Lactic acid fermentation
Lactic acid fermentation describes the energy metabolism pathways in living organisms in which glucose and other monosaccharides are broken down into lactic acid alone or other end products . They are exergonic chemical reactions that serve as an energy source for living beings .
In addition to lactic acid bacteria , which form lactic acid from sugars, lactic acid fermentation is also used in some fungi , plants and animals as well as humans (→ hypoxemia ) when there is a lack of oxygen . The normal case of lactic acid fermentation in humans and other animals, however, is the production of energy from glucose in muscle cells that are not or to a limited extent capable of oxidative further metabolism - cf. see the section on lactic acid fermentation in mammalian cells .
A distinction is made between different types of lactic acid fermentation according to the main end products formed and the degradation routes:
- homofermentative lactic acid fermentation, in which only lactic acid is formed as the main end product,
- heterofermentative lactic acid fermentation, in which as the main end products in addition to lactic acid in the case of hexoses cleardown ethanol and carbon dioxide, in the case of pentoses cleardown acetic acid are formed,
- Lactic acid fermentation by Bifidobacterium , during which lactic acid and acetic acid are formed as the main end products.
Homofermentative lactic acid fermentation
First, glucose is broken down into pyruvate in glycolysis . This is reduced to lactate ( anion of lactic acid) by the enzyme lactate dehydrogenase with the coenzyme NADH formed during the oxidation of glyceraldehyde phosphate to phosphoglycerate , the NADH is oxidized to NAD + .
During the homofermentative fermentation of glucose, two molecules of lactate and two adenosine triphosphate are formed per molecule of glucose , so that the sum equation for homofermentative lactic acid fermentation is as follows:
So a glucose molecule reacts with two adenosine diphosphate (ADP) molecules and two free phosphates to form two lactic acid molecules, two protons, two adenosine triphosphate (ATP) molecules and two molecules of water.
The net energy yield is therefore 2 molecules of ATP per molecule of glucose.
The lactate cannot be further metabolized in mammalian cells; it can only be converted back into pyruvate if there is sufficient NAD available. Hence it is also known as the “metabolic dead end”.
Some microorganisms such as Clostridium propionicum or Megasphaera elsdenii, on the other hand, can break down lactate even further, for example to propionate and acetate in the propionic acid fermentation .
Heterofermentative lactic acid fermentation
In contrast to homofermentative fermentation, the degradation products carbon dioxide , acetate (acetic acid) and ethanol (alcohol) also occur in heterofermentative lactic acid fermentation . In addition to the yeast Kluyveromyces lactis , mainly lactic acid bacteria are active, which lack the enzyme aldolase . This is necessary for the glycolysis to split fructose-1,6-bisphosphate into the two phosphotrioses dihydroxyacetone phosphate and glyceraldehyde phosphate . Heterofermentative lactic acid fermenters can break down both hexoses (such as glucose or fructose) and especially pentoses ( xylose , ribose ) via xylulose-5-phosphate through this special metabolic pathway . Typical representatives are the obligatory heterofermentative lactic acid fermenters Oenococcus oeni and Leuconostoc mesenteroides , as well as the optional fermenters Lactobacillus pentosus and Lactobacillus plantarum . See also: Lactobacillaceae .
Heterofermentative lactic acid bacteria specialize in breaking down pentoses. This is converted into pentose-5-phosphate with consumption of adenosine triphosphate (ATP) and isomerized into xylulose-5-phosphate, which catalyzes an epimerase ( EC 18.104.22.168 ). The product is split into the triose glyceraldehyde phosphate (GAP) and acetyl phosphate by the key enzyme of the metabolic pathway, phosphoketolase ( EC 22.214.171.124 ), including an inorganic phosphate (P i ) . Glyceraldehyde phosphate is regularly converted into pyruvate in the course of glycolysis, whereby two molecules of ATP and one molecule of NADH are obtained. This NADH is reoxidized by reducing pyruvate to lactate. This reaction corresponds to the last step of the homofermentative lactic acid fermentation (see above).
The acetyl phosphate obtained in the cleavage is converted to acetate . The high-energy acid anhydride bond is used to obtain ATP via substrate chain phosphorylation. Therefore, ATP is built up in this step, the reaction is catalyzed by an acetate kinase ( EC 126.96.36.199 ).
The breakdown of pentoses is also known as the phosphoketolase pathway . The net energy yield is therefore 2 molecules of ATP per molecule of pentose.
Hexoses can also be used. Here, this, for example glucose, is first activated by a hexokinase to glucose-6-phosphate through ATP consumption , as was already the case with the entry steps of the Entner-Doudoroff path . This oxidizes a glucose-6-phosphate dehydrogenase to 6-phosphoglucono-δ-lactone with NADP + consumption. The lactone is then hydrolyzed to 6-phosphogluconate by a 6-phosphoglucolactonase. This is then decarboxylated to ribulose-5-phosphate and oxidized with NADP + , which catalyzes a phosphogluconate dehydrogenase ( EC 188.8.131.52 ). Ribulose-5-phosphate is then epimerized to xylulose-5-phosphate and finally cleaved by the phosphoketolase. In contrast to the degradation route of pentoses, however, no acetate is formed, as four additional reduction equivalents are produced that have to be reoxidized again. To do this, the acetyl phosphate formed is reduced to ethanol via acetyl CoA and acetaldehyde .
