Production of optically pure (–)-borneol by Pseudomonas monteilii TCU-CK1 and characterization of borneol dehydrogenase involved

Metabolic engineering of the borneol and camphor degradation pathways in Pseudomonas to produce optically pure bicyclic monoterpenes

Borneol, a medicinally important bicyclic monoterpene, facilitates drug transport across mucous membranes and the blood-brain barrier. Derivatives of borneol and camphor also have numerous biomedical applications. Borneol is currently industrially synthesized via the conversion of turpentine and α-pinene. However, the major product is racemic isoborneol rather than racemic borneol. Both borneol and isoborneol are degraded by the soil bacterium Pseudomonas via a well-established degradation pathway. Two indigenous Pseudomonas strains were used to convert racemic isoborneol to other optically pure bicyclic monoterpenes here. Our results showed that deletion of the camE2,5 gene alone from the strain TCU-HL1 genome led to the complete loss of borneol and camphor degradation ability. Knockout of both camE2,5 and bdh1 (TCU-HL1Δbdh1ΔcamE2,5) restored the degradation capability as the role of Bdh1 was replaced by that of Bdh2. This mutant converted racemic isoborneol into an optically pure bicyclic monoterpene, 2,5-diketocamphane, with a 45 % recovery yield. RT-qPCR results suggested that camE2,5 expression plays a pivotal role in regulating the borneol/camphor degradation cluster. While (+)-borneol, (-)-borneol and (+)-camphor can be obtained from plants for mass production purposes, (-)-camphor cannot be obtained in the same manner. P. monteilii TCU-CK1 converted racemic isoborneol into (-)-camphor and 3,6-diketocamphane, with 15 % and 10 % recovery yields, respectively. In conclusion, we report the role of camE2,5 in regulating the borneol/camphor degradation operon and biotransformation methods to produce several optically pure bicyclic monoterpenes.

Functional Characterization and Catalytic Activity Improvement of Borneol Acetyltransferase from Wurfbainia longiligularis

In plant secondary metabolite biosynthesis, acylation is a diverse physiological process, with BAHD acyltransferases playing an essential role. Borneol acetyltransferase (BAT) is an alcohol acetyltransferase, which catalyzes borneol and acetyl-CoA to synthesize bornyl acetate (BA). However, the enzymes involved in the biosynthesis of BA have so far only been characterized in Wurfbainia villosa, the studies on the WvBATs have only been conducted in vitro, and the catalytic activity was relatively low. In this research, three genes (WlBAT1, WlBAT2, and WlBAT3) have been identified to encode BATs that are capable of acetylating borneol to synthesize BA in vitro. We also determined that WlBAT1 has the highest catalytic efficiency for borneol-type substrates, including (+)-borneol, (-)-borneol, and isoborneol. Furthermore, we found that BATs could catalyze a wide range of substrate types in vitro, but in vivo, they exclusively catalyzed borneol-type substrates. Through molecular simulations and site-directed mutagenesis, it was revealed that residues D32, N36, H168, N297, N355, and H384 are crucial for the catalytic activity of WlBAT1, while the R382I-D385R double mutant of WlBAT1 exhibited an increasing acylation efficiency for borneol-type substrates in vitro and in vivo. These findings offer key genetic elements for the metabolic engineering of plants and synthetic biology to produce BA.

Genome-wide identification and functional characterization of borneol dehydrogenases in Wurfbainia villosa

MAIN CONCLUSION

Genome-wide screening of short-chain dehydrogenases/reductases (SDR) family reveals functional diversification of borneol dehydrogenase (BDH) in Wurfbainia villosa. Wurfbainia villosa is an important medicinal plant, the fruits of which accumulate abundant terpenoids, especially bornane-type including borneol and camphor. The borneol dehydrogenase (BDH) responsible for the conversion of borneol to camphor in W. villosa remains unknown. BDH is one member of short-chain dehydrogenases/reductases (SDR) family. Here, a total of 115 classical WvSDR genes were identified through genome-wide screening. These WvSDRs were unevenly distributed on different chromosomes. Seven candidate WvBDHs based on phylogenetic analysis and expression levels were selected for cloning. Of them, four BDHs can catalyze different configurations of borneol and other monoterpene alcohol substrates to generate the corresponding oxidized products. WvBDH1 and WvBDH2, preferred (+)-borneol to (-)-borneol, producing the predominant ( +)-camphor. WvBDH3 yielded approximate equivalent amount of (+)-camphor and (-)-camphor, in contrast, WvBDH4 generated exclusively (+)-camphor. The metabolic profiles of the seeds showed that the borneol and camphor present were in the dextrorotatory configuration. Enzyme kinetics and expression pattern in different tissues suggested WvBDH2 might be involved in the biosynthesis of camphor in W. villosa. All results will increase the understanding of functional diversity of BDHs.

