AP Syllabus focus:
‘The relative concentrations of substrates and products influence how efficiently an enzymatic reaction proceeds in cells.’
Enzyme-catalysed reactions depend strongly on how much substrate is available and how much product has accumulated. Cells regulate concentrations to control reaction efficiency, pathway flux, and the direction and extent of reversible reactions.
Concentration as a control on enzyme-mediated reactions
Enzymes form enzyme–substrate complexes through random molecular collisions. Because collision frequency depends on concentration, changes in substrate and product levels can change the observed reaction rate without changing the enzyme itself.
Key idea: at any moment, an enzyme’s activity reflects how often it is occupied by substrate versus unoccupied and available to bind.
Effects of substrate concentration on reaction efficiency
Low substrate concentrations: substrate-limited rates
When substrate concentration is low relative to enzyme availability:
This figure plots reaction velocity versus substrate concentration, showing the characteristic hyperbolic rise in rate at low [S] and the approach to a maximum rate (Vmax) as active sites become saturated. The curve makes it clear why adding substrate strongly increases rate when many enzymes are unoccupied, but has diminishing returns once most enzymes are already bound. Source
Few enzyme active sites are occupied at a given time.
Increasing substrate increases the frequency of successful collisions.
Reaction rate rises steeply because more enzyme–substrate complexes form.
This is common in cells when substrates are scarce, compartmentalised, or rapidly consumed by downstream reactions.
High substrate concentrations: saturation and maximum rate
Enzyme saturation: A condition in which virtually all enzyme active sites are occupied by substrate most of the time, so adding more substrate produces little or no further increase in reaction rate.
At high substrate concentrations:
The enzyme becomes the limiting factor (not the substrate).
Rate approaches a maximum rate set by how fast the enzyme can convert bound substrate to product and release it.
Further substrate increases do not meaningfully increase efficiency, because there are few free active sites left to bind additional substrate.
Why saturation matters in cells
Saturation explains why:
Cells can “buffer” reaction rates against small substrate fluctuations once enzymes are near full occupancy.
To increase pathway throughput, cells may need more enzyme molecules (e.g., via gene expression) rather than more substrate.
Competing processes that consume the same substrate can reduce the rate by lowering local substrate concentration.
Effects of product concentration on reaction efficiency
Product accumulation and slowing of net rate
As product concentration rises, efficiency can decrease because:
In many enzyme-catalysed reactions, product can rebind to the enzyme (often at or near the active site), reducing the fraction of enzyme available to bind substrate.
The net forward reaction may slow as the reverse reaction becomes more likely in reversible systems.
Even if an enzyme continues to catalyse individual forward and reverse events, the observed net production can drop when product is abundant.
Reversible reactions, equilibrium, and “driving” the reaction
Many enzymatic reactions are reversible. Concentrations influence direction:
High substrate and low product favour net forward conversion.
High product relative to substrate favours net reverse conversion.
Net reaction tends toward a balance where forward and reverse rates match (dynamic equilibrium).
This OpenStax figure illustrates a reversible reaction approaching dynamic equilibrium: reactant and product concentrations change over time and then level off, while the forward and reverse rates converge to the same value. It supports the idea that equilibrium means no net change in concentrations even though microscopic forward and reverse reactions continue. Source
Cells often keep reactions efficient by preventing product build-up, for example by:
Rapidly using product as the substrate for the next step in a pathway.
Transporting product away from the reaction site.
Storing product in another form, reducing free product concentration.
Interpreting concentration effects in biological systems
Limiting factors and pathway context
In cells, reaction efficiency reflects multiple concentration-dependent constraints:
If substrate is limiting, increasing substrate increases rate.
If enzyme is saturated, increasing substrate has little effect.
If product accumulates, net forward flux can decrease even if substrate is present.
Initial rate logic (conceptual)
To isolate substrate effects experimentally or conceptually, biologists often focus on early time points when:
Substrate is near its starting level.
Product is minimal, so product-related slowing is reduced. This highlights how substrate availability alone can determine early efficiency.
FAQ
Cooperative enzymes often show a sigmoidal (S-shaped) response to substrate concentration rather than a simple hyperbolic rise.
Low [S]: relatively low activity
Intermediate [S]: steep increase in rate as binding becomes more favourable
High [S]: saturation still occurs, producing a plateau
High product concentration can still reduce net flux by increasing the probability of reverse reactions in reversible steps.
In pathways, even weak interactions can matter when concentrations are high and diffusion repeatedly brings product into contact with enzyme.
Cells can reduce free product concentration by:
Rapidly converting it in the next reaction step
Sequestering it in organelles/regions of cytosol
Converting it into storage forms that lower the free, reactive pool
Concentration effects arise from changing amounts of substrate/product participating in the same reaction.
Regulatory inhibition involves additional molecules (not the normal substrate or product) that reduce activity by binding to the enzyme and altering binding or catalysis.
In vivo, effective concentrations depend on compartment size, molecular crowding, diffusion limits, and localised enzyme–substrate proximity.
A cell can create microenvironments where the “local” [S] or [P] near an enzyme differs from the bulk cellular concentration.
Practice Questions
Explain why increasing substrate concentration increases the rate of an enzyme-catalysed reaction at low substrate concentrations but not at very high substrate concentrations. (2 marks)
Low substrate: increasing substrate increases collision frequency/enzyme–substrate complex formation, increasing rate. (1)
High substrate: enzyme active sites become saturated; enzyme becomes limiting so adding substrate has little/no further effect. (1)
A reversible enzyme-catalysed reaction converts S to P. In a cell, S concentration drops while P concentration rises. Describe and explain how these changes affect (i) enzyme occupancy and (ii) the net rate of P formation. (5 marks)
Lower [S] reduces frequency of successful collisions and decreases enzyme–substrate complex formation/occupancy by S. (1)
Higher [P] increases likelihood of product rebinding to enzyme, reducing availability for S binding. (1)
Higher [P] increases reverse reaction likelihood in a reversible system. (1)
Net forward rate of P formation decreases due to reduced forward events and/or increased reverse events. (1)
Clear linkage to concentration effects on net flux (towards balance/dynamic equilibrium) rather than enzyme “stopping”. (1)
