Lactic acid fermentation

Lactic acid fermentation designated routes of energy metabolism in animals in which glucose and other monosaccharides to lactic acid alone or next to yet another final products are degraded. There are exergonic chemical reactions that are used to living organisms as an energy source.

Especially lactic acid bacteria operate the lactic acid fermentations. For lack of oxygen but lactic acid can be formed from sugars also in some fungi, plants and animals and in humans ( → hypoxemia ). However, the rule of lactic acid fermentation in animals and humans is the production of energy from glucose in the oxidative Weiterverstoffwechselung not qualified or restricted muscle - see also the section lactic acid fermentation in mammalian cells.

• 2.1 lactic acid fermentation with lactic acid bacteria
• 2.2 lactic acid fermentation in mammalian cells

Biochemical sequence

After the main end products formed and the degradation pathways, a distinction different types of lactic acid fermentation:

• Homofermentative lactic acid fermentation, in which only lactic acid as the major end-product is formed,
• Heterofermentative lactic acid fermentation, wherein the acetic acid is formed as the major end products of addition of lactic acid in the case of hexoses degradation ethanol and carbon dioxide, in the case of pentoses degradation
• Lactic acid fermentation by Bifidobacterium, wherein lactic acid and acetic acid is formed as main final product.

Disaccharides are first cleaved into monosaccharides.

Homofermentative lactic acid fermentation

In the homofermentative lactic acid monosaccharides are converted to lactic acid which dissociates into protons and lactate ions is present under physiological conditions.

This glucose is first broken down in glycolysis to pyruvate. This is the enzyme lactate dehydrogenase with the formed reduced in the oxidation of coenzyme NADH to phosphoglycerate glyceraldehyde to lactate ( the anion of lactic acid), NADH is oxidized to NAD .

During glycolysis be formed per molecule of glucose per two molecules of lactate, NADH and ATP, such that the sum equation of homofermentative lactic acid fermentation is:

The net energy yield is thus 2 molecules of ATP per molecule of glucose.

The lactate in mammalian cells can not be metabolized anaerobically, but only - if available in sufficient quantities NAD available - be converted back to pyruvate.

Some microorganisms such as Clostridium propionicum, however, may still continue to remove lactate, for example, to propionate in the propionic acid fermentation

Heterofermentative lactic acid fermentation

The path of the heterofermentative lactic acid fermentation is bashed by lactic acid bacteria, which lack the enzyme aldolase. This is necessary for glycolysis for cleavage of fructose -1 ,6- bisphosphate in the two Phosphotriosen dihydroxyacetone and glyceraldehyde. Heterofermentative Milchsäuregärer can degrade over xylulose -5- phosphate by this particular pathway both hexoses ( such as glucose or fructose), and especially pentoses ( xylose, ribose ). Typical representatives are the obligate heterofermentative Oenococcus oeni and Leuconostoc mesenteroides Milchsäuregärer, or the optional leavener Lactobacillus pentosus and Lactobacillus plantarum.

Biochemistry

Heterofermentative lactic acid bacteria are specialized in the degradation of pentoses. This is transferred under ATP consumption in pentose -5 -phosphate and isomerized xylulose -5-phosphate, which catalyzes an epimerase ( EC 5.1.3.1 ). The product is of the key enzyme of the pathway, the phosphoketolase ( EC 4.1.2.9 ), incorporating an inorganic phosphate (Pi ) is cleaved in triose glyceraldehyde phosphate and acetyl phosphate. Glyceraldehyde is converted in the course of regularly glycolysis to pyruvate, two molecules of ATP and one molecule of NADH can be obtained. This NADH is reoxidized by pyruvate is reduced to lactate. This reaction corresponds to the last step of homofermentative lactic acid fermentation ( see above).

The acetyl phosphate obtained in the cleavage is converted to acetate. The high-energy Säureanhydridbindung is used to gain ATP via substrate chain. Therefore, it is constructed in this step ATP, the reaction catalyzed an acetate kinase ( EC 2.7.2.1 ).

The degradation of pentoses is also called Phosphoketolaseweg. The net energy yield is thus 2 molecules of ATP per molecule of pentose.

