Molecular and Enzymatic Characterization of Flavonoid 3'-Hydroxylase of Malus × domestica

De novo transcriptome assembly and functional analysis reveal a dihydrochalcone 3-hydroxylase(DHC3H) of wild Malus species that produces sieboldin in vivo

Sieboldin is a specialised secondary metabolite of the group of dihydrochalcones (DHC), found in high concentrations only in some wild Malus species, closely related to the domesticated apple (Malus × domestica L.). To date, the first committed step towards the biosynthesis of sieboldin remains unknown. In this study, we combined transcriptomic analysis and a de novo transcriptome assembly to identify two putative 3-hydroxylases in two wild Malus species (Malus toringo (K. Koch) Carriere syn. sieboldii Rehder, Malus micromalus Makino) whose DHC profile is dominated by sieboldin. We assessed the in vivo activity of putative candidates to produce 3-hydroxyphloretin and sieboldin by de novo production in Saccharomyces cerevisiae. We found that CYP98A proteins of wild Malus accessions (CYP98A195, M. toringo and CYP98A196, M. micromalus) were able to produce 3-hydroxyphloretin, ultimately leading to sieboldin accumulation by co-expression with PGT2. CYP98A197-198 genes of M. × domestica, however, were unable to hydroxylate phloretin in vivo. CYP98A195-196 proteins exerting 3-hydroxylase activity co-localised with an endoplasmic reticulum marker. CYP98A protein model from wild accessions showed mutations in key residues close to the ligand pocket predicted using phloretin for protein docking modelling. These mutations are located within known substrate recognition sites of cytochrome P450s, which could explain the acceptance of phloretin in CYP98A protein of wild accessions. Screening a Malus germplasm collection by HRM marker analysis for CYP98A genes identified three clusters that correspond to the alleles of domesticated and wild species. Moreover, CYP98A isoforms identified in M. toringo and M. micromalus correlate with the accumulation of sieboldin in other wild and hybrid Malus genotypes. Taken together, we provide the first evidence of an enzyme producing sieboldin in vivo that could be involved in the key hydroxylation step towards the synthesis of sieboldin in Malus species.

Novel flavonoid C-8 hydroxylase from Rhodotorula glutinis: identification, characterization and substrate scope

Background

The regioselective hydroxylation of phenolic compounds, especially flavonoids, is still a bottleneck of classical organic chemistry that could be solved using enzymes with high activity and specificity. Yeast Rhodotorula glutinis KCh735 in known to catalyze the C-8 hydroxylation of flavones and flavanones. The enzyme F8H (flavonoid C8-hydroxylase) is involved in the reaction, but the specific gene has not yet been identified. In this work, we present identification, heterologous expression and characterization of the first F8H ortho-hydroxylase from yeast.

Results

Differential transcriptome analysis and homology to bacterial monooxygenases, including also a FAD-dependent motif and a GD motif characteristic for flavin-dependent monooxygenases, provided a set of coding sequences among which RgF8H was identified. Phylogenetic analysis suggests that RgF8H is a member of the flavin monooxygenase group active on flavonoid substrates. Analysis of recombinant protein showed that the enzyme catalyzes the C8-hydroxylation of naringenin, hesperetin, eriodyctiol, pinocembrin, apigenin, luteolin, chrysin, diosmetin and 7,4ʹ-dihydroxyflavone. The presence of the C7-OH group is necessary for enzymatic activity indicating ortho-hydroxylation mechanism. The enzyme requires the NADPH coenzyme for regeneration prosthetic group, displays very low hydroxyperoxyflavin decupling rate, and addition of FAD significantly increases its activity.

Conclusions

This study presents identification of the first yeast hydroxylase responsible for regioselective C8-hydroxylation of flavonoids (F8H). The enzyme was biochemically characterized and applied in in vitro cascade with Bacillus megaterium glucose dehydrogenase reactions. High in vivo activity in Escherichia coli enable further synthetic biology application towards production of rare highly antioxidant compounds.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01899-x.

First purified recombinant CYP75B including transmembrane helix with unexpected high substrate specificity to (2R)-naringenin

Anthochlor pigments (chalcones and aurones) play an important role in yellow flower colourization, the formation of UV-honey guides and show numerous health benefits. The B-ring hydroxylation of chalcones is performed by membrane bound cytochrome P450 enzymes. It was assumed that usual flavonoid 3′-hydroxlases (F3′Hs) are responsible for the 3,4- dihydroxy pattern of chalcones, however, we previously showed that a specialized F3′H, namely chalcone 3-hydroxylase (CH3H), is necessary for the hydroxylation of chalcones. In this study, a sequence encoding membrane bound CH3H from Dahlia variabilis was recombinantly expressed in yeast and a purification procedure was developed. The optimized purification procedure led to an overall recovery of 30% recombinant DvCH3H with a purity of more than 84%. The enzyme was biochemically characterized with regard to its kinetic parameters on various substrates, including racemic naringenin, as well as its enantiomers (2S)-, and (2R)-naringenin, apigenin and kaempferol. We report for the first time the characterization of a purified Cytochrome P450 enzyme from the flavonoid biosynthesis pathway, including the transmembrane helix. Further, we show for the first time that recombinant DvCH3H displays a higher affinity for (2R)-naringenin than for (2S)-naringenin, although (2R)-flavanones are not naturally formed by chalcone isomerase.