1. An exergonic reaction is a reaction for which G <0, and an endergonic reactio
ID: 955647 • Letter: 1
Question
1. An exergonic reaction is a reaction for which G <0, and an endergonic reaction is a reaction for which G > 0. The following questions investigate the molecular basis for the observation that the hydrolysis of ATP is exergonic at pH = 7.0 and 310 K
ATP4- (aq) + H2O (l) ADP3- (aq) + HPO42- (aq) + H3O+ (aq) rG = -31 kJ/mol rH = -20 kJ/mol
a. One hypothesis is that the exergonicity of ATP hydrolysis is due in part to the fact that the standard entropies of hydrolysis of polyphosphates are positive. Why would an increase in entropy accompany the hydrolysis of a triphosphate group into a diphosphate and a phosphate group?
b. Under identical conditions, the Gibbs energies of hydrolysis of H4ATP and MgATP2- , a complex between the Mg2+ ion and ATP4- , are less negative than the hydrolysis of ATP4- . Explain how this observation might support the hypothesis that electrostatic repulsion between adjacent phosphate groups is a factor that controls the exergonicity of ATP hydrolysis. Do these electrostatic effects contribute to the rH or rS terms that determine the exergonicity? Hint: In the Mg ATP2- complex, the Mg2+ ion and ATP 4- anion form two bonds: one that involves a negatively charged oxygen belonging to the phosphate group of ATP4- and another bond that involves a negatively oxygen belonging to the phosphate group adjacent to the terminal phosphate group of ATP4-.
c. Discuss the merits of another hypothesis that stabilization due to resonance in ATP4- and the HPO4- ion contributes to the exergonicity of ATP hydrolysis. Does stabilization through resonance contribute to the rH or rS terms that determine the exergonicity of the reaction? (Adapted from Atkins problem 2.27)
Explanation / Answer
a ) The entropy means level of dissorderness or disorganization. The entropy of ADP is greater than that of ATP. After hydrolysis the ATP molecule are brocken down in to two molecules ie ADP & AMP. Therefore nunber of molecules increases & thus entropy also increses.
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