Question

One of the important reactions in the biochemical pathway of glycolysis is the reaction of glucose-6-phosphate (G₆P) to for fructose-6-phosphate (F6P).

G₆P <----> F₆P DG0= 1.7 kJ/mol

(a) Is the reaction spontaneous or non-spontaneous under standard thermodynamic conditions?

(b) Standard thermodynamic conditions imply the concentration of G₆P and F₆P to be 1 M, however, in a typical cell, they are not even close to these values. Calculate DG when the concentrations of G₆P and F₆P are 120 mM and 28 mM respectively, and determine the spontaneity of the forward reaction under these conditions. Assume the body temperature is 370C.

Answer 1

The conversion of glucose-6-phosphate (G6P) to fructose-6-phosphate (F6P) has a standard free energy change of 1.7 kJ/mol, indicating it is non-spontaneous under standard conditions. However, under typical cellular conditions with concentrations of 120 μM G6P and 28 μM F6P, the reaction may proceed spontaneously if the actual free energy change (ΔG) calculated using these concentrations is negative.

Step-by-step explanation:

The reaction of glucose-6-phosphate (G6P) to fructose-6-phosphate (F6P) is an essential step in the glycolysis pathway. The standard free energy change (ΔG°) for this reaction is 1.7 kJ/mol. To address the questions:

1. Under standard thermodynamic conditions (where ΔG° = 1.7 kJ/mol), the reaction is non-spontaneous because a positive ΔG° value indicates that the reaction requires an input of energy to proceed.
2. However, in a living cell the concentrations of G6P and F6P are not 1 M. To determine the direction of spontaneity under these conditions, the actual free energy change (ΔG) must be calculated using the formula ΔG = ΔG° + RT ln(Q), where Q is the reaction quotient, R is the gas constant (8.314 J/K·mol), and T is the temperature in Kelvin (310.15 K for 37°C). Substituting the given concentrations into Q (as [F6P]/[G6P]), and with ΔG° as 1.7 kJ/mol, we calculate the actual ΔG for this reaction. If ΔG is negative under these cellular conditions, the forward reaction (G6P to F6P) is spontaneous.

If the calculated ΔG is negative, it suggests that under cellular conditions, contrary to the standard conditions, the conversion of G6P to F6P occurs spontaneously.