How do enzymes lower activation energy, and what changes their rate?
2.1.4 Enzymes: the role of enzymes as biological catalysts in metabolic reactions; the mechanism of enzyme action including the lock-and-key and induced-fit models; the effects of temperature, pH, enzyme and substrate concentration on the rate of reaction; the action of competitive and non-competitive inhibitors; the roles of cofactors, coenzymes and prosthetic groups.
A focused answer to the OCR H420 2.1.4 dot point on enzymes. Covers enzymes as catalysts, the lock-and-key and induced-fit models, activation energy, the effects of temperature, pH and concentration, competitive and non-competitive inhibition, and cofactors.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
OCR wants you to explain how enzymes catalyse reactions by lowering activation energy, describe and contrast the lock-and-key and induced-fit models, predict and explain the effect of temperature, pH and concentration on rate, distinguish competitive from non-competitive inhibition, and state the roles of cofactors, coenzymes and prosthetic groups.
The answer
Enzymes as catalysts
Enzymes are globular proteins that act as biological catalysts: they speed up metabolic reactions by lowering the activation energy without being used up. The substrate binds to the enzyme's active site to form an enzyme-substrate complex; the products are then released and the enzyme is free to catalyse again.
The two models of enzyme action
- Lock-and-key model. The active site is a rigid, exactly complementary shape into which only the correct substrate fits, like a key in a lock.
- Induced-fit model. The active site is initially only approximately complementary; when the substrate binds, the active site changes shape slightly to mould around it, straining the substrate's bonds and lowering activation energy further. Induced fit is the model OCR favours because it better explains specificity and catalysis.
Enzymes are specific because the tertiary structure of the active site is complementary to one substrate (or a small group of similar substrates).
Factors affecting rate
- Temperature. Rate rises with temperature as molecules gain kinetic energy and collide more often, up to an optimum; above the optimum the enzyme denatures (hydrogen and ionic bonds break, the active site changes shape) and rate falls.
- pH. Each enzyme has an optimum pH; moving away from it alters the charges on R groups, breaking ionic and hydrogen bonds, changing the active-site shape and reducing rate. Extreme pH denatures the enzyme.
- Substrate concentration. Rate increases with substrate until all active sites are occupied; the rate then plateaus because enzyme concentration is now limiting (V max).
- Enzyme concentration. With excess substrate, rate is proportional to enzyme concentration because more active sites are available.
Inhibitors and cofactors
- Competitive inhibitor. Similar shape to the substrate; binds the active site and blocks it. More substrate overcomes it, so V max is unchanged but is reached more slowly.
- Non-competitive inhibitor. Binds an allosteric site away from the active site, changing the active-site shape; more substrate cannot overcome it, so V max is reduced.
- Cofactors are non-protein components needed for activity. A coenzyme is an organic cofactor that moves between enzymes carrying chemical groups (for example NAD in respiration). A prosthetic group is a cofactor permanently bound to the enzyme (for example zinc in carbonic anhydrase).
Examples in context
Example 1. Respiration coenzymes. NAD and FAD are coenzymes that accept hydrogen during glycolysis and the Krebs cycle and deliver it to the electron transport chain, linking enzymes to ATP synthesis in Module 5.
Example 2. Statin drugs. Statins are competitive inhibitors of an enzyme in cholesterol synthesis, showing how inhibition is exploited medically, a common applied exam context.
Try this
Q1. Explain why an enzyme is specific to its substrate. [2 marks]
- Cue. The active site has a particular tertiary structure that is complementary in shape (and charge) to one substrate, so only that substrate forms an enzyme-substrate complex.
Q2. Describe how a decrease in pH below the optimum reduces the rate of an enzyme reaction. [2 marks]
- Cue. Extra hydrogen ions alter the charges on R groups, breaking ionic and hydrogen bonds; the active-site shape changes so fewer enzyme-substrate complexes form.
Q3. State the difference between a coenzyme and a prosthetic group. [2 marks]
- Cue. A coenzyme is an organic cofactor that moves between enzymes; a prosthetic group is a cofactor permanently bound to one enzyme.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20184 marksExplain the effect of increasing temperature on the rate of an enzyme-controlled reaction, from low temperatures up to and beyond the optimum.Show worked answer →
Two phases, with a clear reason for each.
