Amino acids and protein structureEdexcel A-Level Biology A: Subtopic test

1. 1
   A biochemist mixes two amino acids, glycine and alanine, in solution and adds a catalyst that allows them to join together to form a dipeptide. Both amino acids have the same general structure, with an amino group, a carboxyl group, a hydrogen atom and an R group attached to a central carbon atom.

   (a)

   What is the name of the bond that joins the two amino acids in the dipeptide, and what other product is formed?

   [1 mark]

   • AA peptide bond, and water
   • BA peptide bond, and carbon dioxide
   • CA glycosidic bond, and water
   • DA hydrogen bond, and water

   (b)

   Which groups react together when the two amino acids join to form the dipeptide?

   [1 mark]

   • AThe amino groups of both amino acids
   • BThe R groups of both amino acids
   • CThe carboxyl group of one amino acid and the amino group of the other
   • DThe carboxyl groups of both amino acids

   (c)

   Explain, in terms of the atoms lost from the two amino acids, why the formation of the dipeptide is a condensation reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A globular protein from a bacterium is a single polypeptide chain. Its primary structure contains two cysteine amino acids that are far apart in the sequence. In the folded protein, the sulfur-containing R groups of these two cysteines lie close together. The interior of the protein is made mostly of amino acids with hydrophobic R groups.

   (a)

   Which type of bond is most likely to form between the sulfur-containing R groups of the two cysteines?

   [1 mark]

   • AA peptide bond
   • BA hydrogen bond
   • CAn ionic bond
   • DA disulfide bridge

   (b)

   Which level of protein structure is the three-dimensional folding of this single polypeptide chain?

   [1 mark]

   • APrimary structure
   • BTertiary structure
   • CQuaternary structure
   • DSecondary structure

   (c)

   Explain how the primary structure of this protein determines its tertiary structure.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Haemoglobin is the protein in red blood cells that carries oxygen. A molecule of haemoglobin is made of four polypeptide chains, each folded into a compact shape and each containing a haem group with an iron(II) ion. Haemoglobin is dissolved in the cytoplasm of the red blood cell at a very high concentration. A single red blood cell contains about 2.7 × 10⁸ haemoglobin molecules.

   (a)

   Explain why haemoglobin is a globular protein that is soluble in the cytoplasm of the red blood cell.

   [3 marks]

   (b)

   Calculate the maximum number of oxygen molecules that one red blood cell can carry, and explain how the structure of haemoglobin allows it to carry oxygen.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Tendons join muscles to bones and must resist very large pulling forces without stretching. Tendons are made mainly of the protein collagen, which is secreted by cells and assembled outside them into long fibres. Collagen is insoluble in water.

   (a)

   Describe the structure of a collagen molecule and explain how this structure makes collagen suitable for its function in tendons.

   [6 marks]

   (b)

   Some people inherit a mutation in the gene for collagen so that, at some positions, glycine is replaced by an amino acid with a much larger R group. These people have weaker tendons. Use your knowledge of the structure of collagen to explain why.

   [6 marks]

   Total for question 4: 12 marks

End of questions