- 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.
Table of Contents
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.
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
- https://www.scribd.com/doc/5424834/Phospho-Ketolase-Pathway
- https://www.ncbi.nlm.nih.gov/books/NBK22416/
- http://textbookofbacteriology.net/metabolism_3.html
- https://jb.asm.org/content/194/19/5413
- https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/phosphoketolase
What is the net gain of ATP from this pathway?