Enzymes and metabolism — IB Diploma Biology HL
IB Biology C1.1: enzymes as catalysts, metabolism, active sites and induced fit, factors affecting rate, activation energy and HL enzyme inhibition.
Revision notes
7 short notes, in the order of the specification. Each one in short:
Enzymes are biological catalysts: they speed up chemical reactions in cells without being used up. Metabolism is the complex network of interdependent and interacting chemical reactions in an organism. Each enzyme is specific, so many enzymes are needed, and cells control metabolism by controlling enzymes. Anabolic reactions build macromolecules; catabolic reactions break them down.
Enzymes are globular proteins with an active site, a small region of a few amino acids where the substrate binds and catalysis happens. Binding is by induced fit: the substrate and the enzyme both change shape. Substrate and active site must collide, so molecular motion matters. Specificity depends on active site shape and chemistry, and denaturation destroys both.
Enzyme activity rises with temperature up to an optimum, because faster molecules collide more often, then falls as the enzyme denatures. Activity is highest at an optimum pH and falls either side as the active site changes. Rate rises with substrate concentration until all active sites are occupied. Rate is measured as product formed or substrate used per unit time.
Activation energy is the energy needed to break bonds within the substrate so that a reaction can start. Enzymes lower the activation energy, so more substrate molecules can react at a given temperature and the rate increases. Energy is released when new bonds form in the products. The overall energy change of the reaction stays the same.
Intracellular enzymes catalyse reactions inside cells, such as glycolysis in the cytoplasm and the Krebs cycle in the mitochondrial matrix. Extracellular enzymes are secreted and act outside cells, as in chemical digestion in the gut. Metabolic reactions are never 100% efficient, so heat is always generated. Pathways are linear, like glycolysis, or cyclical, like the Krebs and Calvin cycles.
A non-competitive inhibitor binds reversibly to an allosteric site, away from the active site, causing a conformational change that alters the active site enough to prevent catalysis; more substrate cannot reverse this. A competitive inhibitor, such as a statin, resembles the substrate and binds reversibly to the active site, so high substrate concentrations can overcome it.
Feedback inhibition is regulation of a metabolic pathway by its end product, which binds to an allosteric site on an enzyme early in the pathway; isoleucine inhibits threonine deaminase this way. Mechanism-based inhibition is irreversible: penicillin binds to the active site of bacterial transpeptidases and chemically changes it. Bacteria with altered transpeptidases that penicillin cannot bind are resistant.
Exam questions
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Quick check questions
What is metabolism?
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The complex network of interdependent and interacting chemical reactions occurring in living organisms.
Give one example of an anabolic and one of a catabolic reaction.
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Anabolic: protein synthesis, glycogen formation or photosynthesis. Catabolic: hydrolysis in digestion or oxidation of glucose in respiration.
In the induced-fit model, what changes shape when the substrate binds?
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Both the substrate and the enzyme's active site.
What happens to the activation energy and the overall energy change when an enzyme catalyses a reaction?
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The activation energy is lowered; the overall energy change is unchanged.
HL only Which enzyme does isoleucine inhibit in feedback inhibition?
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Threonine deaminase, the first enzyme of the pathway, at its allosteric site.
Frequently asked questions
Why do enzymes denature at high temperatures?
Enzymes denature at high temperatures because extra vibration breaks the bonds holding the protein's three-dimensional structure. The active site changes shape and chemical properties, so the substrate can no longer bind and no enzyme–substrate complexes form. This is why the rate falls steeply above the optimum temperature, and the change is usually permanent.
What is the induced-fit model of enzyme action?
The induced-fit model says that both the substrate and the enzyme change shape when the substrate binds to the active site. The active site moulds more tightly around the substrate, which stresses bonds in the substrate and lowers the activation energy. After the products leave, the active site returns to its original shape.
Why does the rate of an enzyme reaction level off at high substrate concentration?
The rate levels off because nearly all the active sites are occupied at any moment. Adding more substrate cannot increase the number of successful substrate–active site collisions, so enzyme concentration becomes the limiting factor. At low substrate concentrations, by contrast, more substrate means more frequent collisions and a faster rate.
What is the difference between competitive and non-competitive inhibition?
HL only A competitive inhibitor resembles the substrate and binds reversibly to the active site, so high substrate concentrations overcome it. A non-competitive inhibitor binds reversibly to an allosteric site, changing the shape of the active site, so extra substrate cannot overcome it and the maximum rate falls. Statins are competitive inhibitors.
How does penicillin kill bacteria?
HL only Penicillin binds irreversibly to the active site of transpeptidases, the bacterial enzymes that cross-link peptidoglycan in the cell wall. This mechanism-based inhibition chemically changes the active site, so cross-links cannot form, the wall weakens and growing cells burst. Bacteria with altered transpeptidases that penicillin cannot bind are resistant.
Written and checked against the IB Biology HL specification · Updated October 2026