Ethanol metabolic yield

A further compUcation is that while wUd-type T. saccharolyticum produces ethanol at yields >50% of the theoretical maximum, wild-type C. thermocellum typically produces ethanol at yields closer to 25-30% of the theoretical maximum, with the excess carbon and electrons going to nontraditional fermentation products such as secreted amino acids organic acids such as pyruvate, fumarate, and malate, and alcohols such as isobutanol and butanediol [30, 31]. This may reflect a fundamental difference in either central metabolism or metabolic control between these two organisms. 

We can see that for type 1 processes, high growth rate is obligately linked to a high rate of product formation. Indeed, this is the case for all products produced by a fermentative mode of metabolism, eg ethanol, lactic add, acetone. Chemostat studies have shown that for most aerobic processes when growth is limited by some nutrient other than the carbon source, the yield of product decreases with increase in spedfic growth rate (p or D p = dilution rate (D) in chemostat culture). Conversely, both the biomass yield and the spedfic rate of substrate utilisation (qs g substrate g biomass-1 h-1) increase with spedfic growth rate. 

Ethanol is almost entirely metabolized in the liver. The first step, oxidation by alcohol dehydrogenase, yields acetaldehyde, a reactive and toxic compound. Essentially all of the acetaldehyde is converted to acetate by the liver enzyme aldehyde dehydrogenase. Aldehyde dehydrogenase is inhibited by the drag disulfiram. Given alone, disulfiram is a nontoxic substance. However, ethanol consumption in the presence of... 

C2H5OH, ethanol is formed by bacteria in the gastrointestinal tract in low amounts. Most of the ethanol of bacterial source is metabolized during the first liver passage yielding acetaldehyde and subsequently acetic acid. 

The chemical oxidation of 1,3,5-triarylformazans to tetrazolium salts was first accomplished in 1894 [127], Almost no attention was given to these compounds for about 50 years after their discovery. This situation began to alter markedly because of the application of tetrazolium salts in histochemical, pharmacological, and other biomedical research areas [128]. Specifically, the tetrazolium salt is reduced to a colored formazan derivative by reducing enzymes found only in metabolically active cells. Anodic transformation of for-mazans to tetrazolium salts was performed in acetonitrile solution using cotrolled potential electrolysis [17,129], In our view this reaction could be considered as a method of choice for the preparation of tetrazolium salts. The products were obtained in high yield and the electrolysis can be performed in a divided cell under constant current and decoloration of the solution indicates the end point of the reaction. Recently the anodic oxidation of formazans to tetrazolium salts was performed successfully in aqueous ethanol solution [130]. 

Oxidation of ethanol. Although the major metabolic pathway for alcohols such as ethanol is oxidation catalyzed by alcohol dehydrogenase (see below), ethanol can also be metabolized by cytochrome P-450. The product, ethanal, is the same as produced by alcohol dehydrogenase. The isoform of cytochrome P-450 is CYP2E1. The mechanism may involve a hydroxylation to an unstable intermediate, which loses water to yield ethanal. Alternatively, a radical mechanism could be responsible. The importance of this route of metabolism for ethanol is that it is inducible (see chap. 5), assuming more importance after repeated exposure to ethanol such as in alcoholics and regular drinkers. 

Triose phosphate isomerase, TIM, as a nearly-diffusion-limited enzyme (Ch. 2, Section 2.2.3), catalyzes the equilibration of GAP and DHAP very efficiently. However, equilibrium concentrations of GAP were metabolized to pyruvate and further to ethanol or acetate, so the stoichiometric yield on glucose to 1,3-PPD of 42.5% was lower than the target of 50%. Thus, TIM was cloned out to prevent equilibration between the desired DHAP and the undesired side-product GAP, which successfully increased the yield beyond the minimum target... 

In fermentation practice, the yields of ethanol and C02 have been reported to vary from 92 to 98 percent of theory. This is attributable to the formation of small amounts of aldehydes, volatile and fixed acids, glycerol, and other substances, to use of sugar in the yeasts metabolism, and to small losses of ethanol during the fermentation. 

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