A metabolic pathway consisting of five reactions U→V→W→X→Y→Z has been proposed. The ΔG°′ for each reaction is shown below. Which of the following statements is TRUE regarding this proposed pathway?
U→V ΔG°′ = –15kJ/mol
V→W ΔG°′ = –2kJ/mol
W→X ΔG°′ = +5kJ/mol
X→Y ΔG°′ = +6kJ/mol
Y→Z ΔG°′ = –7kJ/mol

Question

BlackTom AI · Accepted Answer

Question restatement: A metabolic pathway with five sequential reactions U→V→W→X→Y→Z has given standard free energy changes, ΔG°′, for each step: U→V is –15 kJ/mol, V→W is –2 kJ/mol, W→X is +5 kJ/mol, X→Y is +6 kJ/mol, and Y→Z is –7 kJ/mol. Which statement about this proposed pathway is true? The provided answer claims: “This pathway may occur because the overall ΔG°′ is negative.”

First, compute the overall ΔG°′ for the entire pathway by summing the ΔG°′ values of all steps: ΔG°′ total = (–15) + (–2) + (+5) + (+6) + (–7) = –13 kJ/mol. This indicates that, under standard conditions, the overall reaction sequence would be exergonic.

Understanding the implications: In metabolic pathways, a negative overall ΔG°′ means that, as a whole, the pathway tends to proceed spontaneously in the forward direction from U to Z when reactant and product concentrations are at standard-state conditions. A key nuance is that individual steps can have positive or negative ΔG°′ values. Positive ΔG°′ steps (endergonic steps) can still occur within a pathway if the preceding steps release enough free energy to drive them, typically through coupled reactions and favorable metabolite concentrations.

Option-by-option analysis (conceptual since no explicit options were provided):
- If an option stated that the pathway can occur because the overall ΔG°′ is negative, that aligns with thermodynamic principles: a negative cumulative ΔG°′ supports feasibility of the forward progression of the pathway.
- An option suggesting that the pathway cannot occur because some individual steps have positive ΔG°′ would be misleading. Positive ΔG°′ steps do not preclude the overall pathway from proceeding if the net ΔG°′ is negative and if the cellular context provides coupling and appropriate metabolite levels.
- An option asserting that the pathway’s feasibility depends solely on the sign of each single step would be incorrect, because the emergent property is the sum of all steps, not the sign of each step in isolation.
- An option claiming that a positive total ΔG°′ would allow the pathway would be inconsistent with thermodynamics, as a positive overall ΔG°′ indicates nonspontaneity under standard conditions without coupling.

Conceptual takeaway: The crucial factor is the sum of the ΔG°′ values for all steps. A negative overall ΔG°′ indicates that, thermodynamically, the forward progression of the pathway is favorable, provided cellular conditions permit coupling and the relevant concentrations. Conversely, a positive overall ΔG°′ would imply that the pathway is not favored energetically unless significant coupling or energetic input changes the effective conditions.

Note on data gaps: The question as provided lists the correct-sounding statement but does not present alternative answer choices to evaluate. The analysis above focuses on the thermodynamic rationale for why a negative overall ΔG°′ supports pathway feasibility and why reliance on individual step signs alone can be misleading.

BIOL3611.13541.202610 InClassQuizChapter 15

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