Also hexoses can be recycled. Here, this, for example glucose, as first activated with the input stages of the Entner- Doudoroff pathway by hexokinase to glucose -6-phosphate by ATP consumption. This oxidized a glucose-6 -phosphate dehydrogenase to give 6- Phosphoglucono - δ - lactone to NADP consumption. The lactone is then hydrolyzed by a 6- Phosphoglucolactonase to 6 -phosphogluconate. This is then decarboxylated to ribulose -5-phosphate and NADP is oxidized with which a phosphogluconate dehydrogenase ( EC 1.1.1.44 ) catalyses. Ribulose -5-phosphate is then epimerized to xylulose -5-phosphate phosphoketolase and finally cleaved by. In contrast to the degradation pathway of pentoses but no acetate is formed, as four additional reducing equivalents apply and must be reoxidized. To the resulting acetyl -CoA over acetyl and acetaldehyde is reduced to ethanol.

In the balance sheet is used by the two gained one ATP phosphorylation of glucose degraded, but when Acetatzweig none formed. As a result of the net energy yield in the heterofermentative lactic acid fermentation of hexoses only one molecule of ATP per hexose:

Variations

In the heterofermentative lactic acid fermentation, the phosphoketolase can also accept fructose -6-phosphate as a substrate, thereby arise next acetyl erythrose 4 -phosphate. The latter is reduced to erythritol -4 -phosphate and phosphate cleavage reacted according to erythritol. This as " Erythritweg " designated byway, however, has only a low activity.

Alternatively, the glyceraldehyde are reduced to glycerol. Here, glyceraldehyde is first reduced to glycerol -1-phosphate, and then hydrolyzed to glycerol.

Importance

Since inability to glycolysis because of the absence of aldolase degradation of pentoses is more favorable than that of hexoses, heterofermentative Milchsäuregärer are specialized in the degradation of pentoses. These originate for example from plant material that degrade these bacteria. In addition to hexoses come in larger quantities pentoses also present in the grape must, wine or leaven, so that many heterofermentative Milchsäuregärer grow there.

The growth rates of heterofermentative Milchsäuregärern are lower in the degradation of hexoses as the degradation of pentoses, since the hydrogen transfer agents are reoxidized slowly. The reason for this is the low activity of acetaldehyde dehydrogenase. In addition, coenzyme A is required as a cofactor for this degradation pathway. Therefore, the supply of pantothenic acid for the maintenance of metabolic pathway is necessary. Otherwise, the formation of ethanol is inhibited. Thus, the fermentation can take place, the hydrogen carrier must be reoxidized by the formation of glycerol and erythritol (coenzyme A- independent). Since the degradation of pentoses coenzyme A is not needed, there is a lack of pantothenic acid has no direct influence.

Bifidobacterium fermentation

The lactic acid bacterium Bifidobacterium bifidum has as heterofermentative lactic acid bacteria have no aldolase, but bypasses the aldolase step in another way: fructose -6-phosphate is cleaved phosphorolytisch to erythrose -4 -phosphate and acetylphosphate. Erythrose 4 -phosphate, fructose -6- phosphate in the transaldolase and transketolase reactions to two molecules of xylulose -5-phosphate is reacted with a further molecule, both of which are cleaved by a phosphorolytisch phosphoketolase to glyceraldehyde and acetyl phosphate ( pentose phosphate pathway ). With the three molecules of acetyl phosphate 3 ADP phosphorylated to ATP 3, whereby energy in the form of three molecules of ATP conserved and acetic acid is formed as one of the two final products. The two molecules are glyceraldehyde as other lactic acid bacteria for the lactic acid, the final product of the second fermentation, implemented with four molecules of ADP phosphorylated to ATP. The net energy yield is thus 2.5 molecules of ATP per molecule of glucose.

Examples of the occurrence of the lactic acid fermentation

Lactic acid fermentation with lactic acid bacteria

Bacteria which produce lactic acid as the sole or principal fermentation product are referred to as lactic acid bacteria. They are an order of Gram-positive bacteria and are characterized by the absence of the necessary for the Elektronentransportphosphorylierung porphyrins and cytochromes, so that they can get their energy only by the breakdown of sugar -coupled substrate chain.

