Engineering of 2,3-Butanediol Dehydrogenase To Reduce Acetoin Formation by Glycerol-Overproducing,Low-Alcohol Saccharomyces 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°.A large number of quality wines produced by modern winemaking practices, which favor harvesting fully ripened grapes, frequently contain an excessive ethanol content. This tendency is observed in the majority of the world''s wine-producing areas, and reducing the alcohol levels in wines has become a major concern of the wine industry. Consequently, numerous attempts have been made to engineer Saccharomyces cerevisiae yeast strains with reduced ethanol yields, which would offer faster and less expensive biological alternatives to the current physical processes available for the production of low- and reduced-alcohol wines (29). The biological approaches used so far are all based on diverting sugar metabolism toward by-products other than ethanol by metabolic engineering strategies (7, 8, 18, 19, 21, 22, 30). However, these strategies have so far not satisfied the need to obtain a significant reduction in the ethanol yield without causing the accumulation of undesirable secondary products and/or without affecting yeast physiology. Among these various advances, an efficient strategy is based on the rerouting of the carbon flux toward the production of glycerol. This polyol is a relatively neutral compound from an olfactory perspective, and it is has been demonstrated previously to contribute positively to wine quality through enhanced sweetness and viscosity (27). In yeast, glycerol plays a major role as an osmolyte under osmotic stress conditions and also functions as an essential redox sink in the absence of oxygen, when the reoxidation of excess cytosolic NADH is required (1, 2, 42, 44, 46). This compound is formed by the reduction of dihydroxyacetone phosphate by glycerol-3-phosphate dehydrogenase (encoded by GPD1 and GPD2), followed by dephosphorylation by glycerol-3-phosphatase, which exists as two isoforms: Gpp1 and Gpp2p (see Fig. ​Fig.1).1). By overexpressing GPD1 or GPD2, the production of glycerol has been greatly enhanced, making it possible to decrease the ethanol yield as the result of carbon diversion and reduced NADH availability for the alcohol dehydrogenase reaction. This strategy has been used previously to reduce the ethanol yields in wine and brewer''s yeast (4, 7, 23, 25, 26, 31). For wine, it was shown that these glycerol-overproducing strains have the potential to reduce the ethanol content by 1 to 2°. Nevertheless, major modifications in the production levels of other metabolites, in particular acetate and acetoin (23, 31), which are undesirable at high concentrations in wine, are generated. The production of acetate in glycerol-overproducing wine yeasts has been reduced to a normal level by the deletion of ALD6, coding for an acetaldehyde dehydrogenase (4, 30). The major problem of the accumulation of acetoin, which was shown to accumulate at several grams per liter in commercial GPD1 ald6 wine yeast strains (4), remains to be overcome. At usual concentrations in wine, which vary from undetectable levels to 80 mg/liter (32, 33, 40), this compound has no negative organoleptic influence. However, at concentrations higher than its threshold level (around 150 mg/liter 11]), acetoin can confer an unpleasant buttery flavor on wines. In contrast, the reduced form of acetoin, 2,3-butanediol (2,3-BD), has neutral sensory qualities (data not shown). It is found in wines at concentrations ranging from 0.2 to 3 g/liter (14, 41).Open in a separate windowFIG. 1.Schematic representation of metabolic pathways implicated in our design strategy for a low-alcohol yeast. GP, glycerol phosphatase, encoded by GPP1 and GPP2; GPDH, glycerol phosphate dehydrogenase, encoded by GPD1 and GPD2; PDC, pyruvate decarboxylase, encoded by PDC1, PDC5, and PDC6; ACDH, acetaldehyde dehydrogenase, encoded by ALD4, ALD5, and ALD6; ADH, alcohol dehydrogenase, encoded by ADH1; BDH, Bdh1, encoded by BDH1 (other BDHs exist; however, no other identified gene has been associated with BDH activity); ALS, acetolactate synthase, encoded by ILV2; DS, diacetyl synthetase; DR, diacetyl reductase; G3P, glycerol-3-phosphatase; DHAP, dihydroxyacetone phosphate; acetyl CoA, acetyl coenzyme A; TPP, thiamine PP_i.Bdh1, encoded by BDH1, is the only identified enzyme in yeast catalyzing the reduction of acetoin into 2,3-BD (12). This enzyme has strict stereospecificity for the OHs of carbons in R configuration and acts preferentially as a reductase rather than as a dehydrogenase (11, 12). It is essentially responsible for the formation of (2R,3R)-2,3-BD and part of meso-2,3-BD from (3R)-acetoin and (3S)-acetoin, respectively.The accumulation of acetoin in strains engineered for glycerol overproduction has been attributed to several factors (4). On one hand, it was assumed that the amount of Bdh1 is a rate-limiting factor in the conversion of acetoin into 2,3-BD. On the other hand, it is possible that the Bdh1 reaction is limited by the low level of available NADH since this coenzyme is preferentially used for glycerol synthesis in these strains.The aim of the present study was to investigate in detail the metabolic prerequisites for reducing accumulated acetoin in S. cerevisiae overproducing glycerol and exhibiting reduced acetate formation by promoting the conversion of acetoin into the compound 2,3-BD, which has neutral sensory characteristics. In this study, we first determined the role of Bdh1 in the reduction of acetoin into 2,3-BD during wine fermentation. Next, we studied the impact of the overproduction of NADH-dependent Bdh1 or an engineered NADPH-dependent form of Bdh1 in a model wine yeast, V5, overexpressing GPD1 and lacking ALD6 during fermentation in synthetic must with various sugar concentrations. The NADPH-dependent Bdh1 has been obtained previously by the replacement of three amino acids involved in the NADH binding domain, resulting in the complete reversal of the coenzyme specificity from NADH to NADPH (6). The effects on the growth and fermentation properties of the engineered strains and the levels of by-products and key aromatic compounds formed by the strains were determined.