[show abstract][hide abstract] ABSTRACT: The attachment of sugar to flavonoids enhances their solubility. Glycosylation is performed primarily by uridine diphosphate-dependent glycosyltransferases (UGTs). The UGT from Bacillus cereus, BcGT-1 transferred three glucose molecules into kaempferol. The structural analysis of BcGT-1 showed that its substrate binding site is wider than that of flavonoid monoglucosyltransferase of plant. In order to create monoglucosyltransferase from BcGT-1, error-prone polymerase chain reaction (PCR) was performed. We analyzed 150 clones. Among them, two mutants generated only kaempferol O-monoglucoside, albeit with reduced reactivity. Unexpectedly, the two mutants harbored mutations in the amino acids located outside of the active sites. Based on the modeled structure of BcGT-1, it was proposed that the local change in the secondary structure of BcGT-1 caused the alteration of triglucosyltransferase into monoglucosyltransferase.
Journal of Microbiology and Biotechnology 10/2010; 20(10):1393-6. · 1.40 Impact Factor
[show abstract][hide abstract] ABSTRACT: The gene for one of the glycosyltransferases from Populus deltoids, PGT-3, was cloned and was expressed as a glutathione S-transferase fusion protein in Escherichia coli. Various flavonoids were used as potential substrates of the purified recombinant PGT-3. Flavones having two adjacent hydroxyl groups were served as substrate. The regioselectivity of PGT-3 depends on the hydroxyl groups of the substrate. Flavones having two adjacent hydroxyl groups in the B ring were glucosylated at the 4′-hydroxyl group. However, PGT-3 transferred a glucose group to the 3-hydroxyl group of isorhamnetin. Molecular modeling and docking and site-directed mutagenesis were carried out to engineer a PGT-3 having a specificity for isorhamnetin but not for quercetin. Glu82Leu turned out to display this activity. Using the Glu82Leu mutant and a quercetin 3′-O-methyltransferase, isorhamnetin 3-O-glucoside was synthesized.
Journal of Molecular Catalysis B Enzymatic 05/2010; 63(s 3–4):194–199. · 2.82 Impact Factor
[show abstract][hide abstract] ABSTRACT: Isoflavonoids are a class of phytoestroegens. Isoflavonone synthase (IFS) is responsible for the conversion of naringenin to genistein. IFS is a cytochrome P450 (CYP), and requires cytochrome P450 reductase (CPR) for its activity. Additionally, the majority of cytochrome P450s harbor a membrane binding domain, making them difficult to express in Escherichia coli. In order to resolve these issues, we constructed an in-frame fusion of the IFS from red clover (RCIFS) and CPR from rice (RCPR) after removing the membrane binding domain from RCIFS and RCPR reductase. The resultant fusion gene, RCIFS-RCPR, was expressed in E. coli. The conversion of naringenin into genistein was confirmed using this E. coli transformant. Following the optimization of the medium and cell density for biotransformation, 60 microM of genistein could be generated from 80 microM of naringenin. This fusion protein approach might be applicable to express other P450s in E. coli.
Journal of Microbiology and Biotechnology 12/2009; 19(12):1612-6. · 1.40 Impact Factor
[show abstract][hide abstract] ABSTRACT: O O OH OH OH OH HO O O OCH3 OH OH OH HO O O OCH3 OH OCH3 OH HO A B C Tricetin Selgin Tricin Figure 1. Structure of flavonoids used in this study and reaction scheme of ROMT-9 with tricetin. A, B, and C indicate A , B and C rings of flavonoids. SAM Tricetin Asp275 Phe167 Met184 His170 His328 Asn181 Lys270 Asp235 Thr218 SAM Selgin Asp275 Phe167 Met184 His170 His328 Asn181 Lys270 Asp235 Thr218 (a) (b) Figure 2. Docking structure of tricetin (a) and selgin (b) into the active site of ROMT9. Amino acid residues interacting with either flavonoids or SAM are indicated. Natural products from plants are diverse, and might be used as novel starting materials for pharmatheutical or neutro-theutical purposes. Plant phenylpropanoids, such as lignins, flavonoids, and anthocyanins, are a group containing abundant natural compounds that exert diverse biological effects on humans, as well as plants. 1 Phenylpropanoids derived from the amino acid phenylalanine by phenylalanine ammonia lyase (PAL) are synthesized via the phenylpropanoid pathway. 2 The hydroxyl groups of phenolic compounds are chemically active, and thus undergo several modification reactions, which result not only in structural diversity but in alterations in biolo-gical activities. 3 O-Methylation and O-glycosylation are com-mon modification reactions. 4-5 The O-methyltransferases (OMTs) mediate a transfer reaction in which hydroxyl groups of phenolic compounds serve as methyl group acceptors, and S-adenosylmethionine (SAM) functions as a donor. O-Methyla-tion induces a reduction in the chemical reactivity of phenolic hydroxyl groups, and an increase in antimicrobial activity. 6 OMTs that utilize phenylpropanoids are represented by two groups: caffeoyl coenzyme A OMTs (CCoAOMTs) and caffeic acid OMTs (COMTs). 7 Originally, CCoAOMT and COMT played roles in lignin biosynthesis. Eventually, other substrates were found that utilized both groups of OMTs. The amino acid sequences and substrate ranges of these two OMTs differ. 8 Moreover, CCoAOMTs have a molecular weight ranging from 23,000 to 27,000 and are Mg 2+ dependent, whereas the family of COMTs have molecular weights of approximately 38,000 to 43,000, and do not require Mg 2+ for their catalytic activity. The OMTs that utilize flavonoids and alkaloids are more similar to COMTs. Therefore, it has been generally accepted that the OMTs that are similar to CCoAOMTs have a substrate range distinct from those that are similar to COMT. However, recent studies have demonstrated that OMTs closely related to CCoAOMT from microorganisms and plants utilized flavonoids more efficiently than caffeoryl-CoA. 9-12 However, it remains unclear as to whether these OMTs constitute a new group of OMTs. The structures of OMTs of both groups have been deter-mined and the reaction mechanisms of OMTs have been eluci-dated. 13-15 Previously, we cloned and characterized flavonoid OMT from rice (ROMT9). 16 ROMT9 is a B-ring-specific flavonoid OMT. When tricetin is utilized, ROMT-9 immediately trans-fers two methyl groups to the 3' and 5'-hydroxyl groups of tricetin and forms tricin (3',5'-O-dimethyltricetin) (Fig. 1). The 3'-O-methyltricetin (selgin) (Fig. 1) was rarely observed even after a longer enzyme reaction period (Fig. 3A). In order to evaluate the molecular basis of this phenomenon, the struc-ture of ROMT-9 was reconstructed using molecular modeling techniques, with the caffeic acid/5-hydroxyferulic acid 3/5 O-methyltransferase (COMT) from Medicago truncatula (PDB ID: 1ky) being used as a template. The overall structure of ROMT9 is similar to that of the COMT from M. truncatula. 15 Like COMT, the C-terminal domains of ROMT9 are principally involved in binding to substrates (flavonoid and S-adenosyl-methionine (SAM)). The flavonoid binding site forms a narrow channel, which was also noted in COMT (Fig. 2). The SAM binding site of ROMT9 is more spacious than the flavonoid binding site. This narrow channel appears to be important for the positioning of flavonoids, as no specific interaction between tricetin and ROMT9 was observed, with the exception of a few hydrogen bonds (see below).