Moderate# 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 $F = 96,485 {J/V\cdot mol}$