Ethanol to acetaldehyde

The reverse reaction also occurs m living systems NADH reduces acetaldehyde to ethanol m the presence of alcohol dehydrogenase In this process NADH serves as a hydride donor and is oxidized to NAD" while acetaldehyde is reduced... 

Yeast (qv) metabolize maltose and glucose sugars via the Embden-Meyerhof pathway to pymvate, and via acetaldehyde to ethanol. AH distiUers yeast strains can be expected to produce 6% (v/v) ethanol from a mash containing 11% (w/v) starch. Ethanol concentration up to 18% can be tolerated by some yeasts. Secondary products (congeners) arise during fermentation and are retained in the distiUation of whiskey. These include aldehydes, esters, and higher alcohols (fusel oHs). NaturaHy occurring lactic acid bacteria may simultaneously ferment within the mash and contribute to the whiskey flavor profile. 

When the enzyme alcohol dehydrogenase converts acetaldehyde to ethanol, NADH acts as a reducing agent by transferring a hydride from C4 of the nicotinamide ring to the carbonyl group of acetaldehyde. 

Other organisms are equipped to produce ethanol, by employing a thiamine diphosphate-dependent decarboxylation of pyruvate to acetaldehyde (see Section 15.8) and NAD+ is regenerated by reducing the acetaldehyde to ethanol. This is a characteristic of baker s yeast, and forms the essential process for both bread making (production of CO2) and the brewing industry (formation of ethanol). 

The reduction of acetaldehyde to ethanol could be explained by its chemisorption near rhodium particles and the action of spill over hydrogen. On Rh/La 0, Bell has observed that at low residence times acetaldehyde is the primary product whereas at longer residence times the formation of ethanol becomes the dominant process. They concluded that this pattern is characteristic of the sequential reaction process ... 

Conversely, NADH is one of the important natural reductants, and in yeast cells, for example, it reduces acetaldehyde to ethanol ... 

Alcoholic fermentation occurs when the end product is ethanol, as shown in Figure 4.11. In this process the pyruvate is first converted enzymatically to acetaldehyde. The conversion of acetaldehyde to ethanol produces NAD+ from NADH + H+, and the NAD+ is cycled through the glycolysis process. As with lactic acid fermentation, the glycolysis process produces usable energy contained in two molecules of ATP produced for each molecule of glucose metabolized. 

Possible toxic reactions of sulfur dioxide are also indicated in Table I. The reaction of bisulfite with aldehydes has a classic position in biochemistry since Neuberg demonstrated in 1918 that the products of fermentation by yeast were altered by the addition of sodium sulfite, which caused the production of equal amounts of the bisulfite addition compound of acetaldehyde and of glycerol. This was concomitant with the blockage of conversion of acetaldehyde to ethanol. Addition compounds can also be formed with quinones and with ,/ -unsaturated compounds. None of these reactions has been adequately assessed as a possible contributor to toxicity. 

Ethanol Production in Yeast When grown anaerobically on glucose, yeast (S. cerevisiae) converts pyruvate to acetaldehyde, then reduces acetaldehyde to ethanol using electrons from NADH. Write the equation for the second reaction, and calculate its equilibrium constant at 25 °C, given the standard reduction potentials in Table 13-7. 

Aldehydes also arise as normal by-products of yeast fermentation. Acetaldehyde is the ultimate electron acceptor in the conversion of glucose to ethanol. In this pathway, aldehyde dehydrogenase (ADH) reduces acetaldehyde to ethanol with the corresponding oxidation of NADH. Acetaldehyde levels are therefore dependent on the fermentation conditions, e.g., temperature, O2 levels, pH, SOj levels, and yeast nutrient availability (13, 14). Yeast strain can also affect aldehyde formation and excretion (15-17). For example, film yeasts used in sherry production are selected for their ability to produce very high acetaldehyde levels (18). 

Ethanol is formed from pyruvate in yeast and several other microorganisms. The first step is the decarboxylation of pyruvate. This reaction is catalyzed by pyruvate decarboxylase, which requires the coenzyme thiamine pyrophosphate. This coenzyme, derived from the vitamin thiamine (Bj), also participates in reactions catalyzed by other enzymes (Section 17.1.1). The second step is the reduction of acetaldehyde to ethanol by NADH, in a reaction catalyzed by... 

Ru(bpy)3" ]. This reduced species is oxidized by Rh(bpysa).r, resulting in the formation of the reduced hydridorhodium complex, which provides further reduction of NAD" ". The NADH regenerated in this photochemical system was coupled with the enzymatic reduction of acetaldehyde to ethanol in the presence of alcohol dehydrogenase. 

NADH (nicotinamide adenine dinucleotide) is a biochemical source of hydride. In the following example NADH reduces acetaldehyde to ethanol via minor pathway H t., hydride transfer to a cationic center. A Zn ion acts as a Lewis acid to polarize the acetaldehyde carbonyl (similar to protonating the carbonyl). The Lewis acid makes the carbonyl a better electron sink by increasing the partial positive charge on carbon. In fact, the electrophilic catalysis by 2+ and 3+ metal ions can accelerate additions to carbonyls by over a million times. The formation of the aromatic pyridinium ring in the NAD" product helps balance the energetics of this easily reversible reaction. 

Under forcing conditions, temperatures > 200°, it is possible to hydrogenate acetaldehyde to ethanol, as it is formed, with a cobalt l2-PPh3 catalyst. Yields of ethanol up to 70% have been reported. Better results are obtained by adding Ru to the Co catalyst. Here, Ru serves as a hydrogenation catalyst for the conversion of acetaldehyde to ethanol. For example, with a catalyst consisting of Co Ru.T = 1 5 4, at 140°C and 24.1 MPa (H2 C0 = 2 1) the ethanol selectivity is 86% . 

Ethyl acetate is obtained from methyl acetate if the reductive carbonyiation is carried out with a catalyst capable of in situ hydrogenation of acetaldehyde to ethanol. The reaction sequence is ... 

For example, in the case of the reduction of acetaldehyde to ethanol (as catalyzed by alcohol dehydrogenases) the following sequence of events, where Donor-H represents a hydride donor, is composed of a hydride-transfer mechanism for the step in Eq. (4.2a), a proton-transfer mechanism for the step in Eq. (4.2b), and a hydride-transfer/proton-transfer mechanism for the overall reaction formed by Eqs. (4.2a) and (4.2b) ... 

Under anaerobic conditions the NADH produced by glycolysis is used to reduce p)mivate to lactate in skeletal muscle (lactate fermentation) or to convert acetaldehyde to ethanol in yeast (alcohol fermentation). 

Write a balanced chemical equation for the conversion of acetaldehyde to ethanol. 

The reduction of acetaldehyde to ethanol is coupled to the oxidation of NADH to NAD. ... 

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