Hydroxyapatite as an Enzyme Carrier in Continuous-Flow Biocatalysis: An Improved Synthesis of γ‑Glutamyl Derivatives

Biocatalysis has proved to be of great importance to pursue a more sustainable production of fine chemicals, and enzyme immobilization is a crucial tool to achieve this goal. In this work, hydroxyapatite, an eco-friendly material, was used as a support for the immobilization of a γ-glutamyltransferase from Escherichia coli, an enzyme that catalyzes the synthesis of bioactive γ-glutamyl derivatives, exploiting the noncovalent interactions between the support and the enzyme. After screening the immobilization conditions, the storage (up to 90 days) and the thermal stability (50 °C) of the obtained immobilized biocatalyst were studied, as well as its reuse in up to 10 consecutive reactions of γ-glutamylation of S-allyl-l-cysteine, to give γ-l-glutamyl-S-allyl-l-cysteine (3), a flavor enhancer occurring in garlic extract. Finally, the immobilized enzyme was used to prepare a packed-bed reactor and 3 was synthesized under continuous-flow conditions improving both the productivity (from 19.4 to 35.1 μmol·h^-1) and the space-time yield (9.7 vs 381 μmol·h^-1·mL^-1) of the reaction.