RNA sequence and the nature of the CuA-binding site in cytochrome c oxidase

AbstractFor cytochrome c oxidase subunit II (COXII), DNA and protein sequences suggest that Met-207 (bovine numbering) is conserved in all species except plants. Sequencing of plant mitochondrial COXII mRNAs now indicates that Met-207 is also conserved among plants as a result of a C-to-U type of RNA editing. Considering the strict evolutionary conservation of Met-207 and the homology of COXII to type I (blue) copper proteins and nitrous oxide reductase, we propose a model in which Met-207 is associated with the CuA-binding site (along with Cys-196, Cys-200 and His-204) and plays a role in determining its reduction potential and stability

Making artemisinin

The possibilities for the production of the antimalarial artemisinin by biological and chemical means are explored. These include native biosynthesis, genetic modification of Artemisia annua and other plants, engineering of microbes, total and partial chemical synthesis and combinations of the above.Peer reviewed: YesNRC publication: Ye

Characterization of the Brassica napus Extraplastidial Linoleate Desaturase by Expression in Saccharomyces cerevisiae

The substrate specificity and regioselectivity of the Brassica napus extraplastidial linoleate desaturase (FAD3) was investigated in vivo in a heterologous expression system. A strain of the yeast Saccharomyces cerevisiae producing the plant enzyme was constructed and cultured in media containing a variety of fatty acids. The products of desaturation of these potential substrates were determined by gas chromatographic and mass spectrometric analysis of the yeast cultures. The results indicate that the enzyme has: (a) ω-3, as opposed to Δ-15 or double-bond-related regioselectivity, (b) the ability to desaturate substrates in the 16 to 22 carbon range, (c) a preference for substrates with ω-6 double bonds, but the ability to desaturate substrates with ω-6 hydroxyl groups or ω-9 or ω-5 double bonds, and (d) a relative insensitivity to double bonds proximal to the carboxyl end of the substrate

Saponin Biosynthesis in Saponaria vaccaria. cDNAs Encoding β-Amyrin Synthase and a Triterpene Carboxylic Acid Glucosyltransferase

Saponaria vaccaria (Caryophyllaceae), a soapwort, known in western Canada as cowcockle, contains bioactive oleanane-type saponins similar to those found in soapbark tree (Quillaja saponaria; Rosaceae). To improve our understanding of the biosynthesis of these saponins, a combined polymerase chain reaction and expressed sequence tag approach was taken to identify the genes involved. A cDNA encoding a β-amyrin synthase (SvBS) was isolated by reverse transcription-polymerase chain reaction and characterized by expression in yeast (Saccharomyces cerevisiae). The SvBS gene is predominantly expressed in leaves. A S. vaccaria developing seed expressed sequence tag collection was developed and used for the isolation of a full-length cDNA bearing sequence similarity to ester-forming glycosyltransferases. The gene product of the cDNA, classified as UGT74M1, was expressed in Escherichia coli, purified, and identified as a triterpene carboxylic acid glucosyltransferase. UGT74M1 is expressed in roots and leaves and appears to be involved in monodesmoside biosynthesis in S. vaccaria

Topological study of mechanistic diversity in conjugated fatty acid biosynthesis

Variations on an oxidative theme: The precision with which FAD2-type desaturases carry out C 12H activation reactions on flexible lipidic substrates is astonishing. The conformational space available within the active site of these enzymes has been explored using deuterium-labeled substrates, and evidence for a novel quasi-eclipsed conformer has been uncovered. The scheme shows some prototypical substrate conformations.Peer reviewed: YesNRC publication: Ye

