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

The primary functions of pentosephosphate pathway and steps :

1. To generate reducing equivalents, in the form of NADPH, for reductive biosynthesis reactions within cells

2. To provide the cell with ribose-5-phosphate (R5P) for the synthesis of the nucleotides and nucleic acids.

3. Although not a significant function of the PPP, it 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) about 30% of the oxidation of glucose in the liver occurs via the PPP

The enzymes of the PPP are located in the cytoplasm.

The oxidative steps (1-3) generate the NADPH: For every molecule of glucose that enters the PPP pathway,

2 molecules of NADPH are regenerated from NADP+ The nonoxidative steps of the PPP pathway are designed to generate R5P. If R5P is not needed in the cell,

this 5 carbon sugar 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. (Note: it takes 3 molecules of R5P to

make 2 molecules of F6P and one molecule of GAP)

Transketolase and transaldolase are enzymes involved in the rearrangement of carbon skeletons.

Transketolase is an enzyme which transfers C2 units and requires thyamine pyrophosphate (TPP) as a cofactor

Transaldolase is an enzyme which transfers C3 units. The reaction involves a Schiff base formation between

the substrate and an essential lysine residue in the active site of the enzyme.    

These two enzymes can be reversed in times when R5P need is high in the cell.

The glucose-6-phosphate dehydrogenase enzyme is the main site of regulation. This enzyme is regulated by the

NADP+ concentration in the cell.