In: Chemistry
If the intracellular concentrations of a metabolite (M-COO-) and its phosphorylated form (M-COOPO32-) were 2.9 mM and 0.09 mM, respectively, and if the intracellular concentrations of ATP and ADP were 3.8 mM and 0.16 mM, respectively, what would be the numerical value of \DeltaΔG (in kcal per mol to the nearest hundredth) for the following reaction: M-COO- + ATP <--> M-COOPO32- + ADP? Assume a temperature of 37 °C and a pH of 7.4. To solve this problem, you will need to know the standard free energies of hydrolysis of the phosphorylated metabolite and of ATP. These values are –10.3 kcal/mol and –7.3 kcal/mol, respectively.
Ans. Given:
M-COO-P + H2O --------> M-COO- + Pi ; dG0 = -10.3 Kcal/mol - reaction 1
ATP + H2O --------> ADP + Pi ; dG0 = -7.3 Kcal/mol - reaction 2
Note: M-COO-P = M-COOPO32-
Now,
# Step 1: Reverse reaction 1-
M-COO- + Pi --------> M-COO-P + H2O ; dG0 = +10.0 Kcal/mol - reaction 1’
# Step 2: Add reaction 1’ and 2 to get the desired reaction-
M-COO- + Pi --------> M-COO-P + H2O ; dG0 = +10.0 Kcal/mol
(+) ATP + H2O --------> ADP + Pi ; dG0 = -7.3 Kcal/mol
Net Reaction : M-COO- + ATP --------> M-COO-P + ADP ; dG0net = + 2.7 Kcal/mol
# Step 3: Convert all concentrations in terms of molarity-
[M-COO-P] = 0.09 mM = 0.00009 M
[M-COO-] = 2.9 mM = 0.0029 M
[ATP] = 3.8 mM = 0.0038 M
[ADP] = 0.17 mM = 0.00016 M
Now,
Equilibrium constant, Keq = [M-COO-P] [ADP] / ([M-COO-] [ATP])
Or, Keq = (0.00009 x 0.00016) / (0.0029 x 0.0038)
Hence, Keq = 0.0000000144 / 0.00001102
Or, Keq = 0.0013067
# Step 4: Using, dG = dG0 + RT (ln Keq)
Where, dG = free energy change of the reaction
dG0 = standard free energy change of the reaction
R = universal gas constant = 0.001987 kcal mol-1K-1
T = temperature in kelvin = 310.15 K
Keq = equilibrium constant
Or, dG = +2.7 kcal mol-1 + (0.001987 kcal mol-1K-1) x 310.15 K x ln (0.0013067)
Or, dG = 2.7 Kcal mol-1 + 0.61626805 kcal mol-1 x (-6.640250404)
Or, dG = 2.7 kcal mol-1 – 4.09 kcal mol-1
Or, dG = - 1.39 kcal mol-1
Hence, dG = - 1.39 kcal/ mol