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.

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

Twelve polyphenols from three distinct families (dihydroflavonols, flavan-3-ols, and flavanones) were studied as potential substrates of anthocyanidin synthase from (ANS). Only flavan-3-ols of (2,3) configuration having either a catechol or gallol group on ring B are accepted as substrates. Only dihydroflavonols of (2,3) configuration are accepted as substrates, but a catechol or gallol group is not mandatory.

Oxidative Transformation of Leucocyanidin by Anthocyanidin Synthase from Vitis vinifera Leads Only to Quercetin.

Anthocyanidin synthase from Vitis vinifera ( VvANS) catalyzes the in vitro transformation of the natural isomer of leucocyanidin, 2 R,3 S,4 S- cis-leucocyanidin, into 2 R,4 S-flavan-3,3,4-triol ([M + H], m/ z 323) and quercetin. The C-hydroxylation product 2 R,4 S-flavan-3,3,4-triol is first produced and its C,C-dehydration product is in tautomeric equilibrium with (+)-dihydroquercetin. The latter undergoes a second VvANS-catalyzed C-hydroxylation leading to a 4-keto-2 R-flavan-3,3-gem-diol which upon dehydration gives quercetin.

Production of 3,4-cis- and 3,4-trans-Leucocyanidin and Their Distinct MS/MS Fragmentation Patterns.

(+)-2,3-trans-3,4-cis-Leucocyanidin was produced by acidic epimerization of (+)-2,3-trans-3,4-trans-leucocyanidin synthesized by reduction of (+)-dihydroquercetin with NaBH, and structures of the two stereoisomers purified by C18- and phenyl-reverse-phase high-performance liquid chromatography (HPLC) were confirmed by NMR spectroscopy. We confirm that only 3,4-cis-leucocyanidin is used by leucoanthocyanidin reductase as substrate. The two stereoisomers are quite stable in aqueous solution at -20 °C.

Crystal structure and catalytic mechanism of leucoanthocyanidin reductase from Vitis vinifera.

Leucoanthocyanidin reductase (LAR) catalyzes the NADPH-dependent reduction of 2R,3S,4S-flavan-3,4-diols into 2R,3S-flavan-3-ols, a subfamily of flavonoids that is important for plant survival and for human nutrition. LAR1 from Vitis vinifera has been co-crystallized with or without NADPH and one of its natural products, (+)-catechin. Crystals diffract to a resolution between 1.