Oxygenates, synthesis ethanol, fermentation

When plants experience anoxic conditions there is a shift in carbohydrate metabolism from an oxidative to a fermentative pathway (Fig. 1). In the absence of oxygen, ATP is generated not by the Krebs cycle but by alcoholic fermentation, i.e. glycolysis and ethanol synthesis. 

Ether synthesis produces branched ethers like methyl t-butyl ether (MTBE) by reacting methanol (or ethanol) with branched alkenes such as isobutene. These ethers are valuable for their high octane quality and also, on account of the oxygen they contain, their ability to reduce both CO and hydrocarbon exhaust emissions. Alcohols, such as methanol, manufactured from natural gas or coal, and ethanol, produced by fermentation, are other oxygenated components in limited use. 

At higher ethanol concentrations the intracellular alcohol interferes with membrane organization, increasing its fluidity and permeability to ions and small metabolites and inhibiting transport of nutrients. Especially Ca and Mg ions are able to increase the plasma membrane stability. It has been demonstrated that incorporation of unsaturated fatty acids and/or sterol(s) as well as proteolipids into cellular membrane of yeasts helps to alleviate ethanol tolerance. For the synthesis of the unsaturated fatty acids the presence of traces of oxygen under fermentation conditions is required. Further to Ca and Mg ions, other trace elements such as Co, Cu, Mn and Zn " and vitamins, e.g. pantothenate, thiamine, riboflavin, nicotinic acid, pyridoxine, biotin, folic acid and inositol, are essential for the growth and ethanol production by yeasts. 

Yeasts growing under anaerobic conditions (fermentation) also have a pathway for regenerating NAD from NADH. Under oxygen-deprived conditions, yeasts convert pyruvic acid to acetaldehyde by decarboxylation (CO2 is released, (see The Chemistry of... Thiamine in WileyPLUS), then NADH in alcohol dehydrogenase reduces acetaldehyde to ethanol. As in oxygen-starved muscles, this pathway occurs for the purpose of regenerating NAD needed to continue glycolytic ATP synthesis ... 

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