AP Syllabus focus:
‘Enzymes are protein catalysts that lower activation energy, increasing the rate of biological reactions in cells.’
Enzymes make cellular chemistry fast enough to sustain life. They do this by catalysing reactions: accelerating reaction rates under mild cellular conditions while remaining unchanged, enabling metabolism to proceed efficiently and selectively.
What enzymes are
An enzyme is a biological catalyst. Most enzymes are globular proteins whose three-dimensional shapes create chemical environments that promote specific reactions.
Enzyme: A protein catalyst that increases the rate of a chemical reaction by lowering activation energy, without being consumed by the reaction.
Enzymes are essential because many biologically important reactions are thermodynamically possible yet proceed too slowly at typical cellular temperatures without catalysis.
How enzymes differ from reactants and products
Not consumed: the enzyme is regenerated at the end of the reaction cycle and can catalyse many rounds.
Do not change overall energy change: enzymes affect rate, not the reaction’s overall free-energy difference ().
Reaction-coordinate diagram showing how an enzyme-catalyzed pathway has a lower activation-energy barrier than the uncatalyzed pathway. The key takeaway is that the peak (transition state) is lowered, while the overall free-energy change between reactants and products () remains the same. Source
Do not change equilibrium position: enzymes speed both forward and reverse reactions proportionally, helping a system reach equilibrium faster but not shifting where equilibrium lies.
Activation energy and why lowering it matters
A reaction often requires an initial input of energy to reach a high-energy transition state before products can form. That energy “hill” is the main kinetic barrier in cells.
Activation energy: The minimum energy required for reactants to reach the transition state so that a reaction can proceed.
At a given temperature, only a fraction of reactant molecules have enough kinetic energy to overcome this barrier. By lowering activation energy, enzymes increase the fraction of successful molecular collisions per unit time, dramatically increasing reaction rate under physiological conditions.
How enzymes work: core catalytic strategies
Enzymes lower activation energy by providing an alternative reaction pathway that makes transition-state formation more likely. Key, syllabus-aligned ideas focus on transition-state stabilisation and improved reaction kinetics.
Transition-state stabilisation
Enzymes tend to bind and stabilise the transition state more effectively than they bind reactants. Stabilising this high-energy state effectively “lowers the hill” that reactants must climb, reducing activation energy and increasing the rate.
Proximity and orientation effects
Enzymes bring reacting molecules close together and align them in the correct orientation for bond rearrangements to occur.
In solution, reactants collide randomly.
In an enzyme-catalysed reaction, collisions become more frequent and more productive.
Creating a favourable microenvironment
Enzymes can create local chemical conditions that promote reaction steps, for example by positioning charged or polar groups near reactants to stabilise developing charges during bond breaking/forming. This reduces the energetic cost of reaching the transition state.
Straining bonds and facilitating bond rearrangements
By binding reactants, an enzyme may distort particular bonds, making them easier to break, or position functional groups so new bonds form more readily. These effects lower activation energy by making transition-state geometry easier to achieve.
The enzyme reaction cycle (conceptual overview)
An enzyme-catalysed reaction can be thought of as a repeating cycle:
Binding: reactants associate with the enzyme to form a short-lived enzyme–reactant complex.
Catalysis: the enzyme promotes transition-state formation and conversion toward products.
Release: products leave; the enzyme is restored to its original state.
Induced-fit model diagram illustrating that substrate binding can trigger a conformational change in the enzyme’s active site. This dynamic fit helps position reactants and stabilize the transition state, and the products then dissociate so the enzyme can be reused in another catalytic cycle. Source
Because the enzyme is regenerated, a small amount of enzyme can have a large effect on overall cellular reaction rates, which is why enzyme concentration is often a key determinant of metabolic capacity.
Why enzymes are indispensable in cells
Cellular conditions constrain reaction chemistry:
Temperatures are relatively low and stable compared with many industrial reactions.
Water-based environments can limit reaction types and intermediate stability.
Cells require tight timing; reactions must occur rapidly enough to support growth, repair, and homeostasis.
Enzymes solve these constraints by lowering activation energy, increasing reaction rates so biological reactions proceed on useful timescales while keeping the cell’s overall energy accounting unchanged.
FAQ
No. Some need cofactors (e.g., metal ions) or coenzymes (organic molecules) to complete catalysis, while others are fully functional as protein alone.
Amino-acid substitutions can subtly alter folding or local chemistry, reducing transition-state stabilisation or reactant positioning, which raises effective activation energy and slows the reaction.
Specificity refers to the reaction chemistry promoted. The same catalytic features that lower activation energy for the forward reaction also lower it for the reverse under appropriate concentrations.
Yes. Some RNA molecules act as catalysts (ribozymes), using folded structures to stabilise transition states, though many cellular catalysts are proteins.
They track reaction rate by monitoring product formation or substrate disappearance over time (often via colour change, fluorescence, or absorbance), then compare rates with and without enzyme present.
Practice Questions
Explain what is meant by activation energy and state how enzymes affect it. (2 marks)
Defines activation energy as the minimum energy to reach the transition state. (1)
States that enzymes lower activation energy, increasing reaction rate. (1)
Describe how enzymes increase reaction rates without changing the overall free-energy change of a reaction. (5 marks)
Enzymes provide an alternative pathway with lower activation energy. (1)
They stabilise the transition state more than reactants (transition-state stabilisation). (1)
They increase effective collisions by bringing reactants together and/or orientating them correctly. (1)
They may create a favourable microenvironment and/or strain bonds to facilitate bond rearrangements. (1)
They do not change (and therefore do not shift equilibrium), only the rate. (1)
