BLUEPRINTUNIT 3 · CELLULAR ENERGETICSINTERACTIVE LAB
BINDING · SHAPE · REACTION RATE
Enzyme inhibition & feedback
Can adding more substrate overcome inhibition? Look inside the enzyme, then follow an entire pathway.
Feedback connection: Unit 4
MOLECULES IN MOTION
An enzyme can be used again.
Swipe across the molecular view →
S · substrateI · added inhibitorP · reaction product
Products formed0
Products present0
Substrates bound0
Trial time0.0 model s
Follow one of the enzymes in the moving population.
Active site available
An enzyme can be used again after product release.
Substrate binds at the active site, is converted to product, and leaves. The enzyme is available again.
Free substrate and inhibitor levels are maintained by supply. Released products stay inside the chamber and keep moving throughout the trial. Shapes and motion are schematic.
PREDICTED RELATIONSHIP
What does more substrate change?
Graph view
No inhibitorSelected condition
Increasing substrate raises the rate until enzyme availability limits it.
Both curves compare the same enzyme amount. Inhibitor concentration is held fixed along each curve.
ENZYMES IN A POPULATION
Enzymes occupy different binding states.
Empty active siteSubstrate boundInhibitor bound
These are the same enzymes as in the moving population above. Their binding states change independently as molecules encounter and leave them.
UNIT 4 CONNECTION
Negative feedback · Topic 4.4. Here, the final product inhibits an early enzyme, reducing further product formation. Feedback is explored in more detail in Unit 4.
S → B → C → P
The final product regulates an early step.
Swipe across to follow the pathway →
When production exceeds use, final product accumulates. This product can return to bind enzyme 1.
Conversions move forward: S → B → C → P. The separate P-to-enzyme-1 line shows regulation. Downstream enzymes can keep using intermediates after enzyme 1 slows.
ZOOM IN: ENZYME 1
Product binds at an allosteric site.
Enzyme 1 rate5.3relative units / model s
Product level0.0relative units
Product is initially absent. Run the pathway to make it.
Two copies of enzyme 1 release blue B forward toward enzyme 2. Orange P returns from the end of the pathway to inhibit enzyme 1.
FOLLOW THE CHANGES
Product levels reflect production and use.
End-product levelP · relative units
Product production and useProduction Use
Start the pathway. Product must pass through B and C before it appears.
Production is enzyme 3's output. Use increases with product availability. Both can continue at a steady product level.
Two useful distinctions
Binding and catalysis: in the noncompetitive model, substrate can bind while the enzyme is inhibited.
Inhibition and feedback: inhibition describes an enzyme interaction; feedback describes the product regulating its own pathway.
Teacher notes · CED alignment, model assumptions and explanations
Unit 3 focus
3.1.A.2: active-site compatibility and enzyme–substrate complexes. 3.2.A: structure and function. 3.2.B.1: relative substrate/product concentrations. 3.2.B.3: reversible competitive binding at active sites and noncompetitive binding at allosteric sites.
Unit 4 connection: Topic 4.4, especially 4.4.A.1.i: negative feedback at molecular, cellular and organismal levels. Product accumulation reduces further production in this example. This model has an emergent steady level rather than a separately programmed target concentration.
Comparison experiment
Temperature and pH are held constant. Substrate and inhibitor sliders represent maintained free concentrations. The enzyme selector changes 3, 6 or 9 enzyme molecules; more enzymes increase the ideal catalytic capacity. Both graph curves always use the same selected enzyme amount. Six enzymes define a capacity of 100 rate-index units. The graph can show 0–200 for low-substrate detail or 0–5,000 for high-substrate predictions; the molecular trial slider covers 0–200. The wider graph reveals the shared competitive maximum without changing the inhibition parameters.
Competitive inhibitors occupy the active site until they dissociate. Extra substrate increases the chance of substrate binding to an available site; it does not forcibly eject an inhibitor. At sufficiently high substrate, this ideal reversible model approaches the uninhibited maximum rate.
The selected noncompetitive model has substrate binding unaffected by inhibitor occupancy, while inhibitor-bound enzyme cannot complete catalysis. Its predicted maximum rate is lower at a fixed nonzero inhibitor concentration. Allosteric regulation is broader than this ideal kinetic model; actual enzymes need not follow these curves. Regulatory shape changes are distinct from denaturation.
Feedback experiment
The final product reversibly inhibits enzyme 1. Default: an allosteric, ideal noncompetitive teaching model. Alternative: a generic competitive feedback model. The alternative does not imply all end products resemble their initial substrates or compete at active sites.
Substrate is replenished to maintain the selected concentration. B and C are intermediate pools; no other pathways or regulation are included. Product use happens continuously and increases with available product. Product already in the pathway can continue forming after the early enzyme slows. Added product is an external perturbation.
At dynamic balance, production and use are equal while molecular reactions continue. Higher product use tends to lower the product level and relax inhibition. With feedback off, the pathway still responds to substrate supply and enzyme saturation. Enzyme-bound ligands are negligible relative to the model's free pools.
Visuals and sources
Shapes, copy numbers, rates and model times are schematic, not measured data or molecular structures. The magnifier follows one enzyme in the chamber. The feedback view shows two copies of enzyme 1, both producing intermediate B. Its free P level follows the pathway pool; the small collision view does not calculate the pathway graphs. Molecules follow enlarged two-dimensional random paths and bind upon compatible encounters near unoccupied sites. Enzymes are held in place for visibility. The live snapshot repeats the exact states of the moving chamber. Free S and I are supplied to maintain their settings. Comparison products accumulate and reflect from the chamber boundaries, remaining visible until a new trial is started. In the feedback zoom, B moves forward and leaves on the right toward enzyme 2; substrate and regulatory P can diffuse in either direction. The pathway markers have individual identities and integrated forward progress, so falling reaction rates cannot move existing markers backward. A completed marker leaves the link; new markers appear at its input. Conversions are forward-only in this selected pathway model, and the separate return of P represents regulation. Final-product use still occurs in the pathway model. Displayed product counts belong to this illustrative chamber; the ideal rate curves are calculated separately and are not fitted to these stochastic collision counts. This comparison uses independently supplied inhibitor I; the feedback experiment uses its own final product P as the inhibitor. No kinetic-constant calculations are required in the student view.
Curriculum: supplied AP Biology Course and Exam Description (2025), printed pp. 68–70 and 91.
Scientific references: IUBMB enzyme kinetics and inhibition; RCSB PDB-101: an example of allosteric end-product feedback. The RCSB example is not presented as a pure noncompetitive enzyme.