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

To evaluate the proposed pathway, I’ll examine both the individual steps and the overall thermodynamics of the sequence.
Option 1: This pathway will not occur. This is incorrect. A pathway can still proceed if the overall free energy change is favorable, even if some individual steps are unfavorable. The sum of ΔG°′ values determines the net feasibility, not a blanket statement that the pathway cannot occur.
Option 2: The first two reactions of the pathway will occur, but once W is formed the pathway will halt. While the first two steps (U→V and V→W) have negative ΔG°′ values (–15 and –2 kJ/mol), suggesting they are favorable, this argument ignores subsequent steps. A later step with a positive ΔG°′ could be unfavorable, but a subsequent highly favorable step can still drive the overall process if the net ΔG°′ is negative. Therefore asserting that the pathway halts after W is formed is not justified by the data alone.
Option 3: This pathway may occur because the overall ΔG°′ is negative. This is the key point: summing all five steps gives ΔG°′ total = –15 + (–2) + 5 + 6 + (–7) = –13 kJ/mol, which is negative. A negative overall ΔG°′ indicates the pathway is energetically favorable as a whole, so it could occur under standard conditions, assuming no kinetic or regulatory barriers prevent it.
Option 4: This pathway may occur because the largest ΔG°′ is a negative number. This statement is misleading. The largest magnitude of ΔG°′ does not by itself determine feasibility; what matters for spontaneity is the sum of all steps. There are positive ΔG°′ steps (+5 and +6 kJ/mol) that raise the energy of those steps, and a single largest negative step (–15 kJ/mol) does not guarantee overall feasibility when balanced by the positives. Hence this rationale is flawed.
In summary, when you consider the complete sequence, the net ΔG°′ is negative, making the path energetically favorable overall, while the other options rely on incorrect interpretations of how individual steps relate to the whole pathway.

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