1. Describe the chemical test for the presence of proteins. Describe what a positive result would be.
Answer
Biuret test. Add Biuret reagent – if protein is present colour turns from blue to violet.
2. What is meant by the primary structure of a protein ?
Answer
The order/sequence of amino acids in the polypeptide chain.
3. Why is the tertiary structure of a polypeptide so important ?
Answer
They allow the protein to have a specific 3D structure. This enables proteins to function as enzymes or other functions where they need to fit with other molecules.
4. Some proteins do not have a quaternary structure. Explain why.
Answer
They only consist of one polypeptide chain.
5. The figure below shows the structure of cysteine. Draw a circle around the R group.
Answer
6. The R group of cysteine is able to form strong bonds with cysteine elsewhere in the molecule. Name these strong bonds and explain what makes them strong.
Answer
The R group of cysteine contains sulfur, which can form disulfide bonds (also called disulfide bridges or sulfur bonds) with another cysteine in the protein. These bonds are strong because they are covalent bonds, making them much stronger than hydrogen bonds or ionic interactions, and they play a key role in stabilizing the protein's structure.
7. Keratin is a protein that has large amounts of the amino acid cysteine in its structure. The form of keratin in hair is a slightly more elastic form of keratin than that found in fingernails, hooves and claws. Using your understanding of protein secondary structure, suggest how these forms of keratin may differ in structure.
Answer
Keratin in hair has fewer disulfide bridges, but more hydrogen bonds and more ionic bonds.
8. A student carried out an experiment with two different samples of amylase, enzyme A and enzyme B. The enzymes were mixed with starch and immediately placed in a water bath at 65°C. The rate of reaction was measured over 30 minutes. The figure below shows the results obtained by the student. Describe the results for these two enzymes.
Answer
The rate of activity of both enzymes falls. Enzyme A decreases at a faster rate initially, falling more steeply. Enzyme A stops working sooner, showing no activity at about 15 minutes, or almost no activity after 10 minutes. Both enzymes show no activity at 30 minutes.
9. Use your knowledge of enzyme structure to explain the results above.
Answer
Both enzymes denature over that period due to heat. Hydrogen and ionic bonds break, changing the tertiary structure. The active site changes shape so it is no longer complementary to the substrate. Enzyme A is less heat‑stable, with more hydrogen bonds, whereas enzyme B has more disulfide bridges.
10. The figure below shows the energy changes during the course of a specific reaction without an enzyme. Sketch a line to show the result you would expect if an enzyme was present.
Answer
11. Describe the induced fit model of enzyme action.
Answer
The active site is not exactly complementary to the substrate. When the substrate enters the active site, the active site adjusts to become a close fit and complementary. Once the products are released, the active site reverts to its original shape.