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

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.

Nudix hydrolase WvNUDX24 is involved in borneol biosynthesis in Wurfbainia villosa

Borneol, camphor, and bornyl acetate are highly promising monoterpenoids widely used in medicine, flavor, food, and chemical applications. Bornyl diphosphate (BPP) serves as a common precursor for the biosynthesis of these monoterpenoids. Although bornyl diphosphate synthase (BPPS) that catalyzes the cyclization of geranyl diphosphate (GPP) to BPP has been identified in multiple plants, the enzyme responsible for the hydrolysis of BPP to produce borneol has not been reported. Here, we conducted in vitro and in vivo functional characterization to identify the Nudix hydrolase WvNUDX24 from W. villosa, which specifically catalyzes the hydrolysis of BPP to generate bornyl phosphate (BP), and then BP forms borneol under the action of phosphatase. Subcellular localization experiments indicated that the hydrolysis of BPP likely occurs in the cytoplasm. Furthermore, site-directed mutagenesis experiments revealed that four critical residues (R84, S96, P98, and G99) for the hydrolysis activity of WvNUDX24. Additionally, the functional identification of phosphatidic acid phosphatase (PAP) demonstrated that WvPAP5 and WvPAP10 were able to hydrolyze geranylgeranyl diphosphate (GGPP) and farnesyl diphosphate (FPP) to generate geranylgeranyl phosphate (GGP) and farnesyl phosphate (FP), respectively, but could not hydrolyze BPP, GPP, and neryl diphosphate (NPP) to produce corresponding monophosphate products. These findings highlight the essential role of WvNUDX24 in the first step of BPP hydrolysis to produce borneol and provide genetic elements for the production of BPP-related terpenoids through plant metabolic engineering and synthetic biology.

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.