Background: This study explored the biotransformation of (–)-isopulegol using immobilized cells of Rhodococcus rhodochrous IEGM 1362 to optimize the production of new bioactive compounds. Methods: An efficient biocatalyst based on R. rhodochrous IEGM 1362 cells immobilized in a macroporous polyvinyl alcohol (PVA) cryogel matrix was developed for the production of bioactive derivatives of (–)-isopulegol. The biological characteristics of the immobilized cells were investigated using scanning and transmission electron microscopy and energy-dispersive X-ray spectroscopy methods. Results: The use of the biocatalyst increased the overall yield of target products from 54% with free cells to 87% with immobilized cells in a single cycle. Major derivatives identified included (1R,2S,5R)-5-(hydroxymethyl)-2-(prop-1-en-2-yl)cyclohexanol and (1R,3R,4S)-3-hydroxy-4-(prop-1-en-2-yl)cyclohexanecarboxylic acid, both exhibiting potential pharmacological activity. The biocatalyst retained functional activity toward monoterpenoid over 13 exploitation cycles, meeting industrial biotechnology requirements. Immobilized cells were characterized by the absence of endogenous reserve inclusions (in particular lipids) and a high intracellular iron content. Conclusions: The developed immobilized biocatalyst is promising for scaling up the production of biologically active compounds.

Background: The biotransformation of the monoterpenoid (−)-isopulegol by Rhodococcus rhodochrous yields valuable diol and hydroxy acid derivatives; however, the yields are moderate, and formation of the (S)-epoxide byproduct significantly reduces the overall selectivity. Metal oxide nanoparticles (NPs) can modulate biocatalytic activity; however, their effects on monoterpenoid biotransformation are poorly understood. Methods: We investigated the effects of NiO (50 mg/L) and CuO NPs (5 mg/L) on (−)-isopulegol conversion by R. rhodochrous IEGM 1362. Products were quantified by GC–MS; cellular responses were assessed using respirometry, zeta potential measurements, AFM-CLSM, TEM, SEM, EDX, and cell viability assays. Results: NiO NPs at 50 mg/L significantly increased the total yield of target compounds (diol and hydroxy acid) from 36.1% to 48.3%. Importantly, NiO suppressed the formation of the undesired (S)-epoxide. In contrast, CuO NPs (5 mg/L) completely abolished target product formation. Both NPs reduced cell respiratory activity, shifted zeta potential toward less negative values, and induced cell aggregation and ultrastructural damage. NiO NPs were adsorbed onto the cell surface. Conclusions: NiO NPs selectively enhance the yield of valuable oxygenated metabolites of (−)-isopulegol while inhibiting the competing epoxidation pathway. Conversely, CuO NPs are detrimental and unsuitable for this bioprocess.