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# Calculation: Free Energy Change of a TCA Cycle Reaction

BIOCHM-JAABMF

The last reaction of the citric acid cycle or tricarboxylic acid (TCA) cycle is the oxidation of malate to regenerate oxaloacetate (OAA) using the coenzyme ${ NAD }^{ + }$ and catalyzed by malate dehydrogenase (MDH).

This reaction is followed by the first reaction of the TCA cycle, the condensation of acetyl-CoA and OAA, catalyzed by citrate synthase (${ \Delta G }^{ \circ \prime }$ = -31.5 kJ).

Use the standard reduction potential values for the MDH half-reactions and the Nernst equation shown below, as well as your understanding of thermodynamics and regulation of the TCA cycle, to determine which ONE of the following statements is true regarding the reaction catalyzed by MDH.

Half-Reaction ${E}^{ \circ \prime}$
${{Oxaloacetate}^{ - }\quad +\quad 2{ H }^{ + } }\quad +\quad 2{ e }^{ - }\quad \leftrightharpoons \quad {Malate}^{ - }$ -0.166 V
${NAD }^{+}\quad {+}{\quad H}^{+}\quad {+}\quad {2e}^{-}\quad \leftrightharpoons\quad {NADH}$ -0.315 V

Nernst Equation: ${ \Delta G }^{ \circ \prime }{ =-nF }{ \Delta E }^{ \circ \prime }$ where $F = 96,485 {J/V\cdot mol}$

A

The overall change in free energy for the oxidation of malate to OAA under standard conditions is 92.8 kJ.

B

The concentration of OAA at equilibrium under cellular conditions would be very low compared to malate.

C

MDH and citrate synthase are examples of exergonic TCA cycle reactions.

D

The reaction catalyzed by MDH proceeds with a large -${ \Delta G }^{ \circ \prime }$ under standard conditions.

E

A decrease in mitochondrial [NADH] stimulates MDH activity while inhibiting citrate synthase.