The hallmark reaction of P450 monooxygenase involves activation of C–H bond and the production of a hydroxyl. P450s tailoring natural product could further oxidize the hydroxyl to carboxylic acid. However, heterogeneously expressed plant P450s display poor chemo- and regioselectivity in microbes, restricting the efficient biosynthesis of related natural products. CYP72A63 is a P450 enzyme previously used for the biosynthesis of glycyrrhetinic acid, and its poor selectivity resulted in oxidation of 11-oxo-β-amyrin to a mixture of rare licorice triterpenoids (glycyrrhetol, glycyrrhetaldehyde, glycyrrhetinic acid, and 29-OH-11-oxo-β-amyrin). In this study, we have identified key residues, which influence the enzyme–substrate hydrophobic interaction, in controlling the chemo- and regioselectivity of the enzyme and engineered the enzyme toward selectivity oxidation to hydroxyl and carboxylic acid. Moreover, tuning the redox partner of the P450 leads to selective production of glycyrrhetaldehyde, a good starting point for further modification. In this study, controlling the catalytic property of plant P450s prove to be of great use in the synthesis of desired licorice triterpenoids, which can be used in biosynthesis of other terpenoid natural products.

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During steroid bioconversion, organic solvents are widely used for facilitating hydrophobic substrate dissolution in industry. Thus, strains that tolerate organic solvents are highly desirable. IrrE, a global transcriptional factor, was introduced into Arthrobacter simplex with Δ-dehydrogenation ability. The results evidenced that IrrE did not affect cell biological traits and biotransformation performance under non-stress conditions. However, the recombinant strain achieved a productivity higher than that of the control strain in systems containing more ethanol and substrate, which coincided with cell viability under ethanol stress, the major stress factor during biotransformation. It also demonstrated that IrrE caused genome-wide transcriptional perturbation, and several defense proteins or systems were linked with higher organic solvent tolerance. IrrE simultaneously enhanced cell resistance to various stresses, and its horizontal impacts showed strain and stress dependence. In conclusion, the introduction of exogenous global regulators is an efficient approach to enhance organic solvent tolerance in steroid-transforming strains, resulting in higher productivity.

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Microbial cell factories (MCFs) are typical and widely used platforms in biomanufacturing for designing and constructing synthesis pathways of target compounds in microorganisms. In MCFs, transporter engineering is especially significant for improving the biomanufacturing efficiency and capacity through enhancing substrate absorption, promoting intracellular mass transfer of intermediate metabolites, and improving transmembrane export of target products. This review discusses the current methods and strategies of mining and characterizing suitable transporters and presents the cases of transporter engineering in the production of various chemicals in MCFs.

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Plant natural products are important secondary metabolites with several special properties and pharmacological activities, which are widely used in pharmaceutical, food, perfume, cosmetic, and other fields. However, the production of these compounds mainly relies on phytoextraction from natural plants. Because of the low contents in plants, phytoextraction has disadvantages of low production efficiency and severe environmental and ecological problems, restricting its wide applications. Therefore, microbial cell factory, especially yeast cell factory, has become an alternative technology platform for heterologous synthesis of plant natural products. Many approaches and strategies have been developed to construct and engineer the yeast cells for efficient production of plant natural products. Meanwhile, metabolic mass transfer has been proven an important factor to improve the heterologous production. Mass transfer across plasma membrane (trans-plasma membrane mass transfer) and mass transfer within the cell (intracellular mass transfer) are two major forms of metabolic mass transfer in yeast, which can be modified and optimized to improve the production efficiency, reduce the consumption of intermediate, and eliminate the feedback inhibition. This review summarized different strategies of refining metabolic mass transfer process to enhance the production efficiency of yeast cell factory (Figure 1), providing approaches for further study on the synthesis of plant natural products in microbial cell factory.

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