The half-reactions involved in the lactate dehydrogenase...

The half-reactions involved in the lactate dehydrogenase (LDH) reaction and their standard reduction potentials are $$\begin{array}{l}\text { Pyruvate }+2 \mathrm{H}^{-}+2 r^{-} \longrightarrow \text { luctate } \quad \varepsilon^{0}=-0.185 \mathrm{V} \\\mathrm{NAD}^{\prime}+2 \mathrm{H}^{2}+2 \mathrm{c}^{2} \longrightarrow \mathrm{NADH}+\mathrm{H}^{+} \quad 8^{-1}--0.315 \mathrm{V}\end{array}$$ Calculate $\Delta G$ as $p H$ 7.0 for the LDH-catalyzed reduction of pyruvate under the following conditions: (a) [lactate]/[pyruvate] $=1$ and $\left[\mathrm{NAD}^{+}\right] /[\mathrm{NADH}]=1$ (b) $|$ lactate $|$ pynuvate $= 160$ and $\left|\mathrm{NAD}^{*}\right| /(\mathrm{NAD} \mathrm{H})=160$ (c) $|$ lactare $|$ / pynuvate $|= 1000$ and $\left|\mathrm{NAD}^{*}\right| /|\mathrm{NADH}|=1000$ (d) Discuss the effect of the concentration ration in parts a-c on the direction of the reaction.

The half-reactions involved in the lactate dehydrogenase (LDH) reaction and their standard reduction potentials are
$$\begin{array}{l}\text { Pyruvate }+2 \mathrm{H}^{-}+2 r^{-} \longrightarrow \text { luctate } \quad \varepsilon^{0}=-0.185 \mathrm{V} \\\mathrm{NAD}^{\prime}+2 \mathrm{H}^{2}+2 \mathrm{c}^{2} \longrightarrow \mathrm{NADH}+\mathrm{H}^{+} \quad 8^{-1}--0.315 \mathrm{V}\end{array}$$
Calculate $\Delta G$ as $p H$ 7.0 for the LDH-catalyzed reduction of pyruvate under the following conditions:
(a) [lactate]/[pyruvate] $=1$ and $\left[\mathrm{NAD}^{+}\right] /[\mathrm{NADH}]=1$
(b) $|$ lactate $|$ pynuvate $= 160$ and $\left|\mathrm{NAD}^{*}\right| /(\mathrm{NAD} \mathrm{H})=160$
(c) $|$ lactare $|$ / pynuvate $|= 1000$ and $\left|\mathrm{NAD}^{*}\right| /|\mathrm{NADH}|=1000$
(d) Discuss the effect of the concentration ration in parts a-c on the direction of the reaction.

Key Concepts

Standard Reduction Potential

The standard reduction potential is a measure of the intrinsic tendency of a chemical species to gain electrons and be reduced, measured under standard conditions. It is crucial in comparing half-reactions, predicting the direction of electron flow, and determining the feasibility of redox reactions.

Gibbs Free Energy Change (?G)

Gibbs free energy change quantifies the maximum useful work obtainable from a reaction at constant temperature and pressure. In redox reactions, ?G is related to the electrochemical potential difference by the equation ?G = -nFE, linking the electron transfer steps with the thermodynamic favorability of the process.

Nernst Equation

The Nernst equation allows the calculation of the electrode potential under non?standard conditions by incorporating the effects of reactant and product concentrations (or activities) into the standard reduction potential. It provides insight into how deviations in concentration and pH can shift the equilibrium and affect the reaction’s driving force.

Reaction Quotient and Concentration Ratios

The reaction quotient, defined as the ratio of product concentrations to reactant concentrations, plays an essential role in determining how far a system is from equilibrium. Changes in the concentration ratios alter the cell potential according to the Nernst equation, thereby influencing the direction and extent of the reaction.

pH Effects on Redox Reactions

pH can significantly influence redox reactions, especially those that involve proton transfer. The concentration of hydrogen ions affects the reaction quotient in the Nernst equation, leading to changes in the overall potential and Gibbs free energy, which in turn determine the reaction direction and spontaneity under physiological or altered pH conditions.

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