Question

What is the net yield of ATP produced by the anaerobic fermentation of one molecule of galactose to two molecules of lactate in myocytes.

The answer is 2 ATP but what is the explanation for that? I keep getting 3 ATP since galactose enters the glycolytic pathway at Glucose 6 phosphate and bypasses the hexokinase ATP retirement, thus giving the regular 2 ATP from glycolysis and an additional ATP that was not used in the hexokinase reaction. Thus giving 3 ATP.  

Answer 1

In Anaerobic reaction, following processes occurs –

Glucose is phosphorylated to glucose-6-phosphate by consuming one ATP molecule. Then fructose-6-phosphate is synthesized to produce fructose-6-phosphate. Fructose 6 phosphate consumes one ATP molecule and get converted to fructose-1,6-bisphosphate. Fructose-1,6-bisphosphate then splits into dihydroxyacetone phosphate, and glyceraldehyde-3-phosphate

dihydroxyacetone phosphate – isomerized to produce glyceraldehyde-3-phosphate

glyceraldehyde-3-phosphate – phosphorylated to produce 1,3-bisphosphoglycerate by using NAD+ as a cofactor

Now 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by production of 1 molecule of ATP. 3-phosphoglycerate is converted into 2-phosphoglycerate which is then further converted into phosphoenolpyruvate. Phosphoenolpyruvate is converted to pyruvate along with formation of 1 more molecule of ATP.