- Define and explain Enzymes in Depth in your own words
- Use key terms such as active site accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Enzymes in Depth: explain specificity, the induced-fit model, and the effects of pH and temperature.
Explain specificity, the induced-fit model, and the effects of pH and temperature.
Key ideas
Enzymes and DNA replication
Enzymes are specific because each active site has a shape complementary to one substrate — the induced-fit model adds that the active site moulds slightly around the substrate. Rate rises with temperature and substrate concentration until the enzyme denatures or becomes saturated; each enzyme has an optimum pH. DNA replicates semi-conservatively: the double helix unwinds, each strand acts as a template, and free nucleotides pair by complementary base pairing (A–T, C–G) as DNA polymerase builds the new strands.
The molecules of life
Carbohydrates (sugars and starch) provide energy and structure; lipids store energy and form membranes; proteins do the work of the cell as enzymes, carriers and structural fibres; nucleic acids (DNA and RNA) store and transmit genetic information. Each can be identified by a food test: Benedict's reagent turns orange-red with reducing sugars, iodine turns blue-black with starch, Biuret reagent turns purple with protein, and the emulsion test gives a milky layer with lipids.
Key term — active site: The region of an enzyme with a shape complementary to its substrate, where the reaction is catalysed.
An enzyme-controlled reaction proceeds at 4 arbitrary units at 20 °C. Assuming the simplified Q10 model, where the rate doubles for every 10 °C rise before denaturation, predict the rate at 30 °C and at 50 °C if the enzyme denatures at 45 °C.
From 20 °C to 30 °C is a 10 °C rise, so the rate doubles: 4 × 2 = 8 units. From 30 °C to 40 °C the rate would double again, but at 45 °C the enzyme denatures. Denaturation breaks the bonds holding the tertiary structure, changing the active site shape so the substrate no longer fits.
Answer: 8 units at 30 °C. At 50 °C the rate is near zero because the enzyme has denatured and can no longer bind its substrate.
- Saying high temperature kills enzymes Enzymes are not alive, so they cannot be killed. High temperatures denature them by disrupting the bonds that hold their 3D shape.
- Claiming both DNA strands are brand new after replication Replication is semi-conservative: each daughter molecule keeps one original strand and gains one new strand.
Practice
Carbohydrates – energy source (e.g. glucose) or structure (cellulose); lipids – energy storage and membranes; proteins – enzymes, transport and structure; nucleic acids – storing and transmitting genetic information.
Primary structure is the sequence of amino acids in the polypeptide chain. Tertiary structure is the overall three-dimensional folding of that chain, held by hydrogen, ionic and disulfide bonds.
35 °C is 10 °C higher, so the rate doubles: 6 × 2 = 12 units.
Hydrogen and ionic bonds in the enzyme are disrupted, changing the shape of the active site so the substrate no longer fits precisely; the enzyme is denatured and the rate falls.
Quick check
Which of these best defines "active site"?
Describe the result of a positive Biuret test and what it shows.
- Enzymes in Depth: explain specificity, the induced-fit model, and the effects of pH and temperature.
- Enzymes and DNA replication: Enzymes are specific because each active site has a shape complementary to one substrate — the induced-fit model adds that the active site moulds slightly around the substrate.
- monomer: A small molecule that joins with others to form a polymer; for example, amino acids are the monomers of proteins.
- Watch out for: saying high temperature kills enzymes