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Phosphoketolase (Heterolactic) Pathway
- The phosphoketolase pathway is distinguished by the key cleavage enzyme, phosphoketolase.
- Phosphoketolase in the pathway cleaves pentose phosphate into glyceraldehyde-3-phosphate and acetyl phosphate which is the prime reaction of the pathway.
- As a fermentation pathway, it is employed mainly by the heterolactic acid bacteria which carry out heterolactic fermentation.
- Heterolactic fermentation is a type of lactic acid fermentation in which sugars (e.g. lactose, glucose) are fermented to a range of acidic products.
- Examples include some species of Lactobacillus and Leuconostoc.
Steps in Phosphoketolase Pathway
- Glucose-phosphate is oxidized to 6- phosphogluconic acid.
- 6- phosphogluconic acid becomes oxidized and decarboxylated to form pentose phosphate.
- Pentose phosphate is subsequently cleaved to glyceraldehyde3- phosphate (GAP) and acetyl phosphate.
- GAP is converted to lactic acid by the same enzymes as the E-M pathway.
- The intermediate products formed are 1, 3-diphosphoglyceric acid, 3-phosphoglyceric acid, phosphophenol pyruvic acid, and pyruvic acid.
- This branch of the pathway contains oxidation coupled to a reduction while 2 ATP are produced by substrate-level phosphorylation.
- Acetyl phosphate is reduced in two steps to ethanol through acetaldehyde, which balances the two oxidations before the cleavage but does not yield ATP.
Glucose ———->1 lactic acid + 1 ethanol +1 CO2 with a net gain of 1 ATP.
The efficiency is about half that of the E-M pathway.
Applications of Phosphoketolase Pathway
- Heterolactic species of bacteria are occasionally used in the fermentation industry.
- For example, kefir, a type of fermented milk to yogurt, is produced by is produced using a heterolactic Lactobacillus species which utilize this pathway.
- Likewise, sauerkraut fermentations use Leuconostoc, a heterolactic bacterium, to complete the fermentation.