymes coined by Ψ-5′-phosphate C-glycosidases involves surprising mechanistic plasticity in the formation of C-β-ribosyl linkages. The catalytic reactions and proposed underlying mechanisms of the C-glycosidase-like enzymes are reviewed here. The reverse reaction of Ψ-5′-phosphate C-glycosidase (a catabolic enzyme of the Ψ degradation pathway) is shown to provide access to Ψ and analogues thereof with structural variation in the pentose and nucleobase moieties. Enzyme cascade reactions and engineered in vivo biosynthesis are promising strategies for scalable C-β-riboside synthesis. Multistep one-pot rearrangement of uridine into Ψ represents a promising example of C-β-riboside synthesis at an industrial-level productivity.