Enzyme — a protein that acts as a biological catalyst
Enzyme: a protein that works as a biological catalyst, speeding up one specific reaction without being used up, so a single molecule catalyses that reaction over and over. Four ideas are credited separately: protein, biological catalyst, not used up, specific. Metabolic reactions are the chemical reactions inside living organisms — respiration, photosynthesis, protein synthesis, digestion. Enzyme names end in -ase.
Temperature — a different mechanism on each side of the optimum
The rate-temperature curve is peaked. Below the optimum: heating gives more kinetic energy, so collisions with the active site are more frequent and more enzyme-substrate complexes form each second — reversible, with the active site undamaged. At the optimum: rate is maximum. (Extended) Above it: the enzyme is denatured, the active site changes shape permanently, the substrate no longer fits, and rate falls to zero. Cooling cannot undo it.
(Extended) Active site — complementary shape explains specificity
(Extended) The active site is the region of the enzyme whose 3-D shape is complementary to one particular substrate. The substrate binds there to form a temporary enzyme-substrate complex; the reaction happens, products are released, and the enzyme is left unchanged, free to bind another substrate. Only one substrate shape fits — one key, one lock — so each enzyme catalyses only one reaction, which is why an organism needs many enzymes.
Drawn from real examiner reports.
"Less kinetic energy" above the optimum
Kinetic energy keeps rising as temperature rises, so a fall in rate above the optimum can never be explained by "less kinetic energy" or "fewer collisions". The enzyme has been denatured and its active site no longer fits the substrate. Below the optimum the rise is a reversible speed effect; above it the enzyme itself has stopped working.
pH does not change kinetic energy
Temperature acts through two mechanisms (kinetic energy and collisions below the optimum, denaturation above it). pH acts through one only — active-site shape change and denaturation, on either side of the optimum. Importing "more kinetic energy" into a pH answer scores nothing. Nor is every optimum pH 7: pepsin, in the stomach, works best near pH 2.
Biological catalyst, not chemical
The definition needs the word biological — a catalyst made by, and working inside, living organisms. "Chemical catalyst", "reaction catalyst" or a bare "catalyst" is not credited, because none separates an enzyme from a non-biological catalyst such as the manganese(IV) oxide used on hydrogen peroxide in Chemistry. All enzymes are catalysts; not all catalysts are enzymes.
Flagged Jun 2022 P32 Q4c
Denaturation answers need BOTH ideas
Why activity stops at high temperature is a two-part answer, each part credited separately: (i) the heat changes the shape of the active site — the enzyme is denatured; (ii) the substrate can therefore no longer bind/fit, so no more enzyme-substrate complexes form. Writing only "it is denatured", or only "the substrate cannot fit", scores half the marks.
Flagged Jun 2023 P42 Q4aii · Nov 2022 P43 Q7b
The active site belongs to the enzyme
Only the enzyme has an active site, and only the enzyme is denatured. Answers that give the substrate an active site, or say the substrate has been denatured, are not credited — the substrate's own structure is not changed by the heat in this explanation. Name the enzyme as the owner whenever you use the term "active site".
Flagged Jun 2023 P42 Q4aii · Nov 2022 P43 Q7b
Enzyme names end in -ase
The -ase suffix (amylase, protease, lipase, catalase, maltase) is a two-way clue. In a passage, the word ending in -ase is the enzyme, not the substrate or product — starch is the substrate, amylase the enzyme. Asked to suggest a name, take the substrate and swap its ending: sucrose gives sucrase, maltose gives maltase, lactose gives lactase.
Flagged Nov 2023 P21 Q6
Enzymes contain nitrogen too
Enzymes are proteins, so they contain carbon, hydrogen, oxygen and nitrogen. Nitrogen is persistently left out when candidates list the elements in an enzyme or a protein, which shows the B4 biological-molecules content has not been linked to B5. Carbohydrates and fats contain C, H and O only — nitrogen is the protein marker.
Flagged Nov 2022 P11 Q4 · Nov 2023 P41 Q7ci
Say which side of the optimum you are on
Name the region first — below, at, or above the optimum — then use the matching mechanism: kinetic energy and collisions below it, active-site shape change and denaturation above it (or at any extreme pH). Mixing the two is the commonest way to lose the explanation marks.
Match the command word
"Describe" wants the pattern only — the rate rises to a peak at the optimum, then falls steeply to zero. "Explain" wants that pattern plus the mechanism for that side of the peak. A described graph with no mechanism cannot score the explanation marks.
Finish the chain at the rate
End every mechanism on the number of enzyme-substrate complexes formed per second, and then on the rate of reaction. Answers that stop at "the active site changes shape" leave the final mark unclaimed — the examiner wants the consequence, not only the cause.
Cambridge 0654 spec reference: Section B5 "Enzymes" (Core + Extended). This leaf covers the definition of enzymes as biological catalysts, the effect of temperature and pH on enzyme activity, and (Extended) the active-site/enzyme-substrate-complex model that explains enzyme action and specificity.
Out of scope for B5.1: Michaelis-Menten kinetics, enzyme inhibitors, "induced fit" terminology, and the detailed substrates/products of named digestive enzymes (covered in B7).
| Term | Mark-scheme-precise definition |
|---|---|
| Enzyme | A protein that functions as a biological catalyst in a metabolic reaction, speeding up the rate of a specific reaction without being used up |
| Catalyst | A substance that increases the rate of a reaction without being used up or permanently changed itself |
| Active site (Extended) | The region of an enzyme molecule with a shape complementary to a specific substrate, where the substrate binds |
| Enzyme-substrate complex (Extended) | The temporary structure formed when a substrate molecule binds to the active site of an enzyme |
| Denaturation | A permanent change to the shape of an enzyme's active site (caused by high temperature or extreme pH) so it can no longer bind its substrate |
| Specificity (Extended) | An enzyme normally catalyses only one reaction, because only one substrate shape is complementary to its active site |
Enzyme names conventionally end in "-ase" (amylase, protease, lipase, catalase). Enzymes are proteins, so (linking to B4) they contain carbon, hydrogen, oxygen and nitrogen.
Define enzyme.
(a) Define the term enzyme. (2 marks)
(b) State the type of biological molecule that all enzymes are made of. (1 mark)
(c) Suggest the name of the enzyme that breaks down the substrate "lactose". (1 mark)
