Question

Why would an increase in entropy accompany the hydrolysis of a triphosphate group into a diphosphate and a phosphate group?

Answer 1

In adenosine triphosphate (ATP), two high-energy phosphoanhydride bonds three phosphate groups.The useful free energy in an ATP molecule is contained in phosphoanhydride bonds, which are formed from the condensation of two molecules of phosphate by the loss of water:

An ATP molecule has two phosphoanhydride bonds and is often written as adenosine – p~p~p, or simply Ap~p~p, where p stands for a phosphate group and ~ denotes a high-energy bond.

Hydrolysis of a phosphoanhydride bond in each of the following reactions has a highly negative ΔG°′ of about −7.3 kcal/mol:

In these reactions, P_i stands for inorganic phosphate and PP_i for inorganic pyrophosphate, two phosphate groups linked by a phosphoanhydride bond. As the top two reactions show, the removal of a phosphate or a pyrophosphate group from ATP leaves adenosine diphosphate (ADP) or adenosine monophosphate (AMP), respectively.

The phosphoanhydride bond is an ordinary covalent bond, but it releases about 7.3 kcal/mol of free energy (under standard biochemical conditions) when it is broken. In contrast, hydrolysis of the phosphoester bond in AMP, forming inorganic phosphate and adenosine, releases only about 2 kcal/mol of free energy. Phosphoanhydride bonds commonly are termed “high-energy” bonds, even though the ΔG°′ for the reaction of succinate with oxygen is much higher (−37 kcal/mol)