1. 1
   Soya protein is widely eaten by vegans. A food technologist fully hydrolyses a purified soya polypeptide of 64 amino acids in a single chain, releasing its free amino acids for analysis. The analysis shows that the polypeptide contains every amino acid that humans are unable to synthesize.

   (a)

   How many water molecules are used in the complete hydrolysis of the polypeptide?

   [1 mark]

   • A32
   • B63
   • C64
   • D65

   (b)

   Which groups are bonded to the alpha carbon atom of every amino acid released?

   [1 mark]

   • AAn amine group, a carboxyl group, a hydrogen atom and an R-group
   • BTwo amine groups and two hydrogen atoms
   • CAn amine group, a phosphate group, a hydrogen atom and an R-group
   • DA carboxyl group, an R-group and two hydrogen atoms

   (c)

   Explain why soya protein is useful in a vegan diet.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ribonuclease A is a small, water-soluble globular enzyme made of a single chain of 124 amino acids, including eight cysteines that form four disulfide bonds. Researchers treated samples of the enzyme separately with a reducing agent that breaks disulfide bonds, with a buffer at pH 2, and with a buffer at pH 7 as a control. They also made a mutant enzyme in which one amino acid with a hydrophobic R-group, normally buried in the core of the enzyme, was replaced by an amino acid whose R-group is negatively charged at pH 7.

   (a)

   Which bonds does the reducing agent break?

   [1 mark]

   • APeptide bonds between adjacent amino acids
   • BHydrogen bonds between the amine and carboxyl groups of the backbone
   • CIonic bonds between positively and negatively charged R-groups
   • DCovalent bonds between the sulfur atoms of the R-groups of pairs of cysteines

   (b)

   Why can the buffer at pH 2 disrupt ionic bonds in the enzyme?

   [1 mark]

   • AAmine groups in R-groups lose hydrogen ions and become negatively charged
   • BDisulfide bonds are converted into ionic bonds
   • CCarboxyl groups in R-groups gain hydrogen ions and lose their negative charge
   • DHydrophobic R-groups gain a positive charge and repel each other

   (c)

   Suggest why the mutant enzyme fails to fold into its normal conformation.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A researcher measured the secondary structure of a purified soluble protein in a dilute solution. At 25 °C, 42% of its amino acids were in alpha helices and 18% in beta-pleated sheets. After heating to 90 °C for five minutes these values fell to 6% and 3%. When the solution was cooled slowly back to 25 °C, the values returned to 40% and 17%, and the protein regained 95% of its original biological activity. When the experiment was repeated with a concentrated solution of the same protein, the protein molecules clumped together at 90 °C and only 20% of the activity returned after cooling.

   (a)

   Explain how alpha helices and beta-pleated sheets are stabilized and why heating reduced the percentage of the protein in these structures.

   [3 marks]

   (b)

   Evaluate the claim that these data show that the primary structure of a protein determines its three-dimensional conformation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Insulin is a small globular hormone made of two polypeptide chains linked by disulfide bonds. Collagen is a fibrous protein made of three long polypeptide chains wound around each other into a rope-like triple helix; almost every third amino acid is glycine, which has the smallest R-group. Haemoglobin is made of four polypeptide chains, each bound to a non-polypeptide haem group containing iron. In sickle-cell disease, one amino acid on the outer surface of each beta chain of haemoglobin, which has a charged R-group, is replaced by one with a hydrophobic R-group. Images obtained by cryogenic electron microscopy show that the altered haemoglobin molecules link together into long, stiff fibres when oxygen concentration is low.

   (a)

   Compare and contrast insulin and collagen, and explain how the form of each protein is related to its function.

   [6 marks]

   (b)

   Using the information about sickle-cell haemoglobin, discuss how the primary structure and R-groups of a protein determine its conformation and function.

   [6 marks]

   Total for question 4: 12 marks

End of questions