Yeast ethanol catabolism

Yeasts generally catabolize ethanol as follows ethanol —- acetaldehyde — acetic acid — acetyl-CoA —- other oxidative pathways [Figure 4]. It has been speculated that C. utilis forms ethyl acetate by the reaction of acetyl-CoA with ethanol (5). Thus if the flow of metabolites into the TCA cycle is inhibited or limited, as might occur under iron-limited conditions, acetyl-CoA would be expected to accumulate thus providing a precursor pool for esterification of ethanol. 

The third major route of pyruvate catabolism leads to ethanol. In some plant tissues and in certain invertebrates, protists, and microorganisms such as brewer s yeast, pyruvate is converted under hypoxic or anaerobic conditions into ethanol and C02, a process called ethanol (alcohol) fermentation (Fig. 14-3). 

Some catabolic reactions depend upon ADP, but under most conditions its concentration is very low because it is nearly all phosphorylated to ATP. Reactions utilizing ADP may then become the rate-limiting pacemakers in reaction sequences. Depletion of a reactant sometimes has the effect of changing the whole pattern of metabolism. Thus, if oxygen is unavailable to a yeast, the reduced coenzyme NADH accumulates and reduces pyruvate to ethanol plus C02 (Fig. 10-3). The result is a shift from oxidative metabolism to fermentation. 

Hydrolysis of lignocellulose is necessary to enable its use for ethanol production. However, when lignocellulosic materials are hydrolyzed with acid, compounds toxic to the yeast cells are released. The inhibitors are of three main types aldehydes, organic acids, and phenolic compounds. Among the aldehydes, furfural and hydroxymethylfurfural (HMF) are typically found in high concentrations—particularly in dilute-acid hydrolysates (1,2). These compounds have been shown to inhibit certain enzymes in the catabolism necessary for cell growth (3-9). 

The main reason for the low productivity numbers of yeast can be seen from Scheme 1. In reductions catalysed by yeasts carbohydrates are normally used as reducing agents (source for electrons). NADH is formed as a catabolic intermediate. The reaction is accompanied with the formation of acetaldehyde most of which is reduced with NADH to ethanol. This miwanted reaction may consume as much as 95-99 % of the NADH, and the intended reduction of a CX-double bond takes place only to an extent of <1-5 %. 

ANAEROBIC CARBOHYDRATE METABOLISM Yeasts growing in media containing high concentrations of fermentable carbohydrate invariably metabolize it fermentatively to produce ethanol and CO2. If air is present, and when the sugar concentration has been lowered, the ethanol is respired using the metabolic routes described above. Under the anaerobic conditions of a brewery fermentation the hexoses derived from wort fermentable carbohydrates are catabolized by the EMP pathway (Fig. 17.2) to pyruvic acid. The pyruvate produced is decarboxylated by the enzyme pyruvate decarboxylase, with the formation of acetaldehyde and CO2. The enzyme requires the cofactor thiamine pyrophosphate (TPP) for activity and the reaction is shown in Fig. 17.10. The acetaldehyde formed acts (in the absence of the respiratory chain) as an electron acceptor and is used to oxidize NADH with the formation of ethanol ... 

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