Lactobacilli have the ability to ferment glucose to fructose 1,6 bisphosphate but no further because it lacks the enzyme fructose 1,6 bisphosphate aldolase.
The absence of this enzyme is a special feature of Lactobacilli and is compensated for by the presence of the enzyme phosphoketolase. This is essential to enable glycolysis to continue to allow ATP generation and NAD and NADP regeneration.
Lactobacilli process glucose 6 phosphate via the Pentose Phosphate Pathway to Xylulose 5 phosphate which is a substrate for phosphoketolase.
From here ATP generation occurs from acetyl phosphate, and butyrate production from acetyl CoA results in NADP regeneration.
Lactate is produced from glyceraldehyde 3 phosphate in the normal glycolytic sequence resulting in ATP production and NAD regeneration.
The following four pictures detail the fermentation process of Lactobacilli:
1. the overall pathway showing the funnelling of hexose and pentose sugars into a common pathway that results in the production of acetate, butyrate and lactate.
2. shows the products of hemicellulose and cellulose breakdown being converted to xylulose 5 phosphate.
3. shows the common intermediate xylulose 5 phosphate being converted to acetyl phosphate and glyceraldehyde 3 phosphate. This is converted to lactate with NAD regeneration.
4. shows acetyl phosphate being converted to butyrate (achieving regeneration of NADP) and producing acetate (achieving additional ATP generation).
are used by the host ruminant.