Abstract
Marine-derived fungus Geotrichum candidum AS 2.361 was previously reported by our group as an active strain for the enantioselective reduction of ketones. Although some other Geotrichum strains were also found from the terrestrial sources, information on their stability and reusability is scarce. Herein, the stabilities—in terms of pH tolerance, thermostability, and storage stability, and reusability—of G. candidum AS 2.361 were described for the asymmetric reduction of a series of aromatic ketones. Two differently immobilized cells (agar immobilization and calcium alginate immobilization) as well as free cells were prepared. For three substrates (1-(3-bromophenyl) ethan-1-one (1b), 1-(2-chlorophenyl) ethan-1-one (1d), and acetophenone (1g)) immobilized cells on agar showed a great improvement in the bioreduction activities compared to the free cells, increasing yields up to 97% with ee values of 99%. Cells immobilized on agar/calcium alginate could maintain more than 90% of the original activities within the assayed pH ranges of 3.5–11, while free cells were highly sensitive to alkaline and acidic conditions. Concerning thermostability, immobilized cells on agar kept 99% of their original activities after incubation at 60 °C for 1 h, while almost no activity was detected for the free cells under the same condition. Immobilized cells were stable at 4 °C for 80 days without any activity loss, while free cells started to decrease the activity after storage at 4 °C for six days. The immobilized cells retained almost 99% activity after four reuse cycles, while free cells lost almost all the activities at on the third cycle.
Highlights
Pure alcohols are important building blocks that are often found as common structural motifs in the production of bioactive compounds [1]
The characterization of immobilized cells and non-immobilized cells was performed by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM)
The results indicated that the profile curves for both immobilizations on agar and calcium alginate shifted towards higher temperatures suggesting that between enzyme and the tested supports there is existing a 23, strong interaction, enhancing the conformation stability of the free cells
Summary
Pure alcohols are important building blocks that are often found as common structural motifs in the production of bioactive compounds [1]. With the increasing demand for the chiral alcohols, their synthesis has continuously attracted great research interest. Among the methods used for the production of chiral alcohols, asymmetric reduction of prochiral carbonyl compounds is generally considered to be a reliable, scalable, and straightforward route to enantiomerically pure alcohols. Whole-cell biocatalysts are more attractive rather than isolated enzymes used in the asymmetric reduction of ketones for the production of chiral alcohols because the whole-cell system can generate sufficient necessary cofactors for the reduction by cellular metabolism [6]. To the best of our knowledge, most of the known microorganisms have not been applied for industrial preparation of chiral alcohols, owing in large part to the relatively low catalytic activity and stereoselectivity of these microorganisms
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