In nature, one common strategy to make thermodynamically unfavorable reactions p

Question

In nature, one common strategy to make thermodynamically unfavorable reactions proceed is to couple them chemically to reactions that are thermodynamically favorable. As long as the overall reaction is thermodynamically favorable, even the unfavorable reaction will proceed.

Part A

Consider these hypothetical chemical reactions:

AB,G= 14.8 kJ/mol

BC,G= -29.7 kJ/mol

CD,G= 8.10 kJ/mol

What is the free energy, G, for the overall reaction, AD?

Part C

Firefly luciferase is the enzyme that allows fireflies to illuminate their abdomens. Because this light generation is an ATP-requiring reaction, firefly luciferase can be used to test for the presence of ATP. In this way, luciferase can test for the presence of life. The coupled reactions are

luciferin+O2ATPoxyluciferin+lightAMP+PPi

If the overall G of the coupled reaction is -7.50 kJ/mol , what is the equilibrium constant, K, of the first reactions at 11 C ? The G for the hydrolysis of ATP to AMP is 31.6 kJ/mol.

Explanation / Answer

Part A:

DG= 14.8 -29.7 +8.1= -6.8kJ/mol

Part C:

luciferin + O2 <-----> oxyluciferin + light

ATP <------> AMP + PPi

DG for luciferin reaction= -7.50 +31.6 = 24.1 kJ/mol

DG= DG0 + RTLnQ

Q is not possible to calculate because the question does not provide concentrations of the reagents. So, i will assume that the concentrations are 1M. This makes DG=DG0

DG0= -RTLnK= 24.1 ----> K=3.69 x 10-5

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