Self-Immobilizing Fusion Enzymes for Compartmentalized Biocatalysis

The establishment of microfluidic enzyme cascades is a topical field of research and development, which is currently hampered by the lack of methodologies for mild and efficient immobilization of isolated enzymes. We here describe the use of self immobilizing fusion enzymes for the modular configuration of microfluidic packed-bed reactors. Specifically, three different enzymes, the (R)-selective alcohol dehydrogenase LbADH, the (S)-selective methylglyoxal reductase Gre2p and the NADP(H) regeneration enzyme glucose 1-dehydrogenase GDH, were genetically fused with streptavidin binding peptide, Spy and Halo-based tags, to enable their specific and directional immobilization on magnetic microbeads coated with complementary receptors. The enzyme-modified beads were loaded in four-channel microfluidic chips to create compartments that have the capability for either (R)- or (S) -selective reduction of the prochiral C-s-symmetrical substrate S-nitrononane-2,8-dione (NDK). Analysis of the isomeric hydroxyketone and diol products by chiral HPLC was used to quantitatively characterize the performance of reactors configured with different amounts of the enzymes. Long operating times of up to 14 days indicated stable enzyme immobilization and the general robustness of the reactor. Even more important, by fine-tuning of compartment size and loading, the overall product distribution could be controlled to selectively produce a single meso diol with nearly quantitative conversion (>95%) and excellent stereoselectivity (d.r. > 99:1) in a continuous flow process. We believe that our concept will be expandable to a variety of other biocatalytic or chemo-enzymatic cascade reactions.