Question

Compare the ATP yield of three glucose molecules that enter glycolysis and are converted to pyruvate with that of three glucose molecules that proceed through the pentose phosphate pathway such that their carbon skeletons (as two F6P and one GAP) reenter glycolysis and are metabolized to pyruvate. Indicate the steps where ATP was used or produced and the overall ATP totals.

Answer 1

Glycolysis: Glycolysis is the metabolic process of converting 1 molecule of glucose to 2 molecules of pyruvate through a series of 10 enzyme catalyzed reactions.

Overall Reaction:

Glucose + 2ADP + 2NAD⁺ + 2Pi -------> 2 pyruvate + 2ATP + 2NADH + 2H⁺ + 2H₂O

Hence, from 3 molecules of glucose 6 molecules of ATP's are produced.

Pentose Phosphate Pathway (PPP): PPP can operate to metabolize dietary pentose sugars derived from the digestion of nucleic acids as well as to rearrange the carbon skeletons of dietary carbohydrates into glycolytic/gluconeogenic intermediates (GAP, fructose 6-phosphate).

R5P is generated after nonoxidative steps of the PPP pathway. If R5P (ribose pentose phosphate) is not needed in the cell, it can be converted into a 6-carbon sugar (fructose-6-phosphate) and a 3-carbon sugar (GAP) which can be utilized in the glycolysis or gluconeogenesis pathways.

                 From 3 molecules of R5P, 2 molecules of F6P and one molecule of GAP) is produced. Transketolase and transaldolase are enzymes involved in the rearrangement of carbon skeletons.

The PP pathway is a major producer of NADPH for the synthesis of fatty acids. The pathway is not designed to produce ATP.

Steps:

1) Glucose → 2 Pyruvate (2 ATP, 4ATP)

2) 2 Pyruvate→2 acetyl-CoA (6 ATP)

3) 2 acetyl-CoA into TCA cycle (2 ATP, 18 ATP, 4 ATP)

                                               Total 36 or 38 ATP are formed.