How is ATP hydrolysis reaction a coupling process?

1 Answer
Jun 25, 2015

By itself, #"ATP"# hydrolysis is not a coupling process.

Explanation:

The hydrolysis of #"ATP"# is highly exergonic.

#"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 #"ATP"# to drive a thermodynamically unfavourable reaction.

This process is #"ATP"# coupling.

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

#"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 #"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"#

#bar("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.

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