Question

Metal ions such as Mg²⁺ bind to ADP and P_i more tightly than to ATP. The effect of the presence of Mg²⁺ on the hydrolysis of ATP would be:

		To increase the ΔG°’ for hydrolysis
		To decrease the rate of hydrolysis and affect ΔG°’
		To speed up the rate of hydrolysis but not affect ΔG°’
		To decrease the rate of hydrolysis but not affect ΔG°’
		To decrease the ΔG°’ for hydrolysis
		To speed up the rate of hydrolysis and affect ΔG°’ ( this option is incorrect)

Answer 1

Ans) To decrease the ΔG°’ for hydrolysis

(Structure of ATP)

Hydrolysis of the phosphate groups in ATP is especially exergonic, because the resulting orthophosphate group is greatly stabilized by multiple resonance structures, making the products (ADP and P_i) much lower in energy than the reactant (ATP). The high negative charge density associated with the three adjacent phosphate units of ATP also destabilizes the molecule, making it higher in energy. Hydrolysis relieves some of these electrostatic repulsions, liberating useful energy in the process by causing conformational changes in enzyme structure.

Hydrolysis of the terminal phosphoanhydridic bond is a highly exergonic process, releasing 30.5 kJ mol⁻¹ energy. This reaction can then be coupled with thermodynamically unfavorable reactions to give an overall negative (spontaneous) ΔG for the reaction sequence. The actual value of ΔG for ATP hydrolysis varies, primarily depending on Mg²⁺ concentration, and under normal physiologic conditions is actually closer to -50 kJ mol⁻¹.