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.
Practice Questions
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.
