The ADH2 gene codes for the Arabidopsis glutathione-dependent formaldehyde dehydrogenase (FALDH), an enzyme involved in formaldehyde metabolism in eukaryotes. In the present work, we have investigated the potential role of FALDH in detoxification of exogenous formaldehyde. We have generated a yeast (Saccharomyces cerevisiae) mutant strain (sfa1⌬) by in vivo deletion of the SFA1 gene that codes for the endogenous FALDH. Overexpression of Arabidopsis FALDH in this mutant confers high resistance to formaldehyde added exogenously, which demonstrates the functional conservation of the enzyme through evolution and supports its essential role in formaldehyde metabolism. To investigate the role of the enzyme in plants, we have generated Arabidopsis transgenic lines with modified levels of FALDH. Plants overexpressing the enzyme show a 25% increase in their efficiency to take up exogenous formaldehyde, whereas plants with reduced levels of FALDH (due to either a cosuppression phenotype or to the expression of an antisense construct) show a marked slower rate and reduced ability for formaldehyde detoxification as compared with the wild-type Arabidopsis. These results show that the capacity to take up and detoxify high concentrations of formaldehyde is proportionally related to the FALDH activity in the plant, revealing the essential role of this enzyme in formaldehyde detoxification.

The completion of the Saccharomyces cerevisiae genome project in 1996 showed that almost 60% of the potential open reading frames of the genome had no experimentally determined function. Using a conserved sequence motif present in the zinc-containing mediumchain alcohol dehydrogenases, we found several potential alcohol dehydrogenase genes with no defined function. One of these, YAL060W, was overexpressed using a multicopy inducible vector, and its protein product was purified to homogeneity. The enzyme was found to be a homodimer that, in the presence of NAD ؉ , but not of NADP, could catalyze the stereospecific oxidation of (2R,3R)-2,3-butanediol (K m ‫؍‬ 14 mM, k cat ‫؍‬ 78,000 min ؊1 ) and meso-butanediol (K m ‫؍‬ 65 mM, k cat ‫؍‬ 46,000 min ؊1 ) to (3R)-acetoin and (3S)-acetoin, respectively. It was unable, however, to further oxidize these acetoins to diacetyl. In the presence of NADH, it could catalyze the stereospecific reduction of racemic acetoin ((3R/3S)-acetoin; K m ‫؍‬ 4.5 mM, k cat ‫؍‬ 98,000 min ؊1 ) to (2R,3R)-2,3-butanediol and meso-butanediol, respectively. The substrate stereospecificity was determined by analysis of products by gas-liquid chromatography. The YAL060W gene product can therefore be classified as an NAD-dependent (2R,3R)-2,3-butanediol dehydrogenase (BDH). S. cerevisiae could grow on 2,3-butanediol as the sole carbon and energy source. Under these conditions, a 3.5-fold increase in (2R,3R)-2,3-butanediol dehydrogenase activity was observed in the total cell extracts. The isoelectric focusing pattern of the induced enzyme coincided with that of the pure BDH (pI 6.9). The disruption of the YAL060W gene was not lethal for the yeast under laboratory conditions. The disrupted strain could also grow on 2,3-butanediol, although attaining a lesser cell density than the wild-type strain. Taking into consideration the substrate specificity of the YAL060W gene product, we propose the name of BDH for this gene. The corresponding enzyme is the first eukaryotic (2R,3R)-2,3-butanediol dehydrogenase characterized of the medium-chain dehydrogenase/reductase family.

