Phosphoketolase (Heterolactic) Pathway

Table of Contents

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.

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.