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

1. Glucose-phosphate is oxidized to 6- phosphogluconic acid.
2. 6- phosphogluconic acid becomes oxidized and decarboxylated to form pentose phosphate.
3. Pentose phosphate is subsequently cleaved to glyceraldehyde3- phosphate (GAP) and acetyl phosphate. 
4. GAP is converted to lactic acid by the same enzymes as the E-M pathway.
5. The intermediate products formed are 1, 3-diphosphoglyceric acid, 3-phosphoglyceric acid, phosphophenol pyruvic acid, and pyruvic acid.
6. This branch of the pathway contains oxidation coupled to a reduction while 2 ATP are produced by substrate-level phosphorylation.
7. Acetyl phosphate is reduced in two steps to ethanol through acetaldehyde, which balances the two oxidations before the cleavage but does not yield ATP.

Overall Reaction

 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.

References

1. https://www.scribd.com/doc/5424834/Phospho-Ketolase-Pathway
2. https://www.ncbi.nlm.nih.gov/books/NBK22416/
3. http://textbookofbacteriology.net/metabolism_3.html
4. https://jb.asm.org/content/194/19/5413
5. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/phosphoketolase

Phosphoketolase (Heterolactic) Pathway