A new NADP(H)-dependent alcohol dehydrogenase (the YCR105W gene product, ADHVII) has been identified in Saccharomyces cerevisiae. The enzyme has been purified to homogeneity and found to be a homodimer of 40 kDa subunits and a pI of 6.2-6.4. ADHVII shows a broad substrate specificity similar to the recently characterized ADHVI (64% identity), although they show some differences in kinetic properties. ADHVI and ADHVII are the only members of the cinnamyl alcohol dehydrogenase family in yeast. Simultaneous deletion of ADH6 and ADH7 was not lethal for the yeast. Both enzymes could participate in the synthesis of fusel alcohols, ligninolysis and NADP(H) homeostasis.Keywords: cinnamyl alcohol dehydrogenase; fusel alcohols; NADP(H) homeostasis; ligninolysis.The current version of the Yeast Proteome Database (http://www.proteome.com) lists approximately 260 oxidoreductases (160 of them have been characterized experimentally, and the rest predicted by sequence similarity or by other analysis) [1]. Our group is interested in the identification and characterization of novel alcohol dehydrogenase (ADH) gene products from Saccharomyces cerevisiae [2,3]. ADHs are oxidoreductases that catalyze the reversible oxidation of alcohols to aldehydes or ketones, with the corresponding reduction of NAD or NADP. ADHs constitute a large group of enzymes that can be subdivided into at least three distinct enzyme superfamilies: medium-chain and short-chain dehydrogenases/reductases, and iron-activated alcohol dehydrogenases [4,5]. The medium-chain dehydrogenase/ reductase (MDR) superfamily consists of enzymes with a subunit size of approximately 350 residues, dimeric or tetrameric, with two domains in each subunit: one catalytic and one responsible for the binding of the nucleotide (NAD or NADP). Many enzymes of the MDR family have zinc in their active site, and have a sequence motif known as the zinc-containing ADH signature: GHEX 2 GX 5 (G,A)X 2 (I,V,A,C,S) [6]. According to the Pfam and COG databases [7,8], the S. cerevisiae genome codes for 21 potential MDR enzymes, with 12 of them showing the zinc ADH signature described above. These 12 zinc-containing yeast MDR include ADH1, ADH2, ADH3, ADH5, ADH6, SFA1, SOR1 and its 99% identical YDL246C, XYL2, BDH1, YAL061W and YCR105W. All these yeast MDRs, except YCR105W and YAL061W, have enzymatic activities experimentally determined. In the present study, we report the characterization of the YCR105W gene from S. cerevisiae as a new alcohol dehydrogenase. The gene was overexpressed in yeast cells and the corresponding protein product purified to homogeneity. The enzyme showed a wide substrate specificity, using NADP(H) as coenzyme. Given the similar substrate specificities and the sequence identity (64%) between the Ycr105p and the recently characterized ADHVI [3], we propose the name of ADH7, for the YCR105W gene and ADHVII for its coded protein. A null adh7 yeast strain and a double mutant adh6D adh7D were constructed and their growths compared with a wild-type strain. M A T E R I A L S A N ...

YMR318C represents an open reading frame from Saccharomyces cerevisiae with unknown function. It possesses a conserved sequence motif, the zinc-containing alcohol dehydrogenase (ADH) signature, specific to the medium-chain zinc-containing ADHs. In the present study, the YMR318C gene product has been purified to homogeneity from overexpressing yeast cells, and found to be a homodimeric ADH, composed of 40 kDa subunits and with a pI of 5.0-5.4. The enzyme was strictly specific for NADPH and was active with a wide variety of substrates, including aliphatic (linear and branched-chain) and aromatic primary alcohols and aldehydes. Aldehydes were processed with a 50-fold higher catalytic efficiency than that for the corresponding alcohols. The highest k(cat)/K(m) values were found with pentanal>veratraldehyde > hexanal > 3-methylbutanal >cinnamaldehyde. Taking into consideration the substrate specificity and sequence characteristics of the YMR318C gene product, we have proposed this gene to be called ADH6. The disruption of ADH6 was not lethal for the yeast under laboratory conditions. Although S. cerevisiae is considered a non lignin-degrading organism, the catalytic activity of ADHVI can direct veratraldehyde and anisaldehyde, arising from the oxidation of lignocellulose by fungal lignin peroxidases, to the lignin biodegradation pathway. ADHVI is the only S. cerevisiae enzyme able to significantly reduce veratraldehyde in vivo, and its overexpression allowed yeast to grow under toxic concentrations of this aldehyde. The enzyme may also be involved in the synthesis of fusel alcohols. To our knowledge this is the first NADPH-dependent medium-chain ADH to be characterized in S. cerevisiae.

Engineered Saccharomyces cerevisiae strains overexpressing GPD1, which codes for glycerol-3-phosphate dehydrogenase, and lacking the acetaldehyde dehydrogenase Ald6 display large-scale diversion of the carbon flux from ethanol toward glycerol without accumulating acetate. Although GPD1 ald6 strains have great potential for reducing the ethanol contents in wines, one major side effect is the accumulation of acetoin, having a negative sensory impact on wine. Acetoin is reduced to 2,3-butanediol by the NADH-dependent 2,3-butanediol dehydrogenase Bdh1. In order to investigate the influence of potential factors limiting this reaction, we overexpressed BDH1, coding for native NADH-dependent Bdh1, and the engineered gene BDH1 221,222,223 , coding for an NADPH-dependent Bdh1 enzyme with the amino acid changes 221 EIA 223 to 221 SRS 223, in a glycerol-overproducing wine yeast. We have shown that both the amount of Bdh1 and the NADH availability limit the 2,3-butanediol dehydrogenase reaction. During wine fermentation, however, the major limiting factor was the level of synthesis of Bdh1. Consistent with this finding, the overproduction of native or engineered Bdh1 made it possible to redirect 85 to 90% of the accumulated acetoin into 2,3-butanediol, a compound with neutral sensory characteristics. In addition, the production of diacetyl, a compound causing off-flavor in alcoholic beverages, whose production is increased in glycerol-overproducing yeast cells, was decreased by half. The production of higher alcohols and esters, which was slightly decreased or unchanged in GPD1 ald6 cells compared to that in the control cells, was not further modified in BDH1 cells. Overall, rerouting carbons toward glycerol and 2,3-butanediol represents a new milestone in the engineering of a low-alcohol yeast with desirable organoleptic features, permitting the decrease of the ethanol contents in wines by up to 3°.