The production of artemisinin precursors in tobacco

Artemisinin, in the form of artemisinin-based combination therapies (ACTs), is currently the most important compound in the treatment of malaria. The current commercial source of artemisinin is Artemisia annua, but this represents a relatively expensive source for supplying the developing world. In this study, the possibility of producing artemisinin in genetically modified plants is investigated, using tobacco as a model. Heterologous expression of A. annua amorphadiene synthase and CYP71AV1 in tobacco led to the accumulation of amorphadiene and artemisinic alcohol, but not artemisinic acid. Additional expression of artemisinic aldehyde \u39411(13) double-bond reductase (DBR2) with or without aldehyde dehydrogenase 1 (ALDH1) led to the additional accumulation dihydroartemisinic alcohol. The above-mentioned results and in vivo metabolic experiments suggest that amorphane sesquiterpenoid aldehydes are formed, but conditions in the transgenic tobacco cells favour reduction to alcohols rather than oxidation to acids. The biochemical and biotechnological significance of these results are discussed.Peer reviewed: YesNRC publication: Ye

Structural control of chemoselectivity, stereoselectivity, and substrate specificity in membrane-bound fatty acid acetylenases and desaturases

The FAD2-like desaturases comprise a group of membrane-bound oxygenases involved in the modification of fatty acyl groups in plants and fungi. This group includes typical oleate desaturases which introduce a \u39412 cis double bond and more unusual enzymes such as Crep1, an acetylenase from the plant Crepis alpina, which introduces a triple bond in linoleate at the \u39412 position. In this study, the structure 12function relationship between FAD2-like acetylenases and desaturases was examined through site-directed mutagenesis and heterologous expression. Eleven amino acid positions were identified that show complete evolutionary conservation within acetylenases or desaturases but have different amino acids in the other class of enzyme. Point mutants in Crep1 were constructed and expressed in yeast to test the role in fatty acid modification of the amino acids at the 11 positions. Results indicate the importance of five amino acid positions within Crep1 with regard to desaturase and acetylenase chemoselectivity, stereoselectivity, and substrate recognition. For example, relative to wild-type Crep1, the Y150F, F259L, and H266Q mutations all favored desaturation over acetylenation. The data indicate that small changes in primary sequence, particularly in the vicinity of the active site, can have profound changes on chemoselectivity and other aspects of the function of membrane-bound desaturase-like enzymes.Peer reviewed: YesNRC publication: Ye

A glandular trichome-specific monoterpene alcohol dehydrogenase from Artemisia annua

The major components of the isoprenoid-rich essential oil of Artemisia annua L. accumulate in the subcuticular sac of glandular secretory trichomes. As part of an effort to understand isoprenoid biosynthesis in A. annua, an expressed sequence tags (EST) collection was investigated for evidence of genes encoding trichome-specific enzymes. This analysis revealed a gene denoted Adh2, that encodes an alcohol dehydrogenase and shows a high expression level in glandular trichomes relative to other tissues. The gene product, ADH2, shows up to 61% amino acid identity to members of the short chain alcohol dehydrogenase/reductase (SDR) superfamily, including Forsythia x intermedia secoisolariciresinol dehydrogenase (49.8% identity). Through in vitro biochemical analysis, ADH2 was found to show a strong preference for monoterpenoid secondary alcohols including carveol, borneol and artemisia alcohol. These results indicate a role for ADH2 in monoterpenoid ketone biosynthesis in A. annua glandular trichomes.Peer reviewed: YesNRC publication: Ye

Probing the mechanism of a cyanobacterial \u3949 fatty acid desaturase from Spirulina platensis C1 (Arthrospira sp. PCC 9438)

The initial and rate determining step in the mechanism of fatty acid desaturases has been proposed to be breakage of one of the C\ue5f8H bonds at the site of the incipient double bond. This has been investigated and supported for a number of eukaryotic fatty acid desaturases through the use of kinetic isotope effect experiments with deuterated substrates. In order to probe the reaction catalyzed by the cyanobacterial \u3949 desaturase and compare it to the eukaryotic desaturases, the desC gene of Spirulina platensis, strain C1 (Arthrospira sp. PCC 9438) was expressed in a desaturase mutant of baker's yeast. Kinetic isotope effects were performed by culturing yeast transformants with deuterated thia-substituted stearic acids. A large kinetic isotope effect was found for the 9 position, in qualitative agreement with results from eukaryotic desaturases.NRC publication: Ye