Synthesis of the activated form of acetate (acetyl-CoA) is...

Synthesis of the activated form of acetate (acetyl-CoA) is carried out in an ATP-dependent process: $$\text { Acetate }+\mathrm{CoA}+\mathrm{ATP} \rightarrow \text { acetyl-CoA }+\mathrm{AMP}+\mathrm{PP}_{1}$$ (a) The $\Delta G^{\prime \circ}$ for hydrolysis of acetyl-CoA to acetate and $\mathrm{CoA}$ is $-32.2 \mathrm{kJ} / \mathrm{mol}$ and that for hydrolysis of ATP to AMP and PP $_{i}$ is -30.5 kJ/mol. Calculate $\Delta G^{\prime \circ}$ for the ATPdependent synthesis of acetyl-CoA. (b) Almost all cells contain the enzyme inorganic pyrophosphatase, which catalyzes the hydrolysis of $\mathrm{PP}_{\mathrm{i}}$ to $\mathrm{P}_{\mathrm{i}}$. What effect does the presence of this enzyme have on the synthesis of acctyl-CoA? Explain.

Synthesis of the activated form of acetate (acetyl-CoA) is carried out in an ATP-dependent process: $$\text { Acetate }+\mathrm{CoA}+\mathrm{ATP} \rightarrow \text { acetyl-CoA }+\mathrm{AMP}+\mathrm{PP}_{1}$$
(a) The $\Delta G^{\prime \circ}$ for hydrolysis of acetyl-CoA to acetate and $\mathrm{CoA}$ is $-32.2 \mathrm{kJ} / \mathrm{mol}$ and that for hydrolysis of ATP to AMP and PP $_{i}$ is -30.5 kJ/mol. Calculate $\Delta G^{\prime \circ}$ for the ATPdependent synthesis of acetyl-CoA.
(b) Almost all cells contain the enzyme inorganic pyrophosphatase, which catalyzes the hydrolysis of $\mathrm{PP}_{\mathrm{i}}$ to $\mathrm{P}_{\mathrm{i}}$. What effect does the presence of this enzyme have on the synthesis of acctyl-CoA? Explain.

Key Concepts

ATP Hydrolysis

ATP hydrolysis is a common source of energy in cells, where the breakdown of ATP to AMP and pyrophosphate (PPi) releases sufficient free energy. This energy release is often harnessed to drive other biochemical reactions that would otherwise be non-spontaneous, making ATP a vital energy currency in metabolism.

Role of Pyrophosphatase in Reaction Equilibrium

Pyrophosphatase is an enzyme that catalyzes the rapid hydrolysis of pyrophosphate (PPi) into two phosphate ions (Pi). By removing PPi from the reaction mixture, it drives the coupled reaction further towards product formation according to Le Chatelier's principle, thereby ensuring the overall reaction proceeds efficiently and irreversibly in the desired direction.

Gibbs Free Energy

Gibbs free energy (?G) is a thermodynamic state function that indicates the amount of work a chemical reaction can perform. In biochemistry, standard free energy changes (?G°') are used to determine the favorability of reactions under standard conditions, and these values are crucial in calculating the net energy change when reactions are coupled.

Reaction Coupling

Reaction coupling involves linking an energetically unfavorable reaction to a favorable one so that the overall process proceeds. In metabolic pathways, a reaction that has a positive ?G°' can be driven forward by coupling it with another reaction that has a strongly negative ?G°', effectively harnessing the energy released to push the overall reaction toward completion.

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