Chemistry

Problem

A researcher is attempting to couple the endergonic synthesis of malate from oxaloacetate (

\Delta G^{\circ}=+29\,\text{kJ mol}^{-1}

) to one of several non-redox driving reactions available inside mitochondria. Table 2 summarizes the candidate reactions. Table 2 Mitochondrial Driving Reactions (non-redox only)

Label	Reaction	$\Delta G^{\circ}\,(\text{kJ mol}^{-1})$
A	GTP $^{4-}$ → GDP $^{3-}$ + P $_\text{i}^{2-}$	−31
B	ATP $^{4-}$ → AMP $^{2-}$ + PP $_\text{i}^{3-}$	−46
C	acetyl-CoA → CoA + acetate $^{-}$	−35

The laboratory has reported that pyrophosphate (PP

_\text{i}^{3-}

) is rapidly hydrolyzed to two inorganic phosphate ions with

\Delta G^{\circ}=−19\,\text{kJ mol}^{-1}

, and that this hydrolysis occurs virtually to completion. (a) Explain why any driving reaction that alters the oxidation state of a carbon atom is disallowed in this experiment. Your answer should refer to how redox changes would confound the interpretation of metabolic-flux data. (b) For each candidate in Table 2, construct the complete sequence of coupled steps necessary to synthesize malate from oxaloacetate, including the spontaneous hydrolysis of pyrophosphate where relevant. Write the final net reaction and compute

\Delta G^{\circ}_{\text{net}}

. (c) Describe, without further calculation, how the rapid hydrolysis of pyrophosphate affects the position of equilibrium for coupling strategy B compared with strategies A and C. (d) Recommend which candidate provides the minimal energy waste while still rendering the malate synthesis favorable. Defend your recommendation using both numerical results and conceptual reasoning about metabolic control. (a) Conceptual explanation of the redox restriction (b) Construction of reaction sequences and

\Delta G^{\circ}_{\text{net}}

calculations (c) Qualitative effect of PP

_\text{i}

hydrolysis on equilibrium (d) Final recommendation with justification

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