# How is ATP hydrolysis reaction a coupling process?

Jun 25, 2015

By itself, $\text{ATP}$ hydrolysis is not a coupling process.

#### Explanation:

The hydrolysis of $\text{ATP}$ is highly exergonic.

${\text{ATP " + "H"_2"O" → "ADP" + "P}}_{i}$; ΔG = "-30.5 kJ/mol"

The process is highly favourable, so it happens spontaneously.

Many enzymes can use the free energy released by the hydrolysis of $\text{ATP}$ to drive a thermodynamically unfavourable reaction.

This process is $\text{ATP}$ coupling.

For example, the conversion of glucose to glucose-6-phosphate is thermodynamically unfavourable.

$\text{glucose " + "P"_i → "glucose-6-P" + "H"_2"O}$; ΔG = "+14.3 kJ/mol"

The enzyme hexokinase brings these two reactions together, so that the energy released by the hydrolysis of $\text{ATP}$ can drive the synthesis of glucose-6-phosphate.

The reactions are

"glucose " + cancel("P"_i) → "glucose-6-P" + cancel("H"_2"O"); ΔG = "+14.3 kJ/mol"

"ATP " + cancel("H"_2"O") → "ADP" + cancel("P"_i); ΔG = "-30.5 kJ/mol"

$\overline{\text{glucose " + "ATP" stackrel("hexokinase")(→) "glucose-6-P" + "ADP}}$; ΔG = "-16.2 kJ/mol"

The coupled reaction has a negative free energy, so the process is thermodynamically favourable.