Question

Write the two chemical reactions that are coupled in step 3 of glycolysis and label each as exergonic or endergonic.
(Write your reactions using words, accepted acronyms or symbols for reactants and products)

How do you know if the reaction is exergonic or endergonic

Answer 1

The 3rd step in the glycolysis is conversion of Fructose 6 phosphate to Fructose 1,6 bisphosphate by Phosphofructokinase as shown below:

Fructose 6 Phosphate + ATP ---------Phosphofructokinase--------> Fructose 1,6 bisphosphate

This reaction consists of two inner coupled reactions which can be shown as follows:

a. Fructose 6 phosphate -----------+ Pi (Inorganic phosphate) + H+ --------------> Fructose 1,6 bisphosphate + H2O (Unfavorable reaction) Delta G⁰ = +16 kJ/mol

b. ATP + H2O ---------------> ADP + Pi + H+ (Favorable reaction due to ATP hydrolysis) Delta G⁰ = -30 kJ/mol

The reaction (a) shown above is endergonic reaction because the energy is utilized( the free energy is positive value) and energeticaly unfavorable. But the reaction (b) is exergonic reaction as the energy is released due to hydrolysis of ATP. So, this reaction is coupled to make the net reaction feasible with Delta G⁰ = -14 kJ/mol

Other worth noting points about Step 3 of glycolysis:

Step 3 is the 2nd priming phosphorylation reaction and a committed irreersible step in glycolysis which involves Magnesium as cofactor. Phosphofructokinase is thus highly regulated as it is responsible in fixing the Fructose 1,6 bisphosphate to convert to pyruvate and yield energy.