Question

What is the connection between importers and pathways. List the lactic acid bacteria in these pathways, the genetics of the pathways and the applications in the fermented food.

Answer 1

• Connection between importers and pathways: The fermentation pathways require an input of glucose molecule to be metabolized into pyruvate, as explained below. Pyruvate is formed either from the glycolytic pathway or the pentose phosphate pathway. But in either case a steady input of glucose inside the cell is essential. Glucose is a six-carbon sugar, which is too large to pass through the plasma membrane with the help of simple diffusion. Instead glucose diffusion occurs through facilitated diffusion (passive) and active transport. In facilitated diffusion, carrier proteins bind to glucose, change their conformation and translocate the glucose to the other side of the membrane along the concentartion gradient. In primary active transport, the cell invests energy in the form of ATP to transport glucose along or against the concentration gradient. In secondary active transport, a symporter protein in the membrane transports two sodium ions for every molecule of glucose that is imported into the cell. The symporter protein utilises the electrochemical gradient of the sodium ions to facilitate this transport. So a steady intake or import of glucose from the extracellular environment is essential for proper functioning of the the pathways.
• List the lactic acid bacteria in these pathways: The lactic acid fermentation pathway involves conversion of pyruvate to lactic acid, a single step reaction carried out by lactic acide bacteria. The fermentation can be of two types: homolactic and heterolactic fermentation. In homolactic fermentation, pyruvate (obtained by glycolysis) is reduced to lactate or lactic acid by the enzyme lactate dehydrogenase. Examples of homolactic bacteria are Streptococcus thermophiles, Streptococcus lactis, Lactobacillus lactis, Lactobacillus bulgarius, Pediococcus, Enterococcus. In heterolactic fermentation, glucose is converted to pyruvate, acetic acid and CO2 by pentose phosphate pathway. Pyruvate is then reduced to lactic acid and acetic acid to ethanol and CO2. Examples of heterolactic bacteria are Leuconostoc mesenteroides, Lactobacillus bifermentous, Leconostoc lactis.
• Genetics of the pathways: In homolactic fermentation, pyruvate (obtained by glycolysis) is reduced to lactate or lactic acid by the enzyme lactate dehydrogenase and coenzyme NADH. In heterolactic fermentation, glucose is converted to pyruvate, acetic acid and CO2 by pentose phosphate pathway. Pyruvate is then reduced to lactic acid (pyruvate dehydrogenase) and acetic acid (pyruvate decarboxylase and alcohol dehydrogenase) to ethanol and CO2.
• Applications in fermented food: The importance of lactic acid bacteria and the fermentation pathways are pronounced in the dairy industry, to produce various fermented foods via souring of milk. Lactobacillus spp. are used as probiotics.