- Below the optimum
- as temperature rises, enzyme and substrate molecules gain kinetic energy and move faster, so there are more frequent successful collisions and more enzyme-substrate complexes form per second, increasing the rate.
- At the optimum
- the rate is highest.
- Above the optimum
- the extra kinetic energy breaks the hydrogen and ionic bonds holding the tertiary structure, so the enzyme denatures. The active site changes shape and is no longer complementary to the substrate, so fewer enzyme-substrate complexes form and the rate falls sharply.
Markers reward kinetic energy and collisions for the rise, and denaturation and a changed active site for the fall.
OCR H420/01 20214 marksCompare the effects of a competitive inhibitor and a non-competitive inhibitor on the rate of an enzyme-controlled reaction, including the effect of increasing substrate concentration in each case.Show worked answer →
Define where each binds, then the effect of more substrate (about 2 marks each).
Competitive inhibitor: has a similar shape to the substrate and binds to the active site, blocking the substrate. Increasing substrate concentration reduces the inhibition because substrate out-competes the inhibitor for the active site, so the maximum rate can still be reached.
Non-competitive inhibitor: binds to a site away from the active site (an allosteric site), changing the shape of the active site so the substrate no longer fits. Increasing substrate concentration does not overcome the inhibition, so the maximum rate is reduced.
Markers reward the binding site for each and the contrasting effect of adding substrate.
Related dot points
- 2.1.2 Biological molecules: the structure and function of triglycerides and phospholipids; the structure of amino acids, the formation of peptide bonds and the four levels of protein structure; the structure of nucleotides, DNA and RNA; the biochemical tests for lipids (emulsion test) and proteins (biuret test).
A focused answer to the OCR H420 2.1.2 dot point on lipids, proteins and nucleic acids. Covers triglycerides and phospholipids, amino acids and the four levels of protein structure, nucleotide and DNA and RNA structure, and the emulsion and biuret tests.
- 2.1.2 Biological molecules: the properties of water and their importance to living organisms; the structure of monosaccharides, the formation of glycosidic bonds by condensation, and the structure and function of starch, glycogen and cellulose; the biochemical tests for reducing and non-reducing sugars and for starch.
A focused answer to the OCR H420 2.1.2 dot point on water and carbohydrates. Covers the biologically important properties of water, monosaccharides and condensation, the structure and function of starch, glycogen and cellulose, and the Benedict's and iodine tests.
- 2.1.1 Cell structure: the ultrastructure of eukaryotic and prokaryotic cells, the function of organelles including the role of the rough endoplasmic reticulum and Golgi apparatus in producing and secreting proteins; the use, calibration and resolution of light and electron microscopes.
A focused answer to the OCR H420 2.1.1 dot point on cell structure and microscopy. Covers every required eukaryotic and prokaryotic organelle, the protein secretory pathway, the three microscopes, eyepiece-graticule calibration and the magnification equation.
- 5.2.2 Respiration: the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation); the role of decarboxylation, dehydrogenation, reduced NAD and FAD, the electron transport chain, chemiosmosis and ATP synthase; the synthesis of ATP and the role of oxygen as the final electron acceptor; and anaerobic respiration in animals (lactate) and in yeast (ethanol).
A focused answer to the OCR H420 5.2.2 dot point on respiration. Covers the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation), chemiosmosis and ATP synthase, the role of oxygen, and anaerobic respiration producing lactate or ethanol.
- 5.1.4 Hormonal communication: the principles of hormonal coordination and the contrast with nervous coordination; the structure and function of the adrenal glands and pancreas; the control of blood glucose concentration by insulin and glucagon (glycogenesis, glycogenolysis and gluconeogenesis); the second messenger model of adrenaline and glucagon; and the causes of type 1 and type 2 diabetes.
A focused answer to the OCR H420 5.1.4 dot point on hormonal communication. Covers hormonal versus nervous coordination, the adrenal glands and pancreas, the control of blood glucose by insulin and glucagon, the second messenger model, and the causes of type 1 and type 2 diabetes.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)