Fermentation homolactic

Variations of the alcoholic and homolactic fermentations. The course of a fermentation is often affected drastically by changes in conditions. Many variations can be visualized by reference to Fig. 17-9, which shows a number of available metabolic sequences. We have already discussed the conversion of glucose to triose phosphate and via reaction pathway a to pyruvate, via reaction c to lactate, and via reaction d to ethanol. 

It is important not to confuse the reactions of Eq. 17-42 as they occur in an aerobic cell with the tightly coupled pair of redox reactions in the homolactate fermentation (Fig. 10-3 Eq. 17-19). Tire reactions of steps a and c of Eq. 17-42 are essentially at equilibrium, but the reaction of step b may be relatively slow. Furthermore, pyruvate is utilized in many other metabolic pathways and ATP is hydrolyzed and converted to ADP through innumerable processes taking place within the cell. Reduced NAD does not cycle between the two enzymes in a stoichiometric way and the "reducing equivalents" of NADH formed are, in large measure, transferred to the mitochondria. The proper view of the reactions of Eq. 17-42 is that the redox pairs represent a kind of redox buffer system that poises the NAD+/NADH couple at a ratio appropriate for its metabolic function. 

As expected, rhodoquinone is also present in H. diminuta, a cestode known to produce succinate as end product. Surprisingly, rhodoquinone is also present in the homolactic fermenter, 5. mansoni, where it is especially present in sporocysts, the snail stage of this parasite, which produces succinate under anaerobic conditions (see section on Transitions in Energy Metabolism during the Life Cycle ). 

Fig. 5.4. Two types of energy metabolism in cestodes. (a) Type 1 homolactate fermentation, (b) Type 2 Malate dismutation. Reaction 3 involves a carboxylation step decarboxylation occurs at 6, 7 and 10. Reducing equivalents are generated at reactions 6 and 7 one reducing equivalent is used at reaction 9. Thus, when the mitochondrial compartment is in redox balance and malate is the sole substrate, twice as much propionate as acetate is produced. Key 1, pyruvate kinase 2, lactate dehydrogenase 3, phosphoenolpyruvate carboxykinase 4, malate dehydrogenase 5, mitochondrial membrane 6 malic enzyme 7, pyruvate dehydrogenase complex 8, fumarase 9, fumarate reductase 10, succinate decarboxylase complex. indicates reactions at which ATP is synthesised from ADP cyt, cytosol mit, mitochondrion. (After Bryant Flockhart, 1986.)...

Cestodes produce a range of end-products as a result of their respiratory metabolism (Table 5.4). Bryant Flockhart (104) have usefully divided the patterns of respiratory metabolism among parasitic helminths into three types. The metabolism of larval and adult cestodes fits broadly into the first two categories of this biochemical classification and these are illustrated in Fig. 5.4. Type 1 contains the homolactate fermenters in which carbohydrate is degraded, via glycolysis, to lactate and excreted. The ANU (Australian) strain of H. diminuta tends towards this type of metabolism (see below). 

Lactic acid has been used as a food preservative and food-flavoring compound. Recent attention on lactic acid has been for its use in making polylactic acid (PLA), a biodegradable polymer. As a result, the market for lactic acid is rapidly growing. Under batch fermentation conditions, homolactic fermentative... 

Figure 4.1 Homolactic fermentation. The fermentation of one mole of glucose yields two moles of lactic acid via the Embden-Meyerhof pathway.

Recently it has been shown that strain differences in helminths may lead to differences in end products. Kolhagen and coworkers [26] found a variation from 30-60% in the production of lactate in different strains of H. diminuta. It has been reported by McManus [27] that carbohydrate metabolism in adult schistosomes may exclusively lead to lactate formation and that homolactate fermentation is not per se essential for generating ATP in these worms. 

The production of, say, lactic acid from pyruvic acid is sometimes called homolactic fermentation. The enzyme involved in the one-step conversion is lactate dehydrogenase in one form or another. A schematic diagram for the conversion is shown in Figure 3.2. We omit further details about the conversion. 

In bacteria such situations are very common during fermentation. The end products of fermentation are continuously produced internally and excreted into the external medium (Fig. 11). The energy yield by efflux of fermentation products can be quite considerable. Recently, Michels et al. (30) calculated on theoretical grounds that the additional energy yield during homolactic fermentation is in the order of... 

The industrial strains of lactic acid bacteria can completely ferment a medium of 12-15 % sugar in 2-4 days with greater than a 90 % yield of lactic acid. All of these bacteria are considered homofermentative in that they utilize a homolactic fermentation pathway, producing two molecules of lactic acid for each molecule of hexose. As diagrammed in O Fig. 1.10, the Embden-Meyerhof pathway is used in the homolactic fermentation. 

Homolactic fermentation pathway of giucose (giycoiysis, Embden-Meyerhof pathway)... 

Bianchi MM, Brambilla L, Protani F, Liu C, Lievense J, Porro D (2001) Efficient homolactic fermentation by Kluyveromyces lactis strains defective in pyruvate utilization and transformed with the heterologous LDH gene. Appl Environ Microbiol 67 5621-5625. doi 10.1128/AEM. 67.12.5621-5625.2001... 

---