---
title: "Flux Control and Following Atoms with Labelled Substrates"
description: "Control of a pathway is usually shared between several enzymes rather than held by one, and how much each holds can be measured. Feeding a substrate whose atoms are labelled and seeing where they appe"
canonical: https://lightmysky.com/learn/science/flux-control-and-following-atoms-with-labelled-substrates-mt_PzhsDwD_aW
source: https://lightmysky.com/learn/science/flux-control-and-following-atoms-with-labelled-substrates-mt_PzhsDwD_aW.md
retrieved: 2026-09-12
---

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# Flux Control and Following Atoms with Labelled Substrates

Control of a pathway is usually shared between several enzymes rather than held by one, and how much each holds can be measured. Feeding a substrate whose atoms are labelled and seeing where they appear is how the routes actually taken are established.

Subject: Science · Area: Biochemistry & Molecular Biology · Ages 21 to 22
Page: https://lightmysky.com/learn/science/flux-control-and-following-atoms-with-labelled-substrates-mt_PzhsDwD_aW

## Ready when they can

- Explains why calling one enzyme the rate-limiting step is usually an approximation.
- Predicts where a label ends up given a stated pathway and starting position.
- Uses a labelling result to choose between two proposed routes.

## Lesson: Shared control and atom trails

Control of a pathway is usually shared between several enzymes rather than held by one. Speeding a single enzyme rarely speeds the whole pathway in proportion, because the tightest constraint shifts with fuel, demand, and oxygen. Respiration shows it live: phosphofructokinase, pyruvate dehydrogenase, and the cycle enzymes each answer overlapping signals. Calling one of them the rate-limiting step is an approximation that holds only under specific conditions.

Inhibitors help find where control sits. A competitive inhibitor binds the active site itself, so it raises the apparent substrate need without changing the maximum rate. That kinetic signature reveals which step strains under given conditions, and such patterns are among the oldest tools for dissecting control.

**Example.** Picture two proposed routes between the same start and end, drawn as rival maps. Feed a labelled precursor and watch where the label emerges: if each route sends the atom to a different product, one measurement picks the true road. Tracing atoms this way turns static maps into tested descriptions of living flux, the rate at which material flows through a pathway.

**Tip.** To predict where a label ends up, walk the atom through each reaction of the proposed route in order and land it on its product position.

**Recap.** Control is shared and shifting, inhibition signatures probe it, and labels reveal the road truly taken.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Control Points in Glycolysis and the Committed Step](https://lightmysky.com/learn/science/control-points-in-glycolysis-and-the-committed-step-mt_2vAoKtnmoz)
- [Transamination, Ammonia and the Urea Cycle](https://lightmysky.com/learn/science/transamination-ammonia-and-the-urea-cycle-mt_MFu5uQKyFL)
- [The Calvin Cycle and Carbon Fixation](https://lightmysky.com/learn/science/the-calvin-cycle-and-carbon-fixation-mt_zGOCx9mNrx)

## Opens up

- [Metabolic Engineering: Redirecting Flux Through a Production Host](https://lightmysky.com/learn/science/metabolic-engineering-redirecting-flux-through-a-production-host-mt_fjz3EA-dDa)