Oxidative Transformation of Dihydroflavonols and Flavan-3-ols by Anthocyanidin Synthase from Vitis vinifera

Twelve polyphenols from three distinct families (dihydroflavonols, flavan-3-ols, and flavanones) were studied as potential substrates of anthocyanidin synthase from Vitis vinifera (VvANS). Only flavan-3-ols of (2R,3S) configuration having either a catechol or gallol group on ring B are accepted as substrates. Only dihydroflavonols of (2R,3R) configuration are accepted as substrates, but a catechol or gallol group is not mandatory. Flavanones are not substrates of VvANS. HPLC and MS/MS analyses of the enzymatic products showed that the VvANS-catalyzed oxidative transformation of (+)-dihydroflavonols, such as dihydroquercetin, dihydrokaempferol and dihydromyricetin, leads only to the corresponding flavonols. Among the flavan-3-ols recognized as substrates, (+)-gallocatechin was only transformed into delphinidin by VvANS, whereas (+)-catechin was transformed into three products, including two major products that were an ascorbate–cyanidin adduct and a dimer of oxidized catechin, and a minor product that was cyanidin. Data from real-time MS monitoring of the enzymatic transformation of (+)-catechin suggest that its products are all derived from the initial C3-hydroxylation intermediate, i.e., a 3,3-gem-diol, and their most likely formation mechanism is discussed

6es Rencontres FORMIST - 2006

Actes des 6èmes rencontres FORMIST qui se sont déroulées le 15 juin 2006 à l\u27ENSSIB. Tour d\u27horizon des usages des nouvelles technologies de l\u27information par les étudiants : pratiques documentaires, outils bibliographiques, aides et formation à la recherche d\u27information, réseaux sociaux. Synthèses sur des outils particuliers : cartes heuristiques, wikis, flux RSS, blogs, etc

Biological and Catalytic Properties of Selenoproteins

Selenocysteine is a catalytic residue at the active site of all selenoenzymes in bacteria and mammals, and it is incorporated into the polypeptide backbone by a co-translational process that relies on the recoding of a UGA termination codon into a serine/selenocysteine codon. The best-characterized selenoproteins from mammalian species and bacteria are discussed with emphasis on their biological function and catalytic mechanisms. A total of 25 genes coding for selenoproteins have been identified in the genome of mammals. Unlike the selenoenzymes of anaerobic bacteria, most mammalian selenoenzymes work as antioxidants and as redox regulators of cell metabolism and functions. Selenoprotein P contains several selenocysteine residues and serves as a selenocysteine reservoir for other selenoproteins in mammals. Although extensively studied, glutathione peroxidases are incompletely understood in terms of local and time-dependent distribution, and regulatory functions. Selenoenzymes take advantage of the nucleophilic reactivity of the selenolate form of selenocysteine. It is used with peroxides and their by-products such as disulfides and sulfoxides, but also with iodine in iodinated phenolic substrates. This results in the formation of Se-X bonds (X = O, S, N, or I) from which a selenenylsulfide intermediate is invariably produced. The initial selenolate group is then recycled by thiol addition. In bacterial glycine reductase and D-proline reductase, an unusual catalytic rupture of selenium–carbon bonds is observed. The exchange of selenium for sulfur in selenoproteins, and information obtained from model reactions, suggest that a generic advantage of selenium compared with sulfur relies on faster kinetics and better reversibility of its oxidation reactions

A bacterial RNA polymerase mutant that renders λ growth independent of the N and cro functions at 42°C

SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Nomenclature and location of bacterial mutations modifying the frequency of lysogenization of E. coli by lambdoid phages

SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Kinetic and binding equilibrium studies of dihydroflavonol 4-reductase from Vitis vinifera and its unusually strong substrate inhibition

Dihydroflavonol 4-reductase (DFR), a member of the short-chain dehydrogenase family, catalyzes the last common step in the biosynthesis of flavan-3-ols and condensed tannins. Initial rates of DFR were measured by monitoring the 340- nm absorbance decrease resulting from the joint consumption of dihydroquercetin (DHQ) and NADPH, as a function of pH, temperature and ionic strength. At pH 6.5 and 30°C, substrate inhibition was observed above 30 μM DHQ. At lower/non-inhibitory DHQ concentrations, NADP+ behaves as a competitive inhibitor with respect to NADPH and as a mixed inhibitor with respect to DHQ, which supports a sequential ordered mechanism, with NADPH binding first and NADP+ released last. Binding-equilibrium data obtained by means of the chromatographic method of Hummel and Dreyer at pH 7.5 and by isothermal calorimetric titration at pH 6.5 led to the conclusion that ligands of the apoenzyme included NADPH, NADP+ and DHQ. The mecha-nism which best accounts for substrate inhibi-tion at pH 6.5 in the absence of product involves the formation of a binary non-productive E.DHQ complex. Thus, a productive ternary complex cannot be formed when DHQ binds first. This mechanism of inhibition may prevent the ac-cumulation of unstable leucoanthocyanidins within cells

J. agric. food chem.

Anthocyanidin synthase from Vitis vinifera (VvANS) catalyzes the in vitro transformation of the natural isomer of leucocyanidin, 2R,3S,4S-cis-leucocyanidin, into 2R,4S-flavan-3,3,4-triol ([M + H]+, m/z 323) and quercetin. The C3-hydroxylation product 2R,