Knowledge of saccharomyces cerevisiae
Saccharomyces cerevisiae has long been used in industrial alcohol and alcoholic beverages have the ability to ferment glucose into ethanol. This is an interesting process in the ethanol fermentation is leavened held on condition aerob.
According to Pasteur, the existence of oxygen will prevent fermentation pathways in cells so that leavened the carbon source will be used through respiration. This phenomenon is often referred to as the Pasteur effect (Walker 1998). In the cells prokariota and eukariota, Pasteur found a lot of effect, one example is laktat acid fermentation by human muscle cells when lack of oxygen. Based on this phenomenon, should be leavened by ethanol production occurred in the conditions anaerob. But in fact, Pasteur effect in cell leavened observed in cells that have entered a phase stasioner (resting), while the production of alcohol occurs when the cells are in a growth phase (log phase) (Alexander & Jeffries 1990). This makes the Pasteur effect is not suspected of phenomena that occur during ethanol production by Saccharomyces cerevisiae.
Herbert Crabtree in 1929 to find a phenomenon that occurs in tumor cells where the cell lines is dominant fermentation occurs even in conditions aerob (Alexander & Jeffries 1990). In 1948, Clifton Swanson, and the first time shows that the phenomenon occurs in Saccharomyces cerevisiae cells that are grown and produced ethanol as a product during fermentation there is a certain amount of glucose in the growth medium (Alexander & Jeffries 1990). The phenomenon was originally called the Pasteur effect contre-term Crabtree effect prior to use (de Dekken 1966). Crabtree effect can be observed on leavened when the growth medium containing glucose in a high concentration (above 5 mm) (Walker 1998). Based on the de Dekken (1966), Crabtree effect does not occur at all leavened, but only in some species are, 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. repression katabolit 2. inaktivasi katabolit; and 3. respiration capacity is limited.
Katabolit repression occurs when glucose, or glucose metabolism initial product, a variety of enzyme synthesis respiration (Fietcher et al. 1981). However, detailed mechanisms, such as the compound that gives the signal for the synthesis is still not clear (Walker 1998). Initial idea katabolit dicetuskan repression by von Meyenberg in the year 1969 (Alexander & Jeffries 1990) that the S. cerevisiae in a medium containing glucose method continues with the culture. Results of research shows that when the cell concentration is low, the metabolic pathways of respiration is used, whereas when the cell concentration has reached a critical number, ethanol fermentation occurs. From the results on suspected cells of a low concentration, enzyme-enzyme respiration is sufficient to make a path respiration, but when the concentration of cells increased, the concentration of enzyme does not increase because sintesisnya pressed by glucose, so the path respiration stopped and replaced by fermentation. In addition to repression of enzyme synthesis, a high concentration of sugar will also disrupt the structure of leavened mitokondria, for example in the loss of membrane and kristae. However, the structure will return to its normal path during respiration replace ethanol fermentation (Walker 1998). Changes in the structure will prevent Krebs cycle and fosforilasi oksidatif which took place in the mitokondria.
Inaktivasi katabolit occurs when glucose turn off the key enzyme in the path respiration, for example fruktosa 1.6-bifosfatase (FBPase). Inaktivasi going first through the process fosforilasi enzyme, and then followed by the digestion of protein in the enzyme vakuola (Walker 1998). Mechanism inaktivasi FBPase in S. cerevisiae begins with the increased concentration camp and FBPase in the cells by glucose. The increase in the second molecule will trigger camp-dependent protein kinase to do fosforilasi against FBPase (Francois et al. 1984).
Mechanisms that explain the latest on the Crabtree effect is the limited capacity of leavened leavened respiration proposed by Bardford & Hall (1979). Both researchers conducting research that is similar to the von Meyenberg, but found no evidence of repression by glucose katabolit. Therefore they argued that leavened leavened, that is capable of doing fermentation aerob respiration has limited capacity. When glucose is in high concentration, akan glikolisis running quickly pyruvat resulting in a high number. However, limitations to use leavened pyruvat in the next lane respiration (Krebs cycle and fosforilasi oksidatif) causes pyruvat remaining fermentatif be changed into ethanol. Of the, that is not leavened aerob fermentation is considered to have a capacity of respiration is not limited in its use throughout pyruvat resulting from glikolisis although the amount of glucose in the medium level.
(source:Alexander & Jeffries 1990)
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