Rutas metabolicas

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Figure 8. The Embden Meyerhof pathway for glucose dissimilation. The overall reaction is the oxidation of glucose to 2 pyruvic acid. The two branches of the pathway after the cleavage are identical, drawn in this manner for comparison with other bacterial pathways of glycolysis.

This is the pathway of glycolysis most familiar to biochemists and eukaryotic biologists, as well as tobrewers, breadmakers and cheeseheads. The pathway is operated by Saccharomyces to produce ethanol and CO2. The pathway is used by the (homo)lactic acid bacteria to produce lactic acid, and it is used by many other bacteria to produce a variety of fatty acids, alcohols and gases. Some end products of Embden-Meyerhof fermentations are essential components of foods and beverages, and some are useful fuelsand industrial solvents. Diagnostic microbiologists use bacterial fermentation profiles (e.g. testing an organism's ability to ferment certain sugars, or examining an organisms's array of end products) in order to identify them, down to the genus level.

The first three steps of the pathway prime (phosphorylate) and rearrange the hexose for cleavage into 2 trioses (glyceraldehyde-phosphate).Fructose 1,6-diphosphate aldolase is the key (cleavage) enzyme in the E-M pathway. Each triose molecule is oxidized and phosphorylated followed by two substrate level phosphorylations that yield 4 ATP during the drive to pyruvate.

Lactic acid bacteria reduce the pyruvate to lactic acid; yeast reduce the pyruvate to alcohol (ethanol) and CO2 as shown in Figure 9 below.

The oxidation of glucoseto lactate yields a total of 56 kcal per mole of glucose. Since the cells harvest 2 ATP (16 kcal) as useful energy, the efficiency of the lactate fermentation is about 29 percent (16/56). Ethanol fermentations have a similar efficiency.

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Figure 9. (a) The Embden Meyerhof pathway of lactic acid fermentation in lactic acid bacteria (Lactobacillus) and (b) the Embden Meyerhof pathway ofalcohol fermentation in yeast (Saccharomyces). The pathways yield two moles of end products and two moles of ATP per mole of glucose fermented. The steps in the breakdown of glucose to pyruvate are identical. The difference between the pathways is the manner of reducing pyruvic acid, thereby giving rise to different end products.

Figure 10. Fermentations in bacteria that proceed through theEmbden-Meyerhof pathway.

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1 The Heterolactic (Phosphoketolase) Pathway

The phosphoketolase pathway (Figure 11) is distinguished by the key cleavage enzyme, phosphoketolase, which cleaves pentose phosphate into glyceraldehyde-3-phosphate and acetyl phosphate. As a fermentation pathway, it is employed mainly by the heterolactic acid bacteria, which include some species of Lactobacillus andLeuconostoc.In this pathway, glucose-phosphate is oxidized to 6-phosphogluconic acid, which becomes oxidized and decarboxylated to form pentose phosphate. Unlike the Embden-Meyerhof pathway, NAD-mediated oxidations take place before the cleavage of the substrate being utilized. Pentose phosphate is subsequently cleaved to glyceraldehyde-3-phosphate (GAP) and acetyl phosphate. GAP is converted tolactic acid by the same enzymes as the E-M pathway. This branch of the pathway contains an oxidation coupled to a reduction while 2 ATP are produced by substrate level phosphorylation. Acetyl phosphate is reduced in two steps to ethanol, which balances the two oxidations before the cleavage but does not yield ATP. The overall reaction is Glucose ---------->1 lactic acid + 1 ethanol +1 CO2 with a netgain of 1 ATP. The efficiency is about half that of the E-M pathway.

Heterolactic species of bacteria are occasionally used in the fermentation industry. For example, one type of fermented milk called kefir, analogous to yogurt which is produced by homolactic acid bacteria, is produced using a heterolactic Lactobacillus species. Likewise, sauerkraut fermentations use Leuconostoc species of...