Yeast respiration

When in later years Krebs reviewed the major points which had to be established if the cycle was to be shown to be operative in cells, the obvious needs were to find the presence of the required enzymes and to detect their substrates. As the substrates are present in the cycle in catalytic amounts their accumulation required the use of inhibitors. Krebs also stressed that rates of oxidation of the individual substrates must be at least as fast as the established rates of oxygen uptake in vivo, an argument first used by Slator (1907) with reference to fermentation A postulated intermediate must be fermented at least as rapidly as glucose is. (See Holmes, 1991). This requirement did not always appear to be met. In the early 1950s there were reports that acetate was oxidized by fresh yeast appreciably more slowly than the overall rate of yeast respiration. It was soon observed that if acetone-dried or freeze-dried yeasts were used in place of fresh yeast, rates of acetate oxidation were increased more than enough to meet the criterion. Acetate could not penetrate fresh yeast cell walls sufficiently rapidly to maintain maximum rates of respiration. If the cell walls were disrupted by drying this limitation was overcome, i.e. if rates of reaction are to be... 

Vallejos RH, Roveri OA. Alkaloid inhibition of yeast respiration. Prevention by Cal. Biochem Pharmacol 1972 21 1160-7. 

It has been reported that between 10 and 20 min after candicidin treatment of yeasts that protein syntheses slowed and then ceased. Protein synthesis slowed not as a result of energy deficiency, as ATP was abundant in polyene-treated cells at this stage, but as a direct result of reduced potassium levels [287]. Similar reductions in protein syntheses have been observed in E. coli mutants with defective potassium transport systems [291]. Upon treatment with levels of nystatin that did not affect yeast respiration, the uptake of glycine was rapidly reduced and ceased within 20 min of antibiotic application. Low nystatin concentrations did not produce leakage of amino acids into the environment, suggesting that nystatin at high concentrations damaged the cell membrane to such an extent that all normal uptake processes ceased [292]. 

Under aerobic conditions and in the absence of high concentrations of glucose, yeast respires glucose. Under these circumstances pyruvic acid is oxidized and decarboxylated by the so-called pyruvate dehydrogenase complex. This system comprises three separate enzyme activities and is located within mitochondria. The three separate enzymes are pyruvate dehydrogenase, dihydrolipoyl transacetylase and dihydrolipoyl dehydrogenase (A, B and C in... 

Il chenko AP, Chernyavskaya OG, Shishkanova NV, Finogenova TV. Metabolic characteristics of the mutant Yarrowia lipolytica strain N1 producing alpha-ketoglutaric and citric acids from ethanol and the efiect of NH and O on yeast respiration and acidogenesis. Microbiologiia 2001 70 151-7. 

The role of the old yellow enzyme in metabolism is not known. It is reduced by TPNH2 or DPNH2 and the reduced form is reoxidized by atmospheric oxygen. Its activity in this system, however, is too low to account for more than a fraction of yeast respiration. It is more rapidly reoxidized by methylene blue, but very slowly, if at all, by cytochrome c. 

When excess substrate interferes with growth and/or product formation. One example is the production of baker s yeast. It is known that relatively low concentrations of certain sugars repress respiration and this will make the yeast cells switch to fermentative metabolism, even under aerobic conditions. This, of course, has a negative effect on biomass yield. When maximum biomass production is aimed at, fed batch cultures are the best choice, since the concentration of limiting sugar remains low enough to avoid repression of respiration. 

Fermentation occurs naturally in various microorganisms such as bacteria, yeasts, fungi and in mammalian muscle. Yeasts were discovered to have connection with fermentation as observed by Louis Pasteur and originally defined as respiration without air. However, it does not have to always occur in anaerobic condition. For example, starch when fermented under... 

The third class of haemoproteins, with hexa-coordinate low-spin iron, are the cytochromes. First discovered by McMunn in 1884, they were rediscovered in 1925 by David Keilin. Using a hand spectroscope he observed the characteristic absorption (Soret) bands of the three cytochromes a, b and c in respiring yeast cells, which disappeared upon oxygenation. 

Fluoroethanol itself is innocuous towards a variety of tissue constituents, a series of enzymes in rat-liver mince, and the respiration and metabolism in liver, kidney, heart and brain slice.3 After a period of incubation in those tissues known to contain alcohol dehydrogenase, e.g. liver and kidney, the respiration and pyruvate oxidation were strongly inhibited. Likewise, following a period of incubation with yeast, acetate oxidation was blocked. These inhibitions were similar to those produced by fluoroacetate, and the facts can best be explained by the oxidation of fluoroethanol to fluoroacetic acid by alcohol dehydrogenase. 

Since more ATP is produced by respiration of glucose than by fermentation, and since the ATP requirement for biosynthesis of cell mass is the same, it follows that to obtain the same cell yield from glucose, the yeast should consume less sugar under aerobic conditions than under anaerobic conditions, with a resultant decrease in glycolytic flux (Berry, 1982). These phenomena are referred to as the Pasteur effect. Although this effect is observed in some yeasts, in S. cerevisiae it is either absent (Gancedo and Serrano, 1989) or observed only under certain nutrient-limited conditions. The main reason for the absence of the Pasteur effect is that even under aerobic conditions, fermentation is still the main catabolic route for the utilisation of glucose because of the Crabtree effect (Walker, 1994). The Crabtree effect is the repression... 

Mohler, R.E., Dombek, K.M., Hoggard, J.C., Young, E.T., Synovec, R.E. (2006) Comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry analysis of metabolites in fermenting and respiring yeast cells. Anal. Chem. 78 2700-2709. 

Fermentation broths are suspensions of microbial cells in a culture media. Although we need not consider the enhancement factor E for respiration reactions (as noted above), the physical presence per se of microbial cells in the broth will affect the k a values in bubbling-type fermentors. The rates of oxygen absorption into aqueous suspensions of sterilized yeast cells were measured in (i) an unaerated stirred tank with a known free gas-liquid interfacial area (ii) a bubble column and (iii) an aerated stirred tank [6]. Data acquired with scheme (i) showed that the A l values were only minimally affected by the presence of cells, whereas for schemes (ii) and (iii), the gas holdup and k a values were decreased somewhat with increasing cell concentrations, because of smaller a due to increased bubble sizes. 

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