Saccharomyces cerevisiae has long been used in industrial alcohol and alcoholic beverages because it has the ability to ferment glucose into ethanol. The interesting thing is the process of ethanol on yeast fermentation takes place in aerobic conditions.According to Pasteur, the presence of oxygen will inhibit the fermentation pathway in the yeast cell so that there is a carbon source to be used via the respiratory route. This phenomenon is often referred to as the Pasteur effect (Walker 1998). In cells of prokaryotes and eukaryotes, many found the Pasteur effect, one example is lactic acid fermentation by human muscle cells when deprived of oxygen.
Based on this phenomenon, should the production of ethanol by yeasts occurs in anaerobic conditions. But it turns out, the Pasteur effect in yeast cells was observed in cells that had entered stationary phase (resting), while the production of alcohol occurs when cells are in growth phase (log phase) (Alexander & Jeffries 1990). This makes the Pasteur effect is not a phenomenon that allegedly occurred during the production of ethanol by Saccharomyces cerevisiae.
Herbert Crabtree in 1929 discovered another phenomenon that occurs in tumor cells where the cell is the dominant fermentation pathway occur even in aerobic conditions (Alexander & Jeffries 1990).
In 1948, Swanson and Clifton first to show that this phenomenon occurs in Saccharomyces cerevisiae cells growing and producing ethanol as a fermentation product during a certain amount of glucose contained in the growth medium (Alexander & Jeffries 1990).
This phenomenon was originally called the contre-Pasteur effect before the term used Crabtree effect (de Dekken 1966). Crabtree effect in yeast can be observed when the growth medium containing glucose in concentrations tinggai (above 5 mM) (Walker 1998). Based on the de Dekken (1966),
Crabtree effect does not occur in all yeasts, but only on a few species only, among others Saccahromyces cerevisiae, S. chevalieri, S. italicus, S. oviformis, S. pasteurianus, S. turbidans, S. calsbergensis, Schizosaccharomyces pombe, Debaryomyces globosus, Bretanomyces lambicus, Torulopsis dattila, T. glabrata, and T. colliculosa.
There are three mechanisms that explain the Crabtree effect: 1. catabolite repression; 2. catabolite inactivation, and 3. respiration capacity is limited.
Catabolite repression occurs when glucose, or the initial product of glucose metabolism, suppress the synthesis of various enzymes of respiration (Fietcher et al. 1981). But the detailed mechanisms, such as the compound that gives a signal to suppress the synthesis of these, is still unclear (Walker 1998).
The initial idea catabolite repression triggered by von Meyenberg in 1969 (Alexander & Jeffries 1990) that fosters S. cerevisiae in a medium containing glucose with the method continues culture. The results showed that when low cell concentrations, the metabolic pathways used are respiration, whereas when the cell concentration has reached a critical number, ethanol fermentation occurs.
From these results expected at low cell concentrations, enzymes of respiration was still sufficient to perform respiration path, but when the cell concentration increases, the concentration of enzyme is not increased because the suppressed synthesis by glucose, so the path respiration stopped and replaced by fermentation.
In addition to the repression of enzyme synthesis, a high concentration of sugar will also disrupt the structure of yeast mitochondria, for example loss of membrane in and kristae. However, these structures will be back to normal when respiration path replaces ethanol fermentation (Walker 1998). Changes in the structure will inhibit the Krebs cycle and oxidative phosphorylation takes place in mitochondria.
Catabolite inactivation occurs when glucose disable a key enzyme in the respiratory track, such as fructose 1,6-bifosfatase (FBPase). Inactivation occurs primarily through phosphorylation of the enzyme, followed by protein digestion enzymes in the vacuole (Walker 1998).
The mechanism of inactivation of FBPase in S. cerevisiae begins with the increased concentration of cAMP and FBPase in cells by glucose. The increase in both molecules will trigger cAMP-dependent protein kinase for phosphorylation of FBPase (Francois et al. 1984).
The last mechanism explains the Crabtree effect in yeast is the limited capacity of yeast respiration proposed by Bardford & Hall (1979). Both researchers are conducting research that is similar to von
Meyenberg, but found no evidence of catabolite repression by glucose. Therefore, they argue that the yeast-fermenting yeast that is able to perform aerobic respiration has limited capacity. When present in high concentrations of glucose, glycolysis would run quickly so as to produce a high amount of pyruvat.
But the limitations of these yeasts to use pyruvat in line next respiration (Krebs cycle and oxidative phosphorylation) causes the remaining pyruvat Fermentative revamped into ethanol.
In contrast, the yeast fermentation that is not doing aerobic respiration is considered to have an unlimited capacity to be able to use all pyruvat generated from glycolysis, although the amount of glucose in the medium high (Alexander & Jeffries 1990).
Ethanol fermentation by Saccharomyces cerevisiae: The Crabtree Effect
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