Coupling ATP Cleavage to an Unfavorable Reaction To explore...

Coupling ATP Cleavage to an Unfavorable Reaction To explore the consequences of coupling ATP hydrolysis under physiological conditions to a thermodynamically unfavorable biochemical reaction, consider the hypothetical transformation $\mathrm{X} \rightarrow \mathrm{Y}$, for which $\Delta G^{\prime \circ}=20.0 \mathrm{kJ} / \mathrm{mol} .$ (a) What is the ratio $[\mathrm{Y}] /[\mathrm{X}]$ at equilibrium? (b) Suppose $X$ and $Y$ participate in a sequence of reactions during which ATP is hydrolyzed to ADP and $P_{i}$. The overall reaction is $ \mathbf{x}+\mathbf{A T P}+\mathbf{H}_{2} \mathbf{O} \rightarrow \mathbf{Y}+\mathbf{A D P}+\mathbf{P}_{\mathbf{1}} $ Calculate $[\mathrm{Y}] /[\mathrm{X}]$ for this reaction at equilibrium. Assume that the temperature is $25^{\circ} \mathrm{C}$ and the equilibrium concentrations of ATP, ADP, and $P_{i}$ are 1 M. (c) We know that [ATP], [ADP], and $[\mathrm{Pi}]$ are not $1 \mathrm{M}$ under physiological conditions. Calculate $[\mathrm{Y}] /[\mathrm{X}]$ for the ATP-coupled reaction when the values of [ATP], [ADP], and $\left[\mathrm{P}_{\mathrm{i}}\right]$ are those found in rat myocytes (Table $13-5$ ).

Coupling ATP Cleavage to an Unfavorable Reaction To explore the consequences of coupling ATP hydrolysis under physiological conditions to a thermodynamically unfavorable biochemical reaction, consider the hypothetical transformation $\mathrm{X} \rightarrow \mathrm{Y}$, for which $\Delta G^{\prime \circ}=20.0 \mathrm{kJ} / \mathrm{mol} .$
(a) What is the ratio $[\mathrm{Y}] /[\mathrm{X}]$ at equilibrium?
(b) Suppose $X$ and $Y$ participate in a sequence of reactions during which ATP is hydrolyzed to ADP and $P_{i}$. The overall reaction is $
\mathbf{x}+\mathbf{A T P}+\mathbf{H}_{2} \mathbf{O} \rightarrow \mathbf{Y}+\mathbf{A D P}+\mathbf{P}_{\mathbf{1}}
$ Calculate $[\mathrm{Y}] /[\mathrm{X}]$ for this reaction at equilibrium. Assume that the temperature is $25^{\circ} \mathrm{C}$ and the equilibrium concentrations of ATP, ADP, and $P_{i}$ are 1 M.
(c) We know that [ATP], [ADP], and $[\mathrm{Pi}]$ are not $1 \mathrm{M}$ under physiological conditions. Calculate $[\mathrm{Y}] /[\mathrm{X}]$ for the ATP-coupled reaction when the values of [ATP], [ADP], and $\left[\mathrm{P}_{\mathrm{i}}\right]$ are those found in rat myocytes (Table $13-5$ ).

Key Concepts

Gibbs Free Energy and Equilibrium Constant

This concept relates the standard Gibbs free energy change (?G°) of a reaction to its equilibrium constant (K) via the equation ?G° = -RT ln K. It provides the foundation for understanding how the favorability of a reaction under standard conditions—whether it is spontaneous or not—is quantitatively linked to the concentration ratio of products to reactants at equilibrium.

Reaction Coupling

Reaction coupling is the strategy of coupling an energetically unfavorable reaction to a highly favorable one so that the overall process becomes thermodynamically favorable. In biological systems, ATP hydrolysis is frequently used as the favorable reaction given its large negative free energy change, allowing reactions that would not proceed spontaneously on their own to occur under physiological conditions.

Physiological Versus Standard Conditions

While standard conditions assume concentrations of 1 M for all reactants and products, real biological systems operate under physiological conditions where metabolite concentrations are different. This difference affects the effective Gibbs free energy change and equilibrium position of reactions, highlighting the importance of considering actual in vivo concentrations to accurately predict reaction behaviors.

Mass Action Law

The mass action law describes how the rate and equilibrium state of a chemical reaction depend on the concentrations of the species involved. It underpins the concept of equilibrium ratios by stating that at equilibrium, the ratio of product concentrations to reactant concentrations is a constant (the equilibrium constant) under a given set of conditions.

Transcript

Please give Ace some feedback