A BAHD hydroxycinnamoyltransferase from Actaea racemosa catalyses the formation of fukinolic and cimicifugic acids

Genome-wide identification and expression analysis of the BAHD gene family in Leonurus japonicus

Acylation represents a pivotal biochemical process that is instrumental in the modification of secondary metabolites throughout the growth and developmental stages of plants. The BAHD acyltransferase family within the plant kingdom predominantly utilizes coenzyme A thioester as the acyl donor, while employing alcohol or amine compounds as the acceptor substrates to facilitate acylation reactions. Using bioinformatics approaches, the LjBAHD gene family members in the genome of Leonurus japonicus (L. japonicus) were identified and characterized including gene structure, conserved motifs, cis-acting elements, and potential gene functions. To elucidate the roles of BAHD genes in various tissues of L. japonicus, the expression profiles of LjBAHD family members across different organs were scrutinized. Under drought stress treatment, some LjBAHDs were upregulation, suggesting their potential involvement in drought response. Notably, a detailed study was conducted on a specific HCT gene (i.e., LjBAHD25) within the BAHD gene family. Analysis of its expression patterns suggested a role for LjBAHD25 in the phenylpropanoid metabolism pathway in L. japonicus, contributing to the biosynthesis of secondary metabolites with unique bioactivity. The findings of this study have established a scientific foundation for the subsequent development and functional validation of the BAHD gene family in L. japonicus.

Hydroxy(phenyl)pyruvic acid reductase in Actaea racemosa L.: a putative enzyme in cimicifugic and fukinolic acid biosynthesis

MAIN CONCLUSION

Hydroxy(phenyl)pyruvic acid reductase from Actaea racemosa catalyzes dual reactions in reducing 4-hydroxyphenylpyruvic acid as well as β-hydroxypyruvic acid. It thus qualifies to be part of fukinolic and cimicifugic acid biosynthesis and also photorespiration. The accumulation of fukinolic acid and cimicifugic acids is mainly restricted to Actaea racemosa (Ranunculaceae) and other species of the genus Actaea/Cimicifuga. Cimicifugic and fukinolic acids are composed of a hydroxycinnamic acid part esterified with a benzyltartaric acid moiety. The biosynthesis of the latter is unclear. We isolated cDNA encoding a hydroxy(phenyl)pyruvic acid reductase (GenBank OR393286) from suspension-cultured material of A. racemosa (ArH(P)PR) and expressed it in E. coli for protein production. The heterologously synthesized enzyme had a mass of 36.51 kDa and catalyzed the NAD(P)H-dependent reduction of 4-hydroxyphenylpyruvic acid to 4-hydroxyphenyllactic acid or β-hydroxypyruvic acid to glyceric acid, respectively. The optimal temperature was at 38 °C and the pH optimum at pH 7.5. NADPH is the preferred cosubstrate (Km 23 ± 4 µM). Several substrates are accepted by ArH(P)PR with β-hydroxypyruvic acid (Km 0.26 ± 0.12 mM) followed by 4-hydroxyphenylpyruvic acid (Km 1.13 ± 0.12 mM) as the best ones. Thus, ArH(P)PR has properties of β-hydroxypyruvic acid reductase (involved in photorespiration) as well as hydroxyphenylpyruvic acid reductase (possibly involved in benzyltartaric acid formation).

Complete biosynthesis of the phenylethanoid glycoside verbascoside

Verbascoside, which was first discovered in 1963, is a well-known phenylethanoid glycoside (PhG) that exhibits antioxidant, anti-inflammatory, antimicrobial, and neuroprotective activities and contributes to the therapeutic effects of many medicinal plants. However, the biosynthetic pathway of verbascoside remains to be fully elucidated. Here, we report the identification of two missing enzymes in the verbascoside biosynthesis pathway by transcriptome mining and in vitro enzymatic assays. Specifically, a BAHD acyltransferase (hydroxycinnamoyl-CoA:salidroside hydroxycinnamoyltransferase [SHCT]) was shown to catalyze the regioselective acylation of salidroside to form osmanthuside A, and a CYP98 hydroxylase (osmanthuside B 3,3'-hydroxylase [OBH]) was shown to catalyze meta-hydroxylations of the p-coumaroyl and tyrosol moieties of osmanthuside B to complete the biosynthesis of verbascoside. Because SHCTs and OBHs are found in many Lamiales species that produce verbascoside, this pathway may be general. The findings from the study provide novel insights into the formation of caffeoyl and hydroxytyrosol moieties in natural product biosynthetic pathways. In addition, with the newly acquired enzymes, we achieved heterologous production of osmanthuside B, verbascoside, and ligupurpuroside B in Escherichia coli; this work lays a foundation for sustainable production of verbascoside and other PhGs in micro-organisms.

Characterization of Two BAHD Acetyltransferases Highly Expressed in the Flowers of Jasminum sambac (L.) Aiton

Volatile benzenoid compounds are found in diverse aromatic bouquets emitted by most moth-pollinated flowers. The night-blooming Jasminum sambac is widely cultivated worldwide in the tropics and subtropics for ornamental and industrial purposes owing to its fragrant flowers. Benzylacetate is a characteristic constituent in jasmine scent which makes up to approximately 20–30% of the total emission in the headspace or extract, but the biosynthesis enzymes and the encoding genes have not yet been described. Here, we identify two cytosolic BAHD acyltransferases specifically expressed in the petals with a positive correlation closely to the emission pattern of the volatile benzenoids. Both JsBEAT1 and JsBEAT2 could use benzylalcohol and acetate-CoA as substrates to make benzylacetate in vitro. The recombinant GST-JsBEAT1 has an estimated apparent K_m of 447.3 μM for benzylalcohol and 546.0 μM for acetate-CoA, whereas in the instance of the His-JsBEAT2, the K_m values are marginally lower, being 278.7 and 317.3 μM, respectively. However, the catalytic reactions by the GST-JsBEAT1 are more efficient than that by the His-JsBEAT2, based on the steady-state k_cat parameters. Furthermore, ectopic expression of JsBEAT1 and JsBEAT2 in the transgenic P. hybrida plants, driven by a flower-specific promotor, significantly enhances the biosynthesis of benzylbenzoate and benzylacetate, as well as the total VOCs.

Catalytic function, mechanism, and application of plant acyltransferases

Acyltransferases (ATs) are important tailoring enzymes that contribute to the diversity of natural products. They catalyze the transfer of acyl groups to the skeleton, which improves the lipid solubility, stability, and pharmacological activity of natural compounds. In recent years, a number of ATs have been isolated from plants. In this review, we have summarized 141 biochemically characterized ATs during the period July 1997 to October 2020, including their function, heterologous expression systems, and catalytic mechanisms. Their catalytic performance and application potential has been further discussed.