A. Consider these hypothetical chemical reactions: A<=>B; ?G= 10.7kJ/mol B<=>C;

Question

A. Consider these hypothetical chemical reactions:
A<=>B; ?G= 10.7kJ/mol
B<=>C; ?G= -30.6kJ/mol
C<=>D; ?G= 9.40kJ/mol
What is the free energy, ?G, for the overall reaction, A<=>D?

B. 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+O2<=>oxyluciferin+ light
ATP<=>AMP+PPi

If the overall ?G of the coupled reaction is -8.10 kJ/mol, what is the equilibrium constant,K , of the first reactions at 11

Explanation / Answer

By the extension of Hess's Law to Gibbs free energy, which states that the overall is the sum of the intermediate steps, we obtain

A<=>B; ?G= 10.7kJ/mol
B<=>C; ?G= -30.6kJ/mol
C<=>D; ?G= 9.40kJ/mol

implies that G for A<=>D is the sum of G for A<=>B, G for B<=>C, and G for C<=>D

Thus, G for A<=>D is G = 10.7kj/mol - 30.6kj/mol + 9.40kj/mol ===>   G = -10.5kj/mol

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