ProteinsEdexcel A-Level Biology B: Subtopic test

1. 1
   A biochemist synthesises a short polypeptide in the laboratory by joining amino acids together one at a time. She is checking that the bonds formed are the same as those found in proteins made in cells.

   (a)

   Which groups react together when a peptide bond forms between two amino acids?

   [1 mark]

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

   (b)

   How many water molecules are released when a single unbranched polypeptide of 50 amino acids is formed?

   [1 mark]

   • A50
   • B49
   • C51
   • D100

   (c)

   Explain what is meant by the primary structure of the polypeptide and why a change in the primary structure could change the function of the protein.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Haemoglobin is the protein in red blood cells that carries oxygen. It is soluble in the cytoplasm of the red blood cell and each molecule can carry up to four oxygen molecules.

   (a)

   Which statement describes the quaternary structure of haemoglobin?

   [1 mark]

   • AA single polypeptide coiled into an α-helix
   • BThe sequence of amino acids in one polypeptide
   • CFour polypeptide chains, each with a haem group, associated into one molecule
   • DThree polypeptides wound together into a rope-like triple helix

   (b)

   Which bond between the R groups of two cysteine residues is a strong covalent bond that helps to hold the tertiary structure of a protein?

   [1 mark]

   • ADisulfide bond
   • BHydrogen bond
   • CIonic bond
   • DPeptide bond

   (c)

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

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Tendons connect muscle to bone and must resist large pulling forces without stretching. The main protein in a tendon is collagen, which is a fibrous protein, whereas haemoglobin in the blood is a globular protein.

   (a)

   Explain how the structure of collagen makes it suitable for its role in a tendon.

   [3 marks]

   (b)

   Compare the structure of collagen with the structure of haemoglobin.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A protein chemist purifies a globular enzyme made of a single polypeptide chain 150 amino acids long. She wants to understand how the chain folds into its functional three-dimensional shape and how this could be altered.

   (a)

   Describe the four levels of protein structure and explain the roles of hydrogen, ionic and disulfide bonds in the folding of the polypeptide.

   [6 marks]

   (b)

   A mutation replaces a cysteine in the polypeptide with an amino acid that has a hydrophobic R group. Explain how this could affect the function of the enzyme.

   [6 marks]

   Total for question 4: 12 marks

End of questions