An Integrated Engineering Approach to Intensify the Biocatalytic Metaraminol Synthesis

Metaraminol is a chiral amino alcohol and plays an important role as a precursor molecule and active pharmaceutical ingredient in industry. Its enzymatic synthesis has been developed in recent years and can serve as an alternative to conventional synthesis routes that use toxic, fossil-based resources. Although the enzymatic two-step reaction toward metaraminol has been intensively investigated in the past, full conversion has never been reached in the amine transaminase-catalyzed step. In this study, we focus on identifying and overcoming the hurdles of the transamination step to reach higher metaraminol yields. Photometric and LC-MS analyses revealed side-product formation as a major drawback for the enzymatic metaraminol synthesis. Besides the oxidation of (R)-3-OH-PAC as well as its imine formation with isopropylamine, we demonstrate for the first time the adduct formation of the cofactor pyridoxal-5'-phosphate with metaraminol. Only by changing the amine transaminase formulation to purified enzyme and increasing the concentration by tenfold, >99% product yield with a metaraminol concentration of 75 mM was reached. Further, we successfully integrated the amine donor l-alanine by applying a continuous product extraction system as an alternative to isopropylamine. We believe that our findings and optimization strategies can also serve as a blueprint for other amine-based syntheses.