The conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate is coupl

Question

The conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate is coupled to NAD+ oxidation is shown below. The reaction is believed to occur in 2 steps:

(1) Glyceraldehyde-3-phosphate + NAD+ à 3-phosphoglycerate + NADH (delta G’ = ?)

(2) 3-phosphoglycerate + Pi à 1,3-bisphosphoglycerate (delta G’ = 11.8 kcal/ mol)

overall reaction (delta G’ = 6.3 kJ/mol)

I need help with the following questions:

1. Show the overall reaction

would the overall reaction be C3H7O6P + Pi +NAD+ ---> C3H8O10P2 + NADH + H+ ?

Doesn't seem balanced so not sure if this is correct and I got confused as to what Pi is.

2. What is the equilibrium constant (K) for the reaction of glyceraldehyde-3-phosphate to 3-phosphoglycerate?

3. Give the range of concentrations for GAP when the measured cytosolic concentrations of phosphate is 2 mM (millimolar) and 1,3-BPG is 0.1 micro molar to insure a forward driving reaction.  

Explanation / Answer

Pi is inorganic phospahte which will react to yield 1,3-biphosphoglycerate.

C3H7O6P + Pi + NAD+ + 2H+ ------> C3H8O10P2 + NADH + H+

NAD+ gains 2H+ gets reduced and converted to NADH which is further used to synthesis ATP.

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