Why an Irreversible Step Cannot Simply Run Backward | AI Learning Guide

The comparison on this page separates two ideas that are easy to confuse. A thermodynamic barrier is about direction: a step with a large negative free-energy change simply cannot run backward under physiological conditions, no matter how much enzyme you add. A kinetic barrier is about speed: it depends on enzyme activity and substrate availability, and it controls how fast flux moves, not which way it can go. The example makes this concrete. Step 1 is reversible, so when its enzyme is missing, the accumulating substrate can be partly converted back from the product, buffering the rise. Step 2 is irreversible because it is coupled to ATP hydrolysis, so when its enzyme is missing, the accumulating intermediate cannot flow backward through that step. The takeaway is that reversibility determines whether a blocked step can relieve its own accumulation, and that is a thermodynamic question, not a kinetic one.

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Thermodynamic barrier versus kinetic barrier

Thermodynamic barrier

Determined by the free-energy change \(\Delta G\) of the step
A large negative \(\Delta G\) makes the reverse reaction energetically forbidden
Cannot be overcome by adding more enzyme
Explains why an irreversible step cannot run backward to relieve accumulation

Kinetic barrier

Determined by enzyme activity and substrate availability
Controls how fast flux moves through the step
Can be changed by altering enzyme amount or activity
Explains the rate of accumulation, not its direction

A reversible step versus an irreversible step when blocked

Consider two steps in a pathway. Step 1 interconverts \(X\) and \(Y\) with a \(\Delta G\) near zero — it is reversible. Step 2 converts \(Y\) to \(Z\) and is coupled to ATP hydrolysis, giving it a large negative \(\Delta G\) — it is irreversible. If the enzyme for Step 1 is missing, \(X\) accumulates, but some of it can be converted back from \(Y\) because the reaction can run in reverse, so the accumulation is partially buffered. If the enzyme for Step 2 is missing, \(Y\) accumulates and cannot be converted back to \(X\) through Step 2, because that would require reversing an energy-releasing reaction. The accumulation of \(Y\) is therefore not relieved by backward flow through the blocked step.

A reversible step can partially run backward when blocked and buffer the accumulation of its substrate; an irreversible step cannot, because reversing it is thermodynamically forbidden. This is why the accumulation-depletion pattern differs between the two cases.