Overcoming the challenges of cascade reactions and complex substrates in natural product biocatalysis: Immobilization of a cyclodipeptide synthase

Cyclodipeptide synthases (CDPSs) use aminoacylated tRNAs to produce cyclic dipeptide natural products which can have anticancer and neuroprotective activity. Despite their potential, applications involving CDPSs are hindered by enzyme instability and challenges in producing aminoacylated tRNAs. Immobilizing enzymes can enhance stability and recyclability, yet studies on immobilized enzymes using aminoacylated tRNAs are lacking. Here, we immobilized the CDPS enzyme from Parcubacteria bacterium RAAC4_OD1_1 (PbCDPS) using three sustainable supports: biochar from waste materials, calcium-alginate beads, and chitosan beads. Active PbCDPS immobilization led to production of the cyclodipeptide cyclo (His-Glu) (cHE). Notably, following activation with glutaraldehyde, a five-fold increase in cHE production was observed, while the immobilized enzyme remained active for seven consecutive cycles. Furthermore, we co-immobilized three enzymes required for the cascade reaction yielding cHE, all of which require aminoacyl-tRNA substrates (PbCDPS, histidyl-tRNA synthetase, and glutamyl-tRNA synthetase). This enzymatic cascade successfully generated the cyclic dipeptide of interest, showcasing the potential of immobilizing complex enzymes operating in cascade on a single support. We demonstrated that tRNAs remained free in solution without adsorption onto beads. This work paves the way for the immobilization of enzymes utilizing tRNAs and potentially other complex substrates, expanding the spectrum of reactions exploitable with this technology.