CARBOHYDRATE CATABOLISM - PHOSPHOKETOLASE PATHWAY

PHOSPHOKETOLASE PATHWAY

• Lactic acid is a very common end product of bacterial fermentations.
• The genera which produce large amounts of lactate are called lactic acid bacteria.
• Lactic acid fermenters can be separated into two groups. They are (i) Homolactic fermenters and (ii) Heterolactic fermenters.
• Heterolactic fermenters form substantial amounts of products other than lactate; many produce lactate, ethanol, and CO2 by way of the Phosphoketolase pathway. The heterofermentative bacteria do not use Glycolysis, but use a Phosphoketolase pathway for fermentation. The bacteria Lactobacillus brevis, Lactobacillus fermentum and Leuconostoc sp. are involved in Heterolactic fermentation.
• In the Phosphoketolase pathway, two ATP and two NADH molecule was produced from Glucose by Substrate level Phosphorylation.
• In Substrate - Level Phosphorylation, ATP is usually generated when a high-energy Phosphate group is directly transferred from a Phosphorylated compound (a substrate) to ADP. Generally, the Phosphate group has acquired its energy during an earlier reaction in which the substrate itself was oxidized.
• Phosphoketolase pathway path way is useful in the dairy industry for preparation of kefir (fermented milk), yogurt, etc.

Phosphoketolase Pathway

• Pyruvate and Glyceraldehyde-3-phosphate produced from Glucose by ED pathway.
• At first Glucose is phosphorylated to Glucose -6-phosphate by the enzyme Hexokinase. 
• Glucose-6-phosphate is then oxidized to 6-Phosphogluconic acid by releasing a molecule of NADH. This reaction is catalyzed by the enzyme Glucose-6-phosphate dehydrogenase.
• The 6-Phosphogluconic acid is oxidized to Ribulose-5-phosphate by releasing molecules of NADH and CO2.
• Ribulose-5-phosphate is converted to Xylulose-5-phosphate. The reverse reaction may also occurs. 
• Xylulose-5-phosphate is cleaved to form Glyceraldehyde-3-phosphae and Acetyl phosphate.
• Glyceraldehyde-3-phosphae is further catabolized to Pyruvate through the Triose phosphate portion of the EMP pathway generating two ATP and one NADH/H+. The Pyruvate is then reduced to Lactate by the enzyme Lactate dehydrogenase utilizing one NADH/H+.
• The Acetyl phosphate that is formed can be reductively dephosphorylated to Aetyl – CoA and Acetaldehyde in a CoA - dependent manner by Phosphotransacetylase and Aldehyde dehydrogenase. Acetaldehyde is then reduced to Ethanol by Alcohol dehydrogenase using NADH/H+ and regenerating NAD+. 

Phosphoketolase pathway