Engineering Cofactor Specificity of a Thermostable Phosphite Dehydrogenase for a Highly and Robust NADPH Regeneration System

Advances in aldo-keto reductases immobilization for biocatalytic synthesis of chiral alcohols

Chiral alcohols are essential building blocks of numerous pharmaceuticals and fine chemicals. Aldo-keto reductases (AKRs) constitute a superfamily of oxidoreductases that catalyze the reduction of aldehydes and ketones to their corresponding alcohols using NAD(P)H as a coenzyme. Knowledge about the crucial roles of AKRs immobilization in the biocatalytic synthesis of chiral alcohols is expanding. Herein, we reviewed the characteristics of various AKRs immobilization approaches, the applications of different immobilization materials, and the prospects of continuous flow bioreactor construction by employing these immobilized biocatalysts for synthesizing chiral alcohols. Finally, the opportunities and ongoing challenges for AKR immobilization are discussed and the outlook for this emerging area is analyzed.

Cascade reactions with two non-physiological partners for NAD(P)H regeneration via renewable hydrogen

An attractive application of hydrogenases, combined with the availability of cheap and renewable hydrogen (i.e., from solar and wind powered electrolysis or from recycled wastes), is the production of high-value electron-rich intermediates such as reduced nicotinamide adenine dinucleotides. Here, the capability of a very robust and oxygen-resilient [FeFe]-hydrogenase (CbA5H) from Clostridium beijerinckii SM10, previously identified in our group, combined with a reductase (BMR) from Bacillus megaterium (now reclassified as Priestia megaterium) was tested. The system shows a good stability and it was demonstrated to reach up to 28 ± 2 nmol NADPH regenerated s-1 mg of hydrogenase-1 (i.e., 1.68 ± 0.12 U mg-1, TOF: 126 ± 9 min-1) and 0.46 ± 0.04 nmol NADH regenerated s-1 mg of hydrogenase-1 (i.e., 0.028 ± 0.002 U mg-1, TOF: 2.1 ± 0.2 min-1), meaning up to 74 mg of NADPH and 1.23 mg of NADH produced per hour by a system involving 1 mg of CbA5H. The TOF is comparable with similar systems based on hydrogen as regenerating molecule for NADPH, but the system is first of its kind as for the [FeFe]-hydrogenase and the non-physiological partners used. As a proof of concept a cascade reaction involving CbA5H, BMR and a mutant BVMO from Acinetobacter radioresistens able to oxidize indole is presented. The data show how the cascade can be exploited for indigo production and multiple reaction cycles can be sustained using the regenerated NADPH.

Expression of bacterial phosphite dehydrogenase confers phosphite availability in a unicellular red alga Cyanidioschyzon merolae

　Microalgae are promising cell factories for producing value-added products. Large-scale microalgal cultivation suffers from invasion by contaminating microorganisms. Since most contaminating organisms cannot utilize phosphite as a unique phosphorus source, phosphite-utilizing ability may provide a growth advantage against contaminating organisms and solve this problem. Studies showed that microorganisms, typically unable to metabolize phosphite, can utilize phosphite by expressing exogenous phosphite dehydrogenase. Here, we constructed Cyanidioschyzon merolae strains introduced with the phosphite dehydrogenase gene, ptxD, from Ralstonia sp. 4506. The ptxD-introduced strains grew in a phosphite-dependent manner, with the phosphite-related growth rate almost matching that with phosphate as sole phosphorus source.

A novel salt- and organic solvent-tolerant phosphite dehydrogenase from Cyanothece sp. ATCC 51142

Phosphite dehydrogenase (PtxD) is a promising enzyme for NAD(P)H regeneration. To expand the usability of PtxD, we cloned, expressed, and analyzed PtxD from the marine cyanobacterium Cyanothece sp. ATCC 51142 (Ct-PtxD). Ct-PtxD exhibited maximum activity at pH 9.0°C and 50°C and high stability over a wide pH range of 6.0-10.0. Compared to previously reported PtxDs, Ct-PtxD showed increased resistance to salt ions such as Na+, K+, and NH4 +. It also exhibited high tolerance to organic solvents such as ethanol, dimethylformamide, and methanol when bound to its preferred cofactor, NAD+. Remarkably, these organic solvents enhanced the Ct-PtxD activity while inhibiting the PtxD activity of Ralstonia sp. 4506 (Rs-PtxD) at concentrations ranging from 10% to 30%. Molecular electrostatic potential analysis showed that the NAD+-binding site of Ct-PtxD was rich in positively charged residues, which may attract the negatively charged pyrophosphate group of NAD+ under high-salt conditions. Amino acid composition analysis revealed that Ct-PtxD contained fewer hydrophobic amino acids than other PtxD enzymes, which reduced the hydrophobicity and increased the hydration of protein surface under low water activity. We also demonstrated that the NADH regeneration system using Ct-PtxD is useful for the coupled chiral conversion of trimethylpyruvic acid into L-tert-leucine using leucine dehydrogenase under high ammonium conditions, which is less supported by the Rs-PtxD enzyme. These results imply that Ct-PtxD might be a potential candidate for NAD(P)H regeneration in industrial applications under the reaction conditions containing salt and organic solvent.

