Question

If endergonic reactions are not spontaneous, how do they occur?

Answer 1

An endergonic reaction occurs by coupling with an even more exergonic reaction.

An exergonic reaction is one in which `ΔG` increases, and an endergonic process is one in which `ΔG` decreases.

See Is exergonic the same as endothermic or exothermic?.

The only way that an endergonic reaction can occur spontaneously is if it is coupled with an even more exergonic reaction.

The sum of the `ΔG` values of the two reactions is then negative. The overall reaction becomes exergonic and spontaneous.

The process is called energy coupling.

For example, glutamate and ammonium ions react to form the amino acid glutamine. The reaction is endergonic:

glutamate + NH₄⁺ → glutamine; `ΔG` = +14.2 kJ/mol

The conversion of ATP to ADP is strongly exergonic:

ATP + H₂O → ADP + `"P"_"i"`; `ΔG` = -30.5 kJ/mol

If we add the two reactions, we get

glutamate + NH₄⁺ + ATP + H₂O → glutamine + ADP + `"P"_"i"`; `ΔG` = -16.3 kJ/mol

In the body, the enzyme glutamine synthetase catalyzes the reaction. It proceeds by way of an intermediate phosphate.

glutamate + ATP → glutamyl-`"P"_"i"` + ADP

glutamyl `P_"i"` + NH₄⁺ + H₂O → glutamine + `"P"_"i"`

glutamate + NH₄⁺ + ATP + H₂O → glutamine + ADP + `"P"_"i"`; `ΔG` = -16.3 kJ/mol