Design of the Enzyme–Carrier Interface to Overcome the O₂ and NADH Mass Transfer Limitations of an Immobilized Flavin Oxidase

Understanding how the immobilization of enzymes on solid carriers affects their performance is paramount for the design of highly efficient heterogeneous biocatalysts. An efficient supply of substrates onto the solid phase is one of the main challenges to maximize the activity of the immobilized enzymes. Herein, we apply advanced single-particle analysis to decipher the optimal design of an immobilized NADH oxidase (NOX) whose activity depends both on O₂ and NADH concentrations. Carrier physicochemical properties and its functionality along with the enzyme distribution across the carrier were implemented as design variables to study the effects of the intraparticle concentration of substrates (O₂ and NADH) on the activity. Intraparticle O₂-sensing analysis revealed the superior performance of the enzyme immobilized at the outer surface in terms of effective supply of O₂. Furthermore, the co-immobilization of NADH and NOX within the tuned surface of porous microbeads increases the effective concentration of NADH in the surroundings of the enzyme. As a result, the optimal spatial organization of NOX and its confinement with NADH allow a 100% recovery of the activity of the soluble enzyme upon the immobilization process. By engineering these variables, we increase the NADH oxidation activity of the heterogeneous biocatalyst by up to 650% compared to NOX immobilized under suboptimal conditions. In conclusion, this work highlights the rational design and engineering of the enzyme-carrier interface to maximize the efficiency of heterogeneous biocatalysts.