Efficient chiral synthesis by Saccharomyces cerevisiae spore encapsulation of Candida parapsilosis Glu228Ser/(S)-carbonyl reductase II and Bacillus sp. YX-1 glucose dehydrogenase in organic solvents

Microbial alcohol dehydrogenases: recent developments and applications in asymmetric synthesis

Alcohol dehydrogenases are a well-known group of enzymes in the class of oxidoreductases that use electron transfer cofactors such as NAD(P)+/NAD(P)H for oxidation or reduction reactions of alcohols or carbonyl compounds respectively. These enzymes are utilized mainly as purified enzymes and offer some advantages in terms of green chemistry. They are environmentally friendly and a sustainable alternative to traditional chemical synthesis of bulk and fine chemicals. Industry has implemented several whole-cell biocatalytic processes to synthesize pharmaceutically active ingredients by exploring the high selectivity of enzymes. Unlike the whole cell system where cofactor regeneration is well conserved within the cellular environment, purified enzymes require additional cofactors or a cofactor recycling system in the reaction, even though cleaner reactions can be carried out with fewer downstream work-up problems. The challenge of producing purified enzymes in large quantities has been solved in large part by the use of recombinant enzymes. Most importantly, recombinant enzymes find applications in many cascade biotransformations to produce several important chiral precursors. Inevitably, several dehydrogenases were engineered as mere recombinant enzymes could not meet the industrial requirements for substrate and stereoselectivity. In recent years, a significant number of engineered alcohol dehydrogenases have been employed in asymmetric synthesis in industry. In a parallel development, several enzymatic and non-enzymatic methods have been established for regenerating expensive cofactors (NAD+/NADP+) to make the overall enzymatic process more efficient and economically viable. In this review article, recent developments and applications of microbial alcohol dehydrogenases are summarized by emphasizing notable examples.

Artificially sporulated Escherichia coli cells as a robust cell factory for interfacial biocatalysis

The natural bacterial spores have inspired the development of artificial spores, through coating cells with protective materials, for durable whole-cell catalysis. Despite attractiveness, artificial spores developed to date are generally limited to a few microorganisms with their natural endogenous enzymes, and they have never been explored as a generic platform for widespread synthesis. Here, we report a general approach to designing artificial spores based on Escherichia coli cells with recombinant enzymes. The artificial spores are simply prepared by coating cells with polydopamine, which can withstand UV radiation, heating and organic solvents. Additionally, the protective coating enables living cells to stabilize aqueous-organic emulsions for efficient interfacial biocatalysis ranging from single reactions to multienzyme cascades. Furthermore, the interfacial system can be easily expanded to chemoenzymatic synthesis by combining artificial spores with metal catalysts. Therefore, this artificial-spore-based platform technology is envisioned to lay the foundation for next-generation cell factory engineering.

Structure-based mechanisms: On the way to apply alcohol dehydrogenases/reductases to organic-aqueous systems

Alcohol dehydrogenases/reductases catalyze enantioselective syntheses of versatile chiral compounds relying on direct hydride transfer from cofactor to substrates, or to an intermediate and then to substrates. Since most of the substrates catalyzed by alcohol dehydrogenases/reductases are insoluble in aqueous solutions, increasing interest has been turning to organic-aqueous systems. However, alcohol dehydrogenases/reductases are normally instable in organic solvents, leading to the unsatisfied enantioselective synthesis efficiency. The behaviors of these enzymes in organic solvents at an atomic level are unclear, thus it is of great importance to understand its structure-based mechanisms in organic-aqueous systems to improve their relative stability. Here, we summarized the accessible structures of alcohol dehydrogenases/reductases in Protein Data Bank crystallized in organic-aqueous systems, and compared the structures of alcohol dehydrogenases/reductases which have different tolerance towards organic solvents. By understanding the catalytic behaviors and mechanisms of these enzymes in organic-aqueous systems, the efficient enantioselective syntheses mediated by alcohol dehydrogenases/reductases and further challenges are also discussed through solvent engineering and enzyme-immobilization in the last decade.