One case is different:

• Homofermentative strains of lactic acid bacteria which form lactic acid as a single major end product. These include the genera Streptococcus, Enterococcus, Lactococcus and Pediococcus as well as some members of the genus Lactobacillus.

• Heterofermentative strains of lactic acid bacteria which form lactic acid as the major end products and carbon dioxide in addition to hexoses and pentoses degradation ethanol - acetic acid degradation. These bacteria lack aldolase, the key enzyme of glycolysis. These include the genera Leuconostoc and some members of the genus Lactobacillus, Lactobacillus buchneri mainly.

• Bacteria - type Bifidobacterium bifidum, Bifidobacterium performing the fermentation.

Lactic acid fermentation in mammalian cells

Compared to aerobic respiration is only gained a small amount of energy in fermentation, since it takes the citric acid cycle and subsequent respiratory chain only the substrate chain is utilized. However, the fermentation is a way to quickly to form ATP by substrate chain, without being dependent on oxygen.

In mammals, including man counts, there are numerous examples showing that cells meet their energy from the ( homofermentative ) lactic acid fermentation. To gain fast-twitch white muscle fibers (FT ) fibers because of their smaller facilities with mitochondria and the corresponding enzymes compared to slow-twitch red muscle fibers (ST ) fibers their energy even at low intensity by lactic acid fermentation.

At higher intensity, a higher proportion of FT fibers are recruited. This also falls lactate in larger quantities. As long as the entire organ and muscle system with the transport (see below ) and the further metabolism ( Laktatutilisation ) but not overwhelmed, the body can maintain a lactate steady state in relation to the blood - lactate. At very high intensities ( the sprint from the outset ) a sufficiently fast power supply is only possible by a high glycolytic rate, which results in an exponential increase of blood lactate.

The resulting fermentation in the lactate is partially while partially metabolized further in connection to the increased Leistungsabforderung in various ways. Lactate is delivered through a monocarboxylate transporter 1 in the blood and taken up from this competent liver cells or muscle lactate oxidation in skeletal muscle and the heart muscle, and then oxidized to pyruvate ( " cell-cell lactate shuttle "). Pyruvate can be used via the citric acid cycle for further energy or - in the liver - rebuilt to glucose ( gluconeogenesis ) and the muscles and organs are supplied via the bloodstream ( Cori cycle).

Other organs derive their energy from the lactic acid fermentation, and that is when they are starved of oxygen. The thereby increasing lactate concentration in the blood leads to a decrease in pH, which can lead to lactic acidosis in special circumstances (eg suffocation ).

Other, specialized cells relate ATP exclusively from the anaerobic breakdown of glucose into lactic acid fermentation. Erythrocytes, for example, can metabolize only under anaerobic conditions due to lack of mitochondrial glucose. Since the cornea is avascular, oxygen can pass by diffusion of the corneal cells and not through the blood stream. This limits the supply of oxygen so that a constant energy supply is ensured only on the lactic acid fermentation.

Even in larger animals often come quickly enough oxygen in the tissue, so that enough energy is produced by the fermentation. Alligators and crocodiles are able to start lightning-fast attacks that cost a lot of energy. This energy comes from the lactic acid fermentation. Also, elephants, rhinos, whales and seals are dependent on the lactic acid fermentation.

Lactic acid fermentation for the production of food and feed

For the preservation of food, the lactic acid fermentation is used at least since the Neolithic period. Due to the formation of lactic acid the food is acidified and spoilage are almost completely inhibited in their activity or even killed. Examples are sour milk products such as yogurt, cottage cheese and butter milk, bread drink, sauerkraut, sour beans, the Korean Gimchi, the Japanese Tsukemono and other pickled vegetables.

Furthermore, the lactic acid fermentation is used to make bread with sourdough and maturation of raw sausages as Teewurst, salami and other fermented sausages.

For lactic acid fermentation in dairy products, lactic acid bacteria need the enzyme lactase, which converts using the H2O lactose lactose ( C12H22O11 ) into glucose ( C6H12O6 ) and galactose ( C6H12O6 ). Both sugar so incurred will be implemented through one or more of the ways described above.

The lactic acid fermentation is also used for preservation of plant material as feed in agriculture. This is referred to as a rule carried out in silos as ensiling process and the product as silage.