Yeast cell oxygen requirement

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... 

While enantioselectivity during reduction of ethyl 3-oxobutanoate by baker s yeast (Saccharomyces cerevisiae) to ethyl (S)-3-hydroxybutanoate was found to exceed 99%, yields did not exceed 50-70% (Chin-Joe, 2000). Elimination of two of three causes, evaporation of substrate and product esters and absorption or adsorption of the two esters by the yeast cells, increased the yield to 85%. Alleviation of hydrolysis of the two esters by yeast enzymes could increase the yield even more. Low supply rates of glucose as an electron donor provided the most efficient strategy for electron donor provision and yielded a high enantiomeric excess of ethyl (S)-3-hydroxybutanoate, low by-product formation and biomass increase, with a low oxygen requirement(Chin-Joe, 2001). 

Aeration, Temperature and Duration—Yeast cells require oxygen for carrying on their vital activities, and although fermentation will go on in the absence of air, unless the necessary oxygen is supplied the cells soon lose... 

A key intermediate in the biosynthesis of cholesterol and related sterols is squalene, an open-chain isoprenoid hydrocarbon. It is converted to squalene 2,3-epoxide, which in turn is converted to lanosterol. The conversion of squalene to the 2,3-epoxide is catalyzed by a monooxygenase, and molecular oxygen is a required component for this reaction. Under anaerobic conditions, yeast cells cannot synthesize sterols because they lack oxygen, a substrate for the monooxygenase reaction. 

The membrane is composed of lipid and phospholipid and contains proteins and sterols. The absolute requirement for unsaturated fats and sterols in membranes accounts for much of the oxygen requirement of brewing yeast strains (see Chapter 18). The nature of the unsaturated fats in the cell membrane affects its properties, e.g. in relation to ethanol tolerance [30]. 

Conway adduced no experimental evidence of his own to support his theory of the behavior of oxyntic cells, and his evidence for a redox reaction in the yeast cell that exchanges hydrogen for potassium ions was, in contrast to the elaborate nature of his argument, extremely sketchy. Oxygen is required, and so is an oxidizable substrate secretion is inhibited by azide, monoiodoacetate, and 2,4-dinitrophenol and that is all. 

In fermentation no oxygen is used, so that there is no question as to permeability to oxygen. Glucose, provided in the medium, must permeate the yeast cell before metabolism starts. Metabolism, probably by means of several steps leads to the liberation of carbon dioxide presumably by decarboxylation. To be measured, this carbon dioxide must pass out through the plasma membrane and be freed as a gas from the medium (see Nord and Weichherz, 64). The very great permeability to carbon dioxide of all or most of all the studied types of plasma membrane leads to the conclusion that this step has no measurable influence. The liberation of carbon dioxide from even saturated solutions has been thought to require the use of special methods, such as the addition of large amounts of citric acid as Meyerhof advocates (53). Further study of this step is desirable. 

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