Genome-Wide Identification of BAHD Superfamily and Functional Characterization of Bornyl Acetyltransferases Involved in the Bornyl Acetate Biosynthesis in Wurfbainia villosa

Bornyl acetate (BA) is known as a natural aromatic monoterpene ester with a wide range of pharmacological and biological activities. Borneol acetyltransferase (BAT), catalyzing borneol and acetyl-CoA to synthesize BA, is alcohol acetyltransferase, which belongs to the BAHD super acyltransferase family, however, BAT, responsible for the biosynthesis of BA, has not yet been characterized. The seeds of Wurfbainia villosa (homotypic synonym: Amomum villosum) are rich in BA. Here we identified 64 members of the BAHD gene family from the genome of W. villosa using both PF02458 (transferase) and PF07247 (AATase) as Hidden Markov Model (HMM) to screen the BAHD genes. A total of sixty-four WvBAHDs are distributed on 14 chromosomes and nine unanchored contigs, clustering into six clades; three WvBAHDs with PF07247 have formed a separated and novel clade: clade VI. Twelve candidate genes belonging to clade I-a, I-b, and VI were selected to clone and characterize in vitro, among which eight genes have been identified to encode BATs acetylating at least one type of borneol to synthesize BA. All eight WvBATs can utilize (-)-borneol as substrates, but only five WvBATs can catalyze (+)-borneol, which is the endogenous borneol substrate in the seeds of W. villosa; WvBAT3 and WvBAT4 present the better catalytic efficiency on (+)-borneol than the others. The temporal and spatial expression patterns of WvBATs indicate that WvBAT3 and WvBAT4 are seed-specific expression genes, and their expression levels are correlated with the accumulation of BA, suggesting WvBAT3 and WvBAT4 might be the two key BATs for BA synthesis in the seeds of W. villosa. This is the first report on BAT responsible for the last biosynthetic step of BA, which will contribute to further studies on BA biosynthesis and metabolism engineering of BA in other plants or heterologous hosts.

Engineering of a borneol dehydrogenase from P. putida for the enzymatic resolution of camphor

Several thousand different terpenoid structures are known so far, and many of them are interesting for applications as pharmaceuticals, flavors, fragrances, biofuels, insecticides, or fine chemical intermediates. One prominent example is camphor, which has been utilized since ancient times in medical applications. Especially (-)-camphor is gaining more and more interest for pharmaceutical applications. Hence, a commercial reliable source is needed. The natural sources for (-)-camphor are limited, and the oxidation of precious (-)-borneol would be too costly. Hence, synthesis of (-)-camphor from renewable alpha-pinene would be an inexpensive alternative. As the currently used route for the conversion of alpha-pinene to camphor produces a mixture of both enantiomers, preferably catalytic methods for the separation of this racemate are demanded to yield enantiopure camphor. Enzymatic kinetic resolution is a sustainable way to solve this challenge but requires suitable enzymes. In this study, the first borneol dehydrogenase from Pseudomonas sp. ATCC 17453, capable of catalyzing the stereoselective reduction of camphor, was examined. By using a targeted enzyme engineering approach, enantioselective enzyme variants were created with E-values > 100. The best variant was used for the enzymatic kinetic resolution of camphor racemate, yielding 79% of (-)-camphor with an ee of > 99%. KEY POINTS: • Characterization of a novel borneol dehydrogenase (BDH) from P. putida. • Development of enantioselective BDH variants for the reduction of camphor. • Enzymatic kinetic resolution of camphor with borneol dehydrogenase.

meso-borneol- and meso-carbazole-substituted porphyrins: multifunctional chromophores with tunable electronic structures and antitumor activities

Herein, a series of five C₂ symmetric H₂porphyrins with meso-borneol and meso-carbazole units have been synthesized and isolated. Their electronic structures, photophysical properties and anticancer activity were investigated.

Cloning, expression, identification and characterization of borneol dehydrogenase isozymes in Pseudomonas sp. TCU-HL1

Borneol is a bicyclic plant monoterpene. It can be degraded by soil microorganisms through the conversion of borneol dehydrogenase (BDH) and a known camphor degradation pathway. Recombinant BDH from Pseudomonas sp. TCU-HL1 was produced in the form of inclusion body. The refolded BDH1 tends to precipitate. Insoluble recombinant BDH1 was converted into a soluble form by adding glycerol in LB medium. The k_cat and k_cat/K_m values of soluble form BDH1 for (+)-borneol turned out to be about 34-fold and 45-fold higher, respectively, than those of the refolded enzyme. On the other hand, a gene knockout mutant, TCU-HL1Δbdh, was constructed to investigate the possible presence of a second copy of the bdh gene in TCU-HL1 genome. A new gene, bdh2, encoding a BDH isozyme, was identified, and the recombinant BDH2 protein was produced in a soluble form. Both bdh1 and bdh2 genes are expressed in the crude extract of wild type TCU-HL1, as shown by RT-qPCR results. Both BDH isozymes prefer to degrade (+)-borneol, rather than (-)-borneol, probably because (+)-camphor is the main form present in nature.