Metabolic yeast cells

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. 

Although uptake and accumulation of most amino acids from the external medium seems to be irreversible, amino acids are excreted into the medium whenever they are overproduced above a given threshold by yeast cells [6], This can occur under a number of specific conditions, namely in mutants with impaired regulation of amino acid biosynthesis, or in the presence of mutations preventing substrate catabolism, or when growth occurs in the presence of metabolic intermediates. It can even occur when growth is arrested under conditions where amino acid synthesis can continue. 

Yeast cells are able to metabolize many types of sugars. In this experiment, you will observe the fermentation of sugar by baker s yeast. When yeast cells are mixed with a sucrose solution, they must first hydrolyze the sucrose to glucose and fructose. Then the glucose is broken down in the absence of oxygen to form ethanol and carbon dioxide. You can test for the production of carbon dioxide by using a CBL pressure sensor to measure an increase in pressure. 

Glycolytic oscillations in yeast cells provided one of the first examples of oscillatory behavior in a biochemical system. They continue to serve as a prototype for cellular rhythms. This oscillatory phenomenon, discovered some 40 years ago [36, 37] and still vigorously investigated today [38], was important in several respects First, it illustrated the occurrence of periodic behavior in a key metabolic pathway. Second, because they were soon observed in cell extracts, glycolytic oscillations provided an instance of a biochemical clock amenable to in vitro studies. Initially observed in yeast cells and extracts, glycolytic oscillations were later observed in muscle cells and evidence exists for their occurrence in pancreatic p-cells in which they could underlie the pulsatile secretion of insulin [39]. 

FIGURE 21-8 Subcellular localization of lipid metabolism. Yeast and vertebrate cells differ from higher plant cells in the compartmentation of lipid metabolism. Fatty acid synthesis takes place in the compart-... 

Inside the yeast cell the hexoses are converted principally to ethanol, carbon dioxide, and adenosinetriphosphate (ATP) with the liberation of waste heat. The ATP is an energy source in cell metabolism the ethanol and carbon dioxide diffuse across the cell wall to the exterior where the ethanol dissolves in the juice and the carbon dioxide bubbles... 

With benzaldehyde 144 or halogenated derivatives (Cl, F) as acceptors the yeast-PDC-catalyzed addition proceeds with almost complete stereoselectivity to furnish the corresponding (R)-configurated 1-hydroxy-1-phenylpropanones 145 [447]. For practical reasons, whole yeast cells are most often used as the catalyst, with only small loss of enantioselectivity [423,424]. The conversion of benzaldehyde in particular has gained industrial importance because the acyloin is an important precursor for the synthesis of L-(-)-ephedrine [448]. Otherwise, the substrate tolerance is remarkably broad for aromatic aldehydes on the laboratory scale, however, yields of acyloins are usually low because of the prior or consequent reductive metabolism of aldehyde substrate and product, giving rise to considerable quantities of alcohol 146 and vicinol diols 147, respectively [423,424,449], The range of structural variability covers both higher a-oxo-acids (e.g. -butyrate, -valerate) as the donor component, as well as a,/J-un-saturated aldehydes (e.g. cinnamaldehyde 148) as the acceptor [450]. 

Alcohol dehydrogenases are enzymes that are well known from physiological and biochemical studies on the primary metabolism of cells. Several ADHs are commercially available and for some of them such as the ADH from yeast or liver details concerning the structure and reaction mechanism have been elucidated. For preparative applications however they seldom meet the requirements and new enzymes are needed for this field. 

Peng, X.Y., Li, P.C.H., A three-dimensional flow control concept for single-cell experiments on a microchip. 2. Fluorescein diacetate metabolism and calcium mobilization in a single yeast cell as stimulated by glucose and pH changes. Anal. Chem. 2004, 76, 5282-5292. 

Other forms of vanadium have been implicated in the stimulation of the plasma membrane vanadate-dependent NAD(P)H oxidation reaction. Decavanadate has been shown to be a more potent stimulator of the vanadate-dependent NADH oxidation activity than added orthovanadate [30,31], Interestingly, decavanadate reductase activity has been found to be an alternative activity of an NADP-specific isocitrate dehydrogenase [32], Diperoxovanadium derivatives have also been shown to be involved in this type of reaction [33,34], Decavanadate may play a role in the biological role of vanadium, as it is found in yeast cells growing in the presence of orthovanadate [8] and is a potent inhibitor of phosphofructokinase-1, the control step of glycolysis, and other metabolic reactions [35],... 

After sterilization, yeast is added to initiate fermentation. McConnell and Schramm (1995) recommend inoculation with no less than 10% by volume. Moreover, as the pH of honey is naturally low and because it is poorly buffered, the pH of must may drop during fermentation to a point limiting yeast efficiency. pH reduction can result from the synthesis of acetic and succinic acids by the yeast cells (Sroka and Tuszynski, 2007). While a rapid decline in pH inhibits undesirable microbial activity (Sroka and Tuszynski, 2007), it also reduces the dissociation of fatty acids in the wort, potentially slowing yeast metabolic action. For this, addition of a buffer is important to maintain the pH within a range of 3.7-4.0 throughout fermentation (McConnell and Schramm, 1995). Calcium carbonate, potassium carbonate, potassium bicarbonate, and tartaric acid are potential candidates. However, as some of these salts can add a bitter-salty... 

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.

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