Application of thermodynamic principles in biocatalytic synthesis of novel nucleoside analogues

Nucleosides and analogues thereof are central molecules in modern cancer therapy and in the fight against viral infections. However, their efficient synthesis is a challenging task for synthetic chemistry. This necessitates lengthy routes and results in low atom economy. Nature’s chemistry set, on the other hand, is set up to work with the high density of functional groups of nucleosides. Nucleoside phosphorylases are known to be very regio- and stereospecific enzymes. They are part of the nucleoside salvage pathway and perform nucleoside synthesis in an impressively selective way. This makes them interesting enzymes to study concerning the chemical space they cover. In this regard, the potential of thermophilic nucleoside phosphorylases is especially intriguing due to their high stability under challenging conditions. However, the range of enzymatically accessible nucleoside analogues is underexplored and biocatalytic nucleoside synthesis has not yet reached industrial relevance. Recently, the thermodynamic rules of these reactions have been elucidated, but these findings have not been applied to actual reactions yet. This work aims to highlight the added value of biocatalytic approaches in nucleoside chemistry. To this end, the enzymes were challenged with non-natural substrates and the equilibrium constants of these reaction set-ups were determined. This made it possible to optimize reactions to obtain yield-wastebalanced processes. Using this approach, di-halogenated purine nucleosides, 4’-thionucleosides, an unexpected N7-xanthosine and various ribavirin derivates were synthesized and purified in milligram yields. The cytotoxicity in human cell lines was evaluated for selected examples. In conclusion, it is confirmed that thermophilic nucleoside phosphorylases exhibit an impressive substrate scope. This enables straight-forward diversification reactions from simple starting molecules. In this work this is exemplified by the biocatalytic synthesis of fifteen, so far undescribed, nucleoside analogues. Both sugar- and base modifications are accessible. This signifies a tremendous advantage over traditional chemical syntheses.