Yeast cell aerobic

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. 

Figure 20.7 Volcano plot showing fold change versus f-test p value of eight runs of yeast cell lysates, four runs of aerobically grown yeast and four runs of anaerobically grown yeast. Reproduced with permission from Reference 16.

Yeast cells are cultivated under aerobic conditions at 30 C in an aerated stirred-tank fermentor where air (21 vol% of oxygen) is supplied at a superficial gas velocity of 50 mh . The overall volumetric coefficient for oxygen transfer based on the liquid phase concentration is 80 h T... 

Fig. 3.2 Biological abstraction. Yeast cells reflect anaerobic, reductive metabolism (intestine) as well as aerobic, oxidative metabolism (liver), if glycolysis is regarded as the most active pathway. Therefore, the yeast Saccharomyces cerevisiae is a good model organism for studies of xenobiotic metabolism.

The coupling between assembly of monomers and Fe(ll) oxidation is complex and may be different for frataxin from different soruces. A further complication is that the aerobic oxidation of Fe(ll) in yeast frataxin is sufficiently slow that in vitro it stabilizes Fe(ll) against autooxidation, and under some conditions a 40 60 mixture of Fe(ll) and Fe(lll) is retained by the protein. Iron binding by human frataxin is faster than with yeast frataxin but the resultiug mineral cores are identical and when human frataxin is expressed in yeast cells grown under iron-rich conditions, roughly 50% of the mitochondrial iron is retained as a complex with frataxin (with iron levels of 5 per monomer). We consider further aspects of yeast and human frataxins separately as they appear to have significantly different properties. 

Fig. 12.6. The onset of synthesis of various mitochondrial polypeptides upon transferring anaerobically grown yeast cells to aerobic conditions. Yeast cells were grown overnight under anaerobic conditions. At time zero they were transferred to aerobic conditions, and at the indicated time periods samples of cells were removed and lysed in the presence of NaOH and mercaptoethanol. Samples containing about 50 /ig of protein were electrophoresed in a sodium dodecyl sulfate-polyacrylamide gel. The proteins were electrotransferred to nitrocellulose sheets and decorated with specific antibodies and l-labelled protein A. Paper pieces corresponding to the labelled protein spots were cut out from the immune blot and counted in a y counter. The amount of counts obtained in the samples of 8 h aerobic conditions was taken as 100%. The antibodies used were directed against the following polypeptides porin of the mitochondrial outer membrane (29 k) /8 subunit of the proton-ATPase (iS-F,) subunit IV of cytochrome c oxidase (OxIV) and subunit V of cytochrome c oxidase (OxV).

When yeast cells grow aerobically, they can oxidize the pyruvate entirely to carbon dioxide and water. These reactions take place in structures called mitochondria. The reactions form a cyclic scheme termed the tricarboxylic acid cycle (TCA cycle) or Krebs cycle in which in one cycle, the substrate, pyruvate, is converted entirely to carbon dioxide and water (Fig. 3-10). Both substrate level phosphorylation and electron transport mediated phosphorylation occur during the process. In electron... 

Yeast can grow both aerobically and anaerobically on glucose. Explain why the rate of glucose consumption decreases when yeast cells that have been maintained under anaerobic conditions are exposed to oxygen. 

Yeast cells grovhng aerobically synthesize sterols and incorporate them into membranes. However, under anaerobic conditions yeast cells do not survive unless they are provided "with an exogenous source of sterok. Explain the metabolic basis for this nutritional requirement. 

Fig. 9 gives the curve for At versus glucose concentration. In this example, pure substrate solutions were used and since the test was carried out under aerobic conditions an essential part of the metabolic system in the yeast cells was used. 

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