Question

The main energy-coupling compound in biochemical reactions that allows thermodynamically unfavorable processes to become favorable is ________.

Answer 1

The main energy-coupling compound in biochemical reactions that allows thermodynamically unfavorable processes to become favorable is ATP.

ATP hydrolysis has a large, negative free energy change, dG⁰ = -30.5 kJ.mol^-1. Therefore, an endergonic reaction (thermodynamically unfavorable, due to positive free energy change of the reaction) coupled with ATP hydrolysis (exergonic reaction) becomes thermodynamically favorable.

For example, A ----------> B        , dG¹ = + (X kJmol^-1)      -reaction 1, unfavorable

                     ATP -------> ADP + Pi        ‘dG⁰ = (-30.5 kJmol^-1) – reaction 2

Coupling of reaction 1 and 2

A + ATP -----------> B + ADP      , dG’ = dG¹ + dG⁰ = X kJmol^-1 – 30.5 kJmol^-1 = – Y kJmol^-1

Thus, the overall free energy change of the couple reaction is negative. Therefore, the thermodynamically unfavorable reactions become spontaneous when coupled to hydrolysis of the compound ATP.