Bioprocess Technology- Fundamentals and Applications Chapt 4_Metabolic basis(10)

little energy. Another important product is acetic acid (vinegar) obtained from Acetobactersp. Strictly aerobic acetic acid producing bacteria obtain energy from the partial oxidation ofethanol yielding acetic acid as an end product. This means that the product is spontaneouslyproduced as is the end products of anaerobic fermentations described below.4.6.2 Anaerobic fermentative energy metabolism.

A number of products that are, or have been produced in biotechnical processes are endproducts of anaerobic fermentative energy metabolism. Examples of such products includeethanol, lactic acid, butanediol and acetone-butanol.

Products of this type are spontaneously produced both in growing and non-growing cells. Therate of production increases with growth rate but the yield coefficient varies inversely to thegrowth rate. The principles of these fermentations are outlined in Fig. 4.7. The mixed acidfermentation of E. coli is also shown (Fig 4.8) because it will be triggered if oxygenlimitation occur in any process utilising E. coli (e.g. as host for a plasmid encoded product).Anaerobic fermentative energy metabolism gives a very low energy yield compared toaerobic. ATP is produced only by substrate level phosphorylation. Fermentative organismsobtain 2 ATP/glucose in glycolysis and 1-2 ATP further in other reactions associated withacid production (e.g. acetic or butyric acid). A high flux through glycolysis is required toobtain sufficient energy for growth resulting in excretion of large amounts of end products. Infacultative organisms the rate of glycolysis increases during a shift from aerobic to anaerobicconditions. This was originally observed in yeast by Pasteur and has since then been calledthe Pasteur effect. Today this phenomenon can be explained in terms of regulation of thephosphofructokinase, the rate limiting enzyme of glycolysis. This enzyme responds tochanges in the ATP concentration, or rather to the energy charge. During anaerobicconditions the energy charge tends to fall resulting in an increased enzyme activity.

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