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Enzymes: Class 11 Biology
What enzymes are, how they lower activation energy, the lock and key and induced fit models, factors affecting enzyme rate, cofactors, inhibitors, enzyme classification and uses of enzymes in industry.
Key points
Enzymes are biological catalysts. They are globular proteins that speed up chemical reactions in living things without being used up. Nearly every reaction in a cell needs an enzyme, because without it the reaction would be too slow for life.
A chemical reaction needs a minimum amount of energy to start, called the activation energy. Enzymes lower the activation energy by providing an alternative route for the reaction. They do not change the final amount of product or the energy difference between substrate and product. They only make equilibrium reached faster.
An enzyme has an active site, a small pocket formed by the folding of the protein chain. Only a substrate with a complementary shape can fit in it, which is why enzymes are specific. The substrate binds to form an enzyme substrate complex. The reaction takes place, the products leave, and the enzyme is free to be used again.
Models of enzyme action:
- Lock and key model (Fischer): the active site has a fixed shape that exactly matches the substrate, like a key in a lock.
- Induced fit model (Koshland): the active site is flexible. When the substrate binds, the active site changes shape slightly to fit it better and strains the bonds of the substrate. This model explains observations better and is the one accepted today.
Properties of enzymes: they are needed in small amounts, they are specific, they are not changed by the reaction, they work reversibly in many cases, and they are sensitive to temperature and pH. Most enzymes have names ending in -ase, such as amylase, lipase and catalase.
Factors affecting the rate of enzyme action:
- Temperature: as temperature rises, particles gain kinetic energy, collide more often and the rate increases up to an optimum (about 37 °C for human enzymes). Above the optimum the enzyme loses its shape and the active site no longer fits. This is denaturation, and it is permanent.
- pH: each enzyme has an optimum pH. Pepsin works best at about pH 2 and salivary amylase at about pH 7. Extreme pH changes the charges on amino acids, breaks the bonds that hold the shape and denatures the enzyme.
- Substrate concentration: the rate rises as substrate increases, then levels off when all active sites are busy, so the enzyme is the limiting factor.
- Enzyme concentration: with plenty of substrate, the rate is directly proportional to the amount of enzyme.
Many enzymes need a non-protein helper to work. A cofactor is an inorganic ion such as Mg²⁺, Zn²⁺ or Cl⁻. A coenzyme is a small organic molecule, and many are derived from vitamins, such as NAD from niacin. A tightly bound helper is a prosthetic group. An enzyme with its helper is a holoenzyme, and the protein part alone is the apoenzyme.
Enzyme inhibition:
- Competitive inhibitor: has a shape like the substrate and competes for the active site. The effect can be reduced by adding more substrate.
- Non-competitive inhibitor: binds at another place called the allosteric site, changes the shape of the active site and cannot be overcome by adding more substrate.
- Feedback (end product) inhibition: the final product of a pathway inhibits an earlier enzyme, which prevents waste of materials.
Enzymes are classified by the type of reaction they catalyse into six groups: oxidoreductases (oxidation and reduction), transferases (transfer of a group), hydrolases (hydrolysis using water), lyases (removal of groups without water), isomerases (rearrangement within a molecule) and ligases (joining of molecules using ATP).
Enzymes are used in industry. Amylase converts starch to sugar in brewing and baking, proteases in biological washing powders remove stains, pectinase clears fruit juice, rennin helps make cheese, and immobilised lactase makes lactose free milk. Immobilised enzymes can be reused and do not contaminate the product.
Study tip: Practise sketching the three standard graphs: rate against temperature, pH and substrate concentration, and label the optimum and the plateau. In explanation questions link three steps: the change, the effect on the active site, and the effect on the rate. Use the words active site, complementary, enzyme substrate complex and denatured correctly.
Definitions and flashcards
Say the meaning first, then open the card to check.
Catalyst
A substance that speeds up a reaction and is not used up.
Substrate
The molecule on which an enzyme acts.
Active site
The region of an enzyme where the substrate binds.
Activation energy
The energy needed to start a chemical reaction.
Denaturation
Permanent change in the shape of a protein caused by heat or extreme pH.
Optimum
The temperature or pH at which an enzyme works fastest.
Coenzyme
An organic non-protein molecule that helps an enzyme work.
Allosteric site
A site on an enzyme, away from the active site, where a regulator binds.
Immobilised enzyme
An enzyme fixed to an inert material so that it can be reused.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Why are enzymes called biological catalysts? (2 marks)
They are made by living cells and they speed up chemical reactions without being used up. They work by lowering the activation energy.
What is meant by the specificity of an enzyme? (2 marks)
An enzyme acts on only one substrate or one type of reaction. This is because its active site has a shape complementary to that substrate only.
Explain why enzyme activity falls at high temperature. (3 marks)
Heat gives the enzyme molecule too much vibrational energy. The bonds that hold the folded protein break and the shape of the active site changes. The substrate can no longer fit, so the enzyme is denatured and the rate falls. The change is not reversible.
Differentiate between competitive and non-competitive inhibitors. (3 marks)
A competitive inhibitor looks like the substrate and fits in the active site, so it blocks the substrate. Adding more substrate reduces its effect. A non-competitive inhibitor binds at a different site and changes the shape of the active site. More substrate cannot remove its effect.
Describe the induced fit model of enzyme action and say how it differs from lock and key. (5 marks)
In the induced fit model the active site is not rigid. When the substrate approaches, the enzyme changes shape slightly so the active site moulds around the substrate. This forms the enzyme substrate complex and puts strain on the bonds of the substrate, which lowers the activation energy. The products are released and the enzyme returns to its original shape. In the lock and key model the active site has a fixed shape that already matches the substrate exactly. Induced fit explains how an enzyme can act on related substrates and why binding triggers catalysis.
Explain how substrate concentration affects the rate of an enzyme catalysed reaction. (3 marks)
At low substrate concentration the rate rises in proportion to substrate, because many active sites are free. As more substrate is added, the active sites fill up. At the saturation point all active sites are occupied, so the rate becomes constant. Further increase in substrate has no effect unless more enzyme is added.
Common mistakes
- Saying that high temperature kills enzymes. Enzymes are not alive, they are denatured.
- Saying that cold temperature denatures an enzyme. Cold only slows the reaction, and the enzyme works again on warming.
- Writing that enzymes are used up in the reaction.
- Saying that the enzyme changes shape of the substrate to start the reaction without mentioning the active site and enzyme substrate complex.
- Drawing the rate against temperature graph as a flat line after the optimum. The rate must fall steeply to zero.