In the balance, one of the two ATPs obtained is used to phosphorylate the glucose to be broken down, but none is formed in the acetate branch. As a result, the net energy yield in heterofermentative lactic acid fermentation for hexoses is only one molecule of ATP per hexose:
In heterofermentative lactic acid fermentation, the phosphoketolase can also accept fructose-6-phosphate as a substrate, which in addition to acetyl phosphate also produces erythrose-4-phosphate . The latter is reduced to erythritol-4-phosphate and converted to erythritol after phosphate has been split off . This byway, known as the “Erythritweg”, has little activity.
Alternatively, the glyceraldehyde can also be broken down into glycerine . Glyceraldehyde is first reduced to glycerol-1-phosphate and then hydrolyzed to glycerol.
Since, in the absence of aldolase, the degradation of pentoses is more favorable than that of hexoses in the case of inability to perform glycolysis , heterofermentative lactic acid fermenters are specialized in the degradation of pentoses. These come, for example, from plant material that these bacteria break down. In addition to hexoses, pentoses also occur in large quantities in grape must , wine or sourdough , so that many heterofermentative lactic acid fermenters grow there.
The growth rates of heterofermentative lactic acid fermenters are lower with the breakdown of hexoses than with the breakdown of pentoses, since the hydrogen carriers are reoxidized more 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 is necessary to maintain the degradation pathway. Otherwise the formation of ethanol is inhibited. In order for fermentation to take place, the hydrogen carriers have to be reoxidized through the formation of glycerol or erythritol (independent of coenzyme A). Since coenzyme A is not required for the breakdown of pentoses, a lack of pantothenic acid has no direct influence there.
The lactic acid bacterium Bifidobacterium bifidum , like heterofermentative lactic acid bacteria, has no aldolase , but bypasses the aldolase step in a different way: Fructose-6-phosphate is phosphorolytically cleaved to erythrose-4-phosphate and acetyl phosphate. Erythrose-4-phosphate is converted with another molecule of fructose-6-phosphate in transaldolase and transketolase reactions to two molecules of xylulose-5-phosphate, both of which are phosphorolytically split by a phosphoketolase to glyceraldehyde phosphate and acetyl phosphate ( pentose phosphate pathway ). With the three molecules of acetyl phosphate 3 adenosine diphosphate (ADP) are phosphorylated to 3 adenosine triphosphate (ATP), whereby energy is conserved in the form of three molecules of ATP and acetic acid is formed as one of the two end products. Like other lactic acid bacteria, the two molecules of glyceraldehyde phosphate are converted into lactic acid, the second end product of fermentation, with four molecules of ADP being phosphorylated to ATP . The net energy yield is therefore 2.5 molecules of ATP per molecule of glucose.
Examples of the appearance of lactic acid fermentation
Lactic acid fermentation by lactic acid bacteria
Bacteria that produce lactic acid as the only or main fermentation product are called lactic acid bacteria . They form an order of gram-positive bacteria and are characterized by the lack of the porphyrins and cytochromes required for electron transport phosphorylation , so that they can only gain their energy by means of substrate chain phosphorylation that is coupled to sugar breakdown .
- homofermentative strains of lactic acid bacteria that produce lactic acid as the only main 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, the main end products of which, in addition to lactic acid and carbon dioxide, are ethanol when hexoses are broken down and acetic acid when pentoses are broken down. These bacteria lack aldolase , the key enzyme in glycolysis. These include the genera Leuconostoc and some members of the genus Lactobacillus , mainly Lactobacillus buchneri .
- The type of bacteria Bifidobacterium bifidum that carries out Bifidobacterium fermentation.
Lactic acid fermentation in mammalian cells
Compared to respiration , only a small amount of energy is gained during fermentation, since instead of the citric acid cycle and the subsequent respiratory chain, only the substrate chain phosphorylation is used. However, fermentation is a way to rapidly form adenosine triphosphate (ATP) through substrate chain phosphorylation without relying on oxygen.
In mammals, including humans, there are numerous examples that cells get their energy from (homofermentative) lactic acid fermentation. Fast-twitching white muscle fibers (FT-fibers) gain their energy even at low intensity through lactic acid fermentation due to their lower level of mitochondria and the corresponding enzymes compared to slowly-twitching red muscle fibers (ST-fibers).
With a higher intensity, a higher proportion of FT fibers is recruited. This also results in larger quantities of lactate. As long as the entire organ and muscle system is not overwhelmed with transport (see below) and further metabolism ( lactate utilization ), the body can maintain a lactate steady state with regard to blood lactate. At very high intensities (when sprinting right from the start), a sufficiently fast energy supply is only possible through a high glycolysis rate, which leads to an exponential increase in blood lactate.