Identification and investigation of a novel NADP+-dependent secoisolariciresinol dehydrogenase from Isatis indigotica

Cofactors are crucial for the biosynthesis of natural compounds, and cofactor engineering is a useful strategy for enzyme optimization due to its potential to enhance enzyme efficiency. Secoisolariciresinol dehydrogenase (SIRD) was reported to convert secoisolariciresinol into matairesinol in an NAD⁺-dependent reaction. Here, a SIRD designated as IiSIRD2 identified from Isatis indigotica was found to utilize NADP⁺ as the cofactor. To explore the structural basis for this unique cofactor preference, model-based structural analysis was carried out, and it was postulated that a variation at the GXGGXG glycine-rich motif of IiSIRD2 alters its cofactor preference. This study paves way for future investigations on SIRD cofactor specificity and cofactor engineering to improve SIRD's catalytic efficiency.

Cofactor Self-Sufficient Whole-Cell Biocatalysts for the Relay-Race Synthesis of Shikimic Acid

Shikimic acid (SA) is a key intermediate in the aromatic amino-acid biosynthetic pathway, as well as an important precursor for synthesizing many valuable antiviral drugs. The asymmetric reduction of 3-dehydroshikimic acid (DHS) to SA is catalyzed by shikimate dehydrogenase (AroE) using NADPH as the cofactor; however, the intracellular NADPH supply limits the biosynthetic capability of SA. Glucose dehydrogenase (GDH) is an efficient enzyme which is typically used for NAD(P)H regeneration in biocatalytic processes. In this study, a series of NADPH self-sufficient whole-cell biocatalysts were constructed, and the biocatalyst co-expressing Bmgdh–aroE showed the highest conversion rate for the reduction of DHS to SA. Then, the preparation of whole-cell biocatalysts by fed-batch fermentation without supplementing antibiotics was developed on the basis of the growth-coupled l-serine auxotroph. After optimizing the whole-cell biocatalytic conditions, a titer of 81.6 g/L SA was obtained from the supernatant of fermentative broth in 98.4% yield (mol/mol) from DHS with a productivity of 40.8 g/L/h, and cofactor NADP+ or NADPH was not exogenously supplemented during the whole biocatalytic process. The efficient relay-race synthesis of SA from glucose by coupling microbial fermentation with a biocatalytic process was finally achieved. This work provides an effective strategy for the biosynthesis of fine chemicals that are difficult to obtain through de novo biosynthesis from renewable feedstocks, as well as for biocatalytic studies that strictly rely on NAD(P)H regeneration.

Improved the Activity of Phosphite Dehydrogenase and its Application in Plant Biotechnology

Phosphorus (P) is a nonrenewable resource, which is one of the major challenges for sustainable agriculture. Although phosphite (Phi) can be absorbed by the plant cells through the Pi transporters, it cannot be metabolized by plant and unable to use as P fertilizers for crops. However, transgenic plants that overexpressed phosphite dehydrogenase (PtxD) from bacteria can utilize phosphite as the sole P source. In this study, we aimed to improve the catalytic efficiency of PtxD from Ralstonia sp.4506 (PtxDR4506), by directed evolution. Five mutations were generated by saturation mutagenesis at the 139th site of PtxD R4506 and showed higher catalytic efficiency than native PtxDR4506. The PtxDQ showed the highest catalytic efficiency (5.83-fold as compared to PtxDR4506) contributed by the 41.1% decrease in the K m and 2.5-fold increase in the k cat values. Overexpression of PtxDQ in Arabidopsis and rice showed increased efficiency of phosphite utilization and excellent development when phosphite was used as the primary source of P. High-efficiency PtxD transgenic plant is an essential prerequisite for future agricultural production using phosphite as P fertilizers.