A blocked step does not make flux disappear. The material that would have passed through the missing enzyme is still being produced upstream, and it must go somewhere. If the pathway has a branch point before the block, the accumulating intermediate can be drawn into the alternative branch, so flux through that branch increases. If a bypass route exists — a set of reactions that converts the accumulating intermediate into a downstream product without using the missing enzyme — flux can be rerouted through it, partially restoring supply of the end product. In a purely linear pathway with no branch and no bypass, there is nowhere for the upstream material to go except to accumulate, and the only relief comes from slow side reactions or from the upstream enzyme being slowed by its own rising substrate or product. The key point is that the pathway's topology determines the redistribution: branches and bypasses absorb the flux, a dead-end linear segment does not.
Learn MotionWhen a Key Metabolic Enzyme Is Missing: Pathways, Regulation, and Physiological Consequences
Immediate Consequences of Losing One Catalytic Step
Where the Flux Goes When the Main Route Is Blocked
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Look at the branch point before the block. The intermediate that is piling up has two possible fates: it can continue toward the missing enzyme, or it can be pulled into the side branch. Since the main route is closed, more of it flows into the branch, and you can see the branch arrow thicken. Now look at the bypass route on the right — it carries the intermediate around the block and into the downstream product, so some end product is restored. Compare that with the linear pathway at the bottom, which has no branch and no bypass. There, the accumulating intermediate has nowhere to go, and the only thing that limits its rise is a slow side reaction or the upstream enzyme slowing down. The lesson is that the pathway's shape — where its branches and bypasses are — decides where the flux ends up.
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