RESUMO
A highly efficient di-C-glycosyltransferase GgCGT was discovered from the medicinal plant Glycyrrhiza glabra. GgCGT catalyzes a two-step di-C-glycosylation of flopropione-containing substrates with conversion rates of >98%. To elucidate the catalytic mechanisms of GgCGT, we solved its crystal structures in complex with UDP-Glc, UDP-Gal, UDP/phloretin, and UDP/nothofagin, respectively. Structural analysis revealed that the sugar donor selectivity was controlled by the hydrogen-bond interactions of sugar hydroxyl groups with D390 and other key residues. The di-C-glycosylation capability of GgCGT was attributed to a spacious substrate-binding tunnel, and the G389K mutation could switch di- to mono-C-glycosylation. GgCGT is the first di-C-glycosyltransferase with a crystal structure, and the first C-glycosyltransferase with a complex structure containing a sugar acceptor. This work could benefit the development of efficient biocatalysts to synthesize C-glycosides with medicinal potential.
Assuntos
Glicosiltransferases/química , Glicosiltransferases/metabolismo , Glycyrrhiza/enzimologia , Clonagem Molecular , Cristalografia por Raios X , Glicosilação , Glicosiltransferases/genética , Glycyrrhiza/genética , Ligantes , Modelos Moleculares , Floretina/química , Floretina/metabolismo , Especificidade por Substrato , Transcriptoma , Uridina Difosfato Galactose/química , Uridina Difosfato Galactose/metabolismo , Uridina Difosfato Ácido Glucurônico/química , Uridina Difosfato Ácido Glucurônico/metabolismo , Uridina Difosfato N-Acetilglicosamina/química , Uridina Difosfato N-Acetilglicosamina/metabolismo , Uridina Difosfato Xilose/química , Uridina Difosfato Xilose/metabolismoRESUMO
Glycyrrhizin, a sweet triterpenoid saponin found in the roots and stolons of Glycyrrhiza species (licorice), is an important active ingredient in traditional herbal medicine. We previously identified two cytochrome P450 monooxygenases, CYP88D6 and CYP72A154, that produce an aglycone of glycyrrhizin, glycyrrhetinic acid, in Glycyrrhiza uralensis. The sugar moiety of glycyrrhizin, which is composed of two glucuronic acids, makes it sweet and reduces its side-effects. Here, we report that UDP-glycosyltransferase (UGT) 73P12 catalyzes the second glucuronosylation as the final step of glycyrrhizin biosynthesis in G. uralensis; the UGT73P12 produced glycyrrhizin by transferring a glucuronosyl moiety of UDP-glucuronic acid to glycyrrhetinic acid 3-O-monoglucuronide. We also obtained a natural variant of UGT73P12 from a glycyrrhizin-deficient (83-555) strain of G. uralensis. The natural variant showed loss of specificity for UDP-glucuronic acid and resulted in the production of an alternative saponin, glucoglycyrrhizin. These results are consistent with the chemical phenotype of the 83-555 strain, and suggest the contribution of UGT73P12 to glycyrrhizin biosynthesis in planta. Furthermore, we identified Arg32 as the essential residue of UGT73P12 that provides high specificity for UDP-glucuronic acid. These results strongly suggest the existence of an electrostatic interaction between the positively charged Arg32 and the negatively charged carboxy group of UDP-glucuronic acid. The functional arginine residue and resultant specificity for UDP-glucuronic acid are unique to UGT73P12 in the UGT73P subfamily. Our findings demonstrate the functional specialization of UGT73P12 for glycyrrhizin biosynthesis during divergent evolution, and provide mechanistic insights into UDP-sugar selectivity for the rational engineering of sweet triterpenoid saponins.
Assuntos
Glicosiltransferases/metabolismo , Glycyrrhiza uralensis/enzimologia , Ácido Glicirrízico/metabolismo , Arginina/química , Arginina/metabolismo , Regulação da Expressão Gênica de Plantas/genética , Glicosiltransferases/química , Glicosiltransferases/genética , Glycyrrhiza uralensis/genética , Glycyrrhiza uralensis/metabolismo , Ácido Glicirrízico/química , Cinética , Simulação de Acoplamento Molecular , Mutação , Filogenia , Raízes de Plantas/enzimologia , Raízes de Plantas/genética , Raízes de Plantas/metabolismo , Plantas Medicinais/enzimologia , Plantas Medicinais/genética , Plantas Medicinais/metabolismo , Saponinas/análise , Transcriptoma , Triterpenos/química , Triterpenos/metabolismo , Uridina Difosfato Ácido Glucurônico/química , Uridina Difosfato Ácido Glucurônico/metabolismoRESUMO
UDP-glucuronate: baicalein 7-O-glucuronosyltransferase (UBGAT) catalyzes the transfer of glucuronic acid from UDP-glucuronic acid to the 7-OH of baicalein. UBGAT was purified from cultured cells of Scutellaria baicalensis Georgi (Lamiaceae). It was purified 95-fold using various chromatography and chromatofocusing procedures to apparent homogeneity. The Mr was estimated to be 110 kDa by gel filtration chromatography with a 52 kDa subunit by SDS-PAGE. The isoelectric point was pH 4.8. UBGAT was specific to UDP-glucuronic acid as a sugar donor and flavones with substitution ortho- to the 7-OH group such its baicalein (6-OH), scutellarein (6-OH) and wogonin (8-OMe).
Assuntos
Flavanonas , Glucuronosiltransferase/isolamento & purificação , Lamiaceae/química , Proteínas de Plantas/isolamento & purificação , Cromatografia em Gel , Medicamentos de Ervas Chinesas/química , Eletroforese em Gel de Poliacrilamida , Flavonoides/química , Glucuronosiltransferase/química , Proteínas de Plantas/química , Raízes de Plantas/química , Especificidade por Substrato , Uridina Difosfato Ácido Glucurônico/químicaRESUMO
Pectins are complex polysaccharides that contain 1,4-linked alpha-D-galactosyluronic acid residues found in the primary wall of all higher plant cells. The pectic polysaccharides play critical roles in cell wall structure and in plant growth and development. As a first step in studying pectin biosynthesis a method was developed to routinely generate and purify UDP-[U-14C]galacturonic acid (UDP-[14C]GalA), the nucleotide sugar substrate for homogalacturonan biosynthesis. UDP-[14C]GalA was enzymatically synthesized by 4-epimerization of commercially available UDP-[U-14C]glucuronic acid (UDP-[14C]GlcA) using a particulate preparation from radish roots. The resulting mixture of UDP-[14C]GalA and UDP-[14C]GlcA was separated by high-performance anion-exchange chromatography using a Dionex CarboPac PA1 anion-exchange column. The UDP-sugars were detected by their absorbance at 262 nm or by pulsed amperometric detection following postcolumn addition of NaOH. The yield of UDP-[14C]GalA obtained using this procedure was 16% of the starting UDP-[14C]GlcA. Establishment of a reliable method to synthesize and purify UDP-[14C]GalA will facilitate the identification and purification of the galacturonosyltransferase(s) involved in pectin biosynthesis.