The lactate that occurs during fermentation is further metabolized in various ways during and sometimes after the increased performance demand. Lactate is released into the blood by a monocarboxylate transporter 1 , from which it is absorbed by liver cells or by muscle cells of the skeletal and cardiac muscles that are capable of lactate oxidation and then oxidized to pyruvate ("cell-cell-lactate shuttle"). Pyruvate can be used for further energy production via the citric acid cycle or - in the liver - reconstructed into glucose ( gluconeogenesis ) and supplied to the muscles and organs via the bloodstream ( Cori cycle ).
Other organs also get their energy from lactic acid fermentation when they are undersupplied with oxygen. The increasing lactate concentration in the blood leads to a decrease in the pH value, which under special circumstances (e.g. asphyxiation) can lead to lactic acidosis .
Other specialized cells obtain adenosine triphosphate (ATP) exclusively from the anaerobic breakdown of glucose in lactic acid fermentation. Erythrocytes, for example, can only metabolize glucose under anaerobic conditions due to the lack of mitochondria . Since the cornea is vascular, oxygen can only reach the corneal cells by diffusion and not through the bloodstream. This limits the supply of oxygen so that a constant energy supply can only be ensured through lactic acid fermentation.
Even in larger animals, oxygen often does not get into the tissues quickly enough, so that the necessary energy is obtained through fermentation. Alligators and crocodiles can launch lightning-fast attacks that cost a lot of energy. This energy comes from 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
Lactic acid fermentation has been used to preserve food since at least the Neolithic Age . The lactic acid formation makes the food acidic and spoilage pathogens are almost completely inhibited in their activity or even killed. Examples are sour milk products such as yogurt , quark and buttermilk , bread drink , sauerkraut , pickled cucumbers , sour beans , Korean kimchi , Japanese tsukemono and other pickled vegetables .
For lactic acid fermentation in dairy products, the lactic acid bacteria need the enzyme lactase , which converts the milk sugar lactose (C 12 H 22 O 11 ) into glucose (C 6 H 12 O 6 ) and galactose (C 6 H 12 O 6 ) using H 2 O . Both sugars thus formed are converted in one or more of the ways described above.
Lactic acid fermentation is also used to preserve plant material as animal feed in agriculture . The process, which is usually carried out in silos , is called ensiling and the product is called silage . When preparing silage, the plant material with a dry matter content of 25% - 50% (optimal: around 34%) is usually chopped or stored short-cut (in mobile silos, high silos or large bales wrapped in foil). The immediate exclusion of air after filling is completed, the residual oxygen is consumed by aerobic bacteria and fungi. This is followed by anaerobic fermentation through lactic acid producers (MSB; mostly lactic acid bacteria, but also others). The residual sugar in the plants is converted into lactic acid, acidifying the material and reducing the pH to around 4.0 - 4.5. At this value, the MSB itself are inhibited, the bacterial fermentation pests are already inhibited above a pH value of 4.5. This stops fermentation and the silage is stable.
If the silage is stored too wet, there is a risk that the silage will not become stable. The additional water in the silage has a buffering effect, so that significantly more lactic acid is required to lower the pH value to 4.0 - 4.5. Since lactic acid is produced from the residual sugar, there may not be enough of it as a substrate for the MSB. Fermentation pests (clostridia, yeasts and coliform bacteria) are also not inactivated quickly enough by the slower acidification. Since these fermentation pests use sugar as a substrate and excrete weaker acids (including acetic acid, butyric acid) as products, the existing sugar is not used as effectively for acidification and lowering the pH value - and in the case of butyric acid it is also a very unpleasant one Smell produced by the silage. In addition, some fermentation pests can also convert lactic acid into butyric acid, which is also unfavorable for lowering the pH. The consequence of a pH value that is too high is that the quality of the silage deteriorates due to the metabolic activity of harmful organisms: the energy content drops, and proteins are split off from non-protein nitrogen (NPN), especially ammonia.
With a water content of> 70%, seepage juice also escapes, which is very rich in nutrients and thus also reduces the quality of the silage.
Silage stored too dry leads to insufficient compaction of the plant material of the so-called silo stock. As a result, there is more residual oxygen in the stick. Aerobic yeasts in particular use sugar for growth under these conditions, although this initially has little effect on the pH value, unless there is too little sugar available for lactic acid fermentation. The problem of silage that is stored dry is to be expected above all with maize silages or whole-plant grain silages (GPS), which already have sufficient sugar. As soon as the oxygen is used up, the yeast growth stops. The low pH value caused later by the lactic acid fermentation does not kill the yeast, it only inactivates it. Therefore, further yeast growth is to be expected when opening the silo. Because of the high yeast content due to the previous extensive growth, the yeasts now multiply exponentially, which leads to the heating of the silage (reheating of the silage; low aerobic stability). In warm silage, good living conditions are achieved for coliform bacteria, which in turn convert the lactic acid into butyric acid, thus raising the pH of the silage and ultimately causing it to spoil.
In addition to the deterioration in the quality of the silage due to energy losses and protein breakdown, it is almost always to be expected that the excretion products of fermentation pests, especially yeasts and fungi, are toxic. This leads to a performance depression in the animals to which this silage is fed, if not sufficiently blended.
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