Question

How do lactic acid homofermemtative pathway and heterofermentative pathway differ in their glycolytic pathways? Why do they differ in these pathways and what is the advantage of the two alternate glycolytic pathways vs. the classical EMP glycolytic pathway?

Answer 1

The Lactic acid homo-fermentative pathway utilizes the Embden Meyerhof pathway for glycolysis. This pathway is seen during oxygen deprivation an anaerobic condition. The lactic acid homofermenters such as lactobacilli, enterococci, lactococci, Pediococci, Streptococci etc will use EM pathway to break down glucose to pyruvate producing 2 ATP. The pyruvate is then broken down to lactic acid by lactate dehydrogenase to regenerate the NAD+ for EM pathway.

Lactic acid hetero-fermentative pathway is the phosphoketolase pathway or pentose phosphate pathway. It is seen in organisms such as Leuconostoc, Weisella and a few lactobacilli etc. First, glucose is converted to glucose 6 phosphate, which then forms 6-phosphogluconate. This 6-phosphogluconate is decarboxylated to produce pentose-5 phosphate (ribulose 5 phosphate) and CO2. Ribulose 5 phosphate cleaves to glyceraldehyde phosphate (GAP) an acetyl phosphate. GAP enters homo-fermentative glycolysis to produce lactic acid. Acetyl phosphate will form acetyl CoA. Acetyl CoA is converted to acetaldehyde and CO2 and then ethanol. Thus, it forms both lactate and ethanol along with CO2.

The difference between the two pathways lies in the presence and absence of fructose 1, 6-diphosphate and phosphoketolase enzyme. Homo-fermentative LAB (lactic acid bacteria) has fructose 1, 6-diphosphate but lack phosphoketolase enzyme. Conversely, hetero-fermentative LAB lack fructose 1, 6-diphosphate but possess phosphoketolase enzyme.

The advantage of LAB pathways over EMP pathway is the efficient linking of carbohydrate fermentation to substrate level phosphorylation. The EMB pathway generates 1 ATP, 1 NADP and 1 NADH molecule per glucose. Conversely the homo- and heter0-fermentative LA fermentation generates 2 ATP, 2 NADH per glucose.