Production of hydroxytyrosol from tyrosol via controllable oxidation with immobilized tyrosinase: A comparative study

Hydroxytyrosol (HT) is a novel functional pharmaceutical or food ingredient that can be prepared using tyrosinase (TYR) as the catalyst. In this study, cross-linked agarose resin (AG-NTA-Ni²⁺) was selected as the carrier for the one-step immobilization and purification of the enzyme via specific adsorption of the His-tag on the protein and metal ions on the carrier particles. The immobilized TYR exhibited excellent thermal and pH stabilities and retained 82.3% of its relative activity after seven cycles. Using different reduction methods, immobilized TYR was used as a catalyst to synthesize HT via the controlled oxidation of tyrosol. The enzymatic cascade reaction of co-immobilized TYR and glucose dehydrogenase (GDH) as a biocatalyst served as a bioreduction method with an HT yield of 88%. For the chemical reduction using ascorbic acid (AA) as the reducing agent, an 80% HT yield was achieved. These results indicate that the immobilization method, which relies on the specific coordination between transition metal ions and certain protein amino acids, is a potential strategy for the effective co-immobilization of TYR and GDH. The proposed affinity co-immobilization technique is among the potential strategies for the efficient production of HT.