A reaction with positive ΔG cannot run spontaneously. But if it is coupled to an exergonic reaction (often ATP hydrolysis) and the combined ΔG is negative, the overall process will run. This is how cells drive every biosynthesis and every uphill transport.
The Principle
ΔG values are additive. If Reaction 1 has ΔG = +5 kcal/mol and Reaction 2 has ΔG = -10 kcal/mol, and they share a common intermediate, the coupled reaction has ΔG = -5 kcal/mol - spontaneous.
Example: glutamate + NH3 → glutamine (unfavorable on its own, ΔG°’ = +3.4 kcal/mol). Coupled to ATP → ADP + Pi (ΔG°’ = -7.3 kcal/mol):
Step 1: Glu + ATP → Glu-phosphate + ADP (ΔG°’ ≈ 0, not spontaneous by itself, but drives step 2).
Step 2: Glu-phosphate + NH3 → Glutamine + Pi (strongly exergonic once the phosphate is in place).
Direct ATP hydrolysis: most common. Kinases transfer a phosphate from ATP to substrate, “activating” it for the next step (e.g., hexokinase, pyruvate carboxylase).
ATP → AMP + PPi: the pyrophosphate is then hydrolyzed, producing extra driving force. Used when an especially strong push is needed (tRNA charging, fatty acid activation, DNA synthesis).
NADH / NADPH as reducing equivalents: electrons from glycolysis or TCA drive biosynthesis.
Ion gradients: the proton motive force powers ATP synthase; the sodium gradient powers secondary active transport.
Why ATP Is Perfect for Coupling
ATP sits in the middle of the phosphate transfer hierarchy. It receives phosphate from higher-energy compounds (PEP, creatine-P) in exergonic reactions, and donates phosphate to lower-energy compounds (glucose, amino acids) in reactions that are still favorable. This “intermediate” position makes ATP a universal intermediary.
How do cells drive energetically unfavorable reactions?
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By coupling them to a strongly exergonic reaction (most often ATP hydrolysis) through a shared intermediate. ΔG values add. If the unfavorable reaction has ΔG of +4 and ATP hydrolysis has ΔG of -7.3, the coupled reaction has net ΔG of -3.3 - spontaneous. The enzyme catalyzing the pair binds both reactants and channels the energy flow.
Why does ATP hydrolysis to AMP + PPi release more energy than to ADP + Pi?
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ATP → AMP + PPi releases one phosphoanhydride bond directly. The pyrophosphate (PPi) is then hydrolyzed by ubiquitous pyrophosphatase to 2 Pi, which is also strongly exergonic and effectively irreversible. Together, the two steps release more energy than a simple ATP → ADP + Pi. Cells use this double hit when they need especially high driving force (tRNA charging, fatty acid activation).
Besides ATP hydrolysis, what are two other ways cells couple energy to drive reactions?
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(1) NADH or NADPH as reducing power - electrons transferred from NADPH to a biosynthetic intermediate drive reductive biosynthesis. (2) Ion gradients - the proton gradient across the inner mitochondrial membrane drives ATP synthase; the Na+ gradient drives secondary active transport of glucose, amino acids, and other solutes. All these are forms of energy coupling.
