When a mixture of 3-phosphoglycerate and 2-phosphoglycerate is incubated at 25 °

Question

When a mixture of 3-phosphoglycerate and 2-phosphoglycerate is incubated at 25 °C with phosphoglycerate mutase until equilibrium is reached, the final mixture contains six times as much 2-phosphoglycerate as 3-phosphoglycerate.

1. Which one of the following statements is most nearly correct, when applied to the reaction as written?

  3-Phosphoglycerate 2-phosphoglycerate

a)G'° is –4.44 kJ/mol.

b)G'° is zero.

c)G'°is +12.7 kJ/mol.

d)G'°is incalculably large and positive.

e)G'° cannot be calculated from the information given.

2) This reaction is _______________________ (endergonic or exergonic) and _________________ (will or will not) proceed spontaneously.

Explanation / Answer

The 3-phosphoglycerate is converted into 2-phosphoglycerate due to the intramolecular shift of phosphoryl group from C3 to C2 , by the enzyme phosphoglycerate mutase. This is a reversible reaction with a G0' value = +1.06 kcal/mol.

Phosphoglyceromutase is a dimer of identical subunits. Mg=2 is essential for this reaction. The enzyme requires 2,3 - diphosphoglycerate as cofactor for its action and combines with it t give a phosphoenzyme and either the 2-phosphoglycerate or the 3-phosphglycerate.

At high concentration of 3 - phosphoglycerate, the 3 - phosphoglycerate, and phosphoenzyme react to produce free enzyme and 2,3-phosphoglycerate, which in turn yields the 2-phosphoglycerate.The reaction occurs in reverse direction at high concentration of 2 - phosphoglycerate. At equilibrium, the ratio of 2- to the 3-phosphoglycerate is 50:1.

Hence, the correct answer is G'° is –4.44 kJ/mol. Therefore, the option (a) is correct.

2. If the free energy change for this reaction (G0' value = +1.06 kcal/mol) is represented by a negative sign, there is a loss of free energy. Therefore, the reaction is said to be exergonic and proceeds spontaneously.

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