Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy-Yielding Processes
Fermentation
Lactic Acid Fermentation

This type of Fermentation is also widespread in nature and has been found in representatives of various kingdoms of living organisms: primarily in mammals, Protozoa, some insects, Bacteria of the family Lactobacillaceae, as well as in certain tissue Cells of birds and fish.

There are two MAIN TYPES OF Lactic acid fermentation: homo- and heterofermentative. In homofermentative fermentation, up to 90% of the final products consist of lactic acid. Only a small fraction of the Pyruvate formed during Glycolysis is converted into acetic acid, ethanol, acetoin, and СО2. The pathway of this process is shown in Fig. 10.2. Worth noting is The ratio of oxidized and reduced forms of nicotinamide carriers: just as in Alcoholic Fermentation, equivalence between them is maintained. ATP synthesis does not occur in the actual reactions of lactic acid fermentation; it takes place earlier, prior to pyruvate formation, during glycolysis.

Microorganisms that carry out Heterofermentative lactic acid fermentation (bacteria of the genus Lactobacillus, Leuconostoc mesenteroides) lack the Enzymes of The Glycolytic Pathway—aldolase and Triosephosphate isomerase. In the cells of these bacteria, glucose is catabolized via the Pentose Phosphate Pathways, and ribulose-5-phosphate serves as the fermentation substrate. In this case, the second fermentation pathway shown in Fig. 10.1 is realized, with lactic acid and ethanol predominating among the fermentation products.

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Fig. 10.2. Scheme of the Homofermentative lactic acid fermentation pathway



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