Enzymes are how biology cheats thermodynamics. Knobs and dials on the chemistry of being alive.
Every sub-topic below feeds at least one of these questions.
In what ways do enzymes interact with other molecules?
What are the interdependent components of metabolism?
The required syllabus content for C1.1, in order. Each card is one lesson-sized checkpoint.
Enzymes as catalysts
Because of enzyme specificity, many different enzymes are required by living organisms, and control over metabolism can be exerted through these enzymes.
Anabolic and catabolic reactions
Enzymes as globular proteins with an active site for catalysis
Interactions between substrate and active site to allow induced-fit binding
Role of molecular motion and substrate-active site collisions in enzyme catalysis
Relationships between the structure of the active site, enzyme–substrate specificity and denaturation
Effects of temperature, pH and substrate concentration on the rate of enzyme activity
Measurements in enzyme-catalysed reactions
Effect of enzymes on activation energy
Enzymes speed up specific reactions without being consumed. Without them, the chemistry of being alive would proceed far too slowly to sustain a cell.
Each metabolic reaction is catalysed by a specific enzyme. Because enzymes are specific, the cell can regulate metabolism: by turning specific enzymes on or off, the cell controls which reactions happen and how fast.
All metabolic reactions divide into two groups based on direction.
Smaller molecules → larger ones. Requires energy. Often involves condensation reactions. Examples: protein synthesis (amino acids → polypeptides), glycogen formation (glucose → glycogen), photosynthesis (CO₂ + H₂O → glucose).
Larger molecules → smaller ones. Often releases energy. Hydrolysis reactions are catabolic. Examples: digestion (polymers → monomers), respiration (glucose oxidation).
Older "lock and key" model imagined a rigid active site. The current induced-fit model has both substrate and enzyme adjusting during binding.
The active site is a region of the enzyme — just a few amino acids — but it works because the rest of the protein's 3D structure positions those amino acids exactly. The active site has a complementary shape and chemistry to the substrate.
Collision theory underlies enzyme kinetics. An enzyme catalyses a reaction only when a substrate collides with its active site at the right angle and with enough energy.
Things that increase collisions between substrates and active sites speed up the reaction:
Temperature, pH and substrate concentration each affect enzyme activity in a characteristic, generalisable way.
Low T → low collisions, slow rate. As T rises → rate rises. At optimum (humans: ~37 °C) rate is maximum. Above optimum → enzyme denatures; rate drops sharply.
Each enzyme has an optimum pH. Below or above, R-group charges change → active site shape changes → activity drops. Large pH change → denaturation.
Rate rises with [substrate] (more collisions). Plateaus when all active sites are saturated. Adding more substrate beyond that point doesn't help — you need more enzyme.
These curves are idealised models. Real data from an enzyme experiment will be noisier — but the model predicts the general shape, and lets you evaluate experimental results against it.
Rate of reaction = change in concentration of substrate or product per unit time. In practice, you measure whatever is easiest to observe.
From a graph of product vs time, rate = the gradient of the line. Initial rate (at t = 0) is most accurate — before substrate is depleted.
Every reaction needs activation energy to break old bonds before new ones form. Enzymes lower that barrier without changing the energy of substrate or product.
Enzymes don't change how much energy the substrate or product contains. They change the pathway by which the reaction occurs — providing an alternative route with lower activation energy.
This is why life can happen at 37 °C. Without enzymes, the same reactions would need hundreds of degrees to proceed at biological rates.
An extra 7 sub-topics for HL — same syllabus, deeper mechanism.
Intracellular and extracellular enzyme-catalysed reactions
Generation of heat energy by the reactions of metabolism
Cyclical and linear pathways in metabolism
Allosteric sites and non-competitive inhibition
I have switched the order of C1.1.15 and C.1.1.14 for ease of understanding.
Regulation of metabolic pathways by feedback inhibition
Mechanism-based inhibition as a consequence of chemical changes to the active site caused by the irreversible binding of an inhibitor
Most enzymes work inside the cell that produced them. A specialised subset are secreted to work outside.
Most enzymes. Examples: glycolysis enzymes in cytoplasm, Krebs cycle enzymes in mitochondria, Calvin cycle enzymes in chloroplasts.
Synthesised by ribosomes on rough ER, processed in Golgi, packaged in secretory vesicles, released by exocytosis. Examples: digestive enzymes (amylase, pepsin, trypsin) in the gut lumen.
Heat is an inevitable by-product of metabolism. Endotherms exploit it to maintain constant body temperature.
Every metabolic reaction loses some energy as heat — energy transfers are never 100% efficient (second law of thermodynamics). For ectotherms this is just a loss. For endotherms (mammals, birds), it's their primary source of body heat — used to maintain a constant internal temperature regardless of the environment. A polar bear at -40 °C is still 37 °C inside.
Some metabolic pathways are linear chains; others are cycles. The IB names one of each from respiration and one cycle from photosynthesis.
10 enzyme-catalysed steps from glucose to two pyruvates. Each product is the next reaction's substrate.
Eight enzyme-catalysed steps that return to the starting point (oxaloacetate). Acetyl-CoA enters; CO₂, NADH, FADH₂ and ATP are produced.
The light-independent reactions of photosynthesis. CO₂ is fixed and reduced to glucose; the starting compound (RuBP) is regenerated.
Inhibitors bind to enzymes and reduce their activity. Two main types — distinguished by where they bind and how their effect responds to substrate concentration.
Inhibitor has a similar shape and chemistry to the substrate — competes for the active site. Binding is reversible. Effect: reduces rate. Adding more substrate reduces inhibition (substrate outcompetes inhibitor for the active site). At very high [substrate], rate approaches the un-inhibited maximum.
Inhibitor binds to a separate site (the allosteric site). Binding causes the enzyme to change shape, distorting the active site. Effect: reduces the number of functioning enzymes. Adding more substrate doesn't relieve inhibition — the affected enzymes are non-functional.
Cholesterol biosynthesis runs through the enzyme HMG-CoA reductase, which converts HMG-CoA to mevalonic acid. Statins resemble HMG-CoA and competitively bind to the same active site, blocking it. Mevalonic acid production drops → cholesterol production drops → blood cholesterol falls.
A common regulatory motif: the final product of a metabolic pathway non-competitively inhibits the first enzyme of the pathway. Stops oversupply.
Example: the pathway converting threonine to isoleucine has multiple enzyme-catalysed steps. The end product (isoleucine) is a non-competitive inhibitor of threonine deaminase — the first enzyme in the pathway. When isoleucine accumulates, it binds threonine deaminase's allosteric site, shutting down the pathway. As isoleucine is consumed, it dissociates from the allosteric site and the pathway resumes.
Some inhibitors bind irreversibly — they chemically modify the active site, killing the enzyme permanently. Penicillin is the textbook example.
Penicillin enters the active site of bacterial transpeptidase — the enzyme that cross-links peptidoglycan in bacterial cell walls. Once inside, penicillin covalently binds the active site, permanently inactivating the enzyme. Cell wall building stops; the bacterial cell bursts under osmotic pressure.
Resistant bacteria produce a modified transpeptidase whose active site no longer binds penicillin. The bacterium can still build cell walls; penicillin no longer kills it. This is one of the routes to antibiotic resistance.
If you can't define one of these in a sentence, that's where to revise next.
“What are examples of structure–function relationships in biological macromolecules?”
“What biological processes depend on differences or changes in concentration?”