Engineering microbial cells for biometallic catalysis

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Abstract

This study investigates the use of palladised Escherichia coli as a biometallic catalyst with a view to its use in deracemisation reactions. Some bacteria can reduce metals by their use as the terminal electron acceptor in the respiratory chain, and recent work using Desulfovibrio spp. has led to the generation of a palladium bionanocatalyst with superior activity to commercially available carbon-supported Pd(0) nanoparticles. This current study has investigated the use of formate as an electron donor for the reduction of soluble Pd(II) to insoluble Pd(0) nanoparticles by E. coli, and investigates the biological mechanisms responsible for the bioreduction, and for the extracellular location of the nanoparticles.Previous work has been done at the Manchester Interdisciplinary Biocentre with the directed evolution of the monoamine oxidase enzyme of Aspergillus niger (known as MAO-N), which catalyses the oxidative deamination of terminal amines. The result has been to produce an enzyme with enhanced activity in the production of enantiomerically pure chiral amines. The modified enzyme catalyses the oxidation of the (S)-enantiomer to the corresponding imine, which is then reduced using a chemical reductant back to either the (S)- or the (R)-enantiomer, leading to an enantiomeric excess (e.e.) of the latter. After several cycles, enantiomerically pure amines are produced. In this study, E. coli transformed with a plasmid containing the variant mao-N-D5 gene insert demonstrated good activity in catalysing the oxidation of the (S)-enantiomer of the secondary amine 1-methyltetrahydroisoquinoline (MTQ) to the imine 1-methyldihydroisoquinoline (MDQ), as measured by HPLC. The ability of E. coli to reduce Pd(II) to Pd(0) was exploited in order to use the supported nanoparticles in place of the chemical reductant, catalysing the reduction of the imine back to MTQ. Following five cycles of oxidation and reduction, racemic MTQ was converted to (R)-MTQ with an enantiomeric excess of up to 93%. The activity of the biometallic catalyst immobilised in alginate beads was also assessed, and found to be the same as planktonic cells. The recyclability and long-term storage of the immobilised biocatalyst was also investigated, and it was found that freeze-drying maintained the stability of the beads for up to six weeks, which was the limit of the experiment.

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