Proteins

Question 1

Lysozyme is an enzyme. A molecule of lysozyme is made up of 129 amino acid molecules joined together. In the formation of its active site, the two amino acids that are at positions 35 and 52 in the amino acid sequence need to be close together.

(1)

(ii) Suggest how the amino acids at positions 35 and 52 are held close together to form the active site.

Answer 1

idea that amino acid chain folds / tertiary structure;

named bond holding tertiary structure e.g. ionic disulphide hydrogen;

Question 2

Two proteins have the same number and type of amino acids but different tertiary structures.

Explain why.

Answer 2

Different sequence of amino acids

OR

Different primary structure;

If candidate assumes proteins are the same, accept effect of different pH/ temperature

2. Forms ionic / hydrogen / disulfide bonds in different places;

Question 3

A dipeptide consists of two amino acids joined by a peptide bond. Dipeptides may differ in the type of amino acids they contain.

Describe two other ways in which all dipeptides are similar and one way in which they might differ.

Answer 3

  Amine/NH2 (group at end);

Accept amino/NH3+

2.   Carboxyl/COOH (group at end);

Accept carboxylic / COO−

3.   Two R groups;

4.   All contain C and H and N and O;

Accept examples of different R groups

Difference

5.   Variable/different R group(s);

Question 4

A solution contained a mixture of three different amino acids. A scientist passed an electric current through the solution to separate the amino acids.

She placed a drop of the mixture at one end of a piece of filter paper, attached an electrode to each end of the paper and switched on the current. She switched off the current after 20 minutes and stained the paper to show spots of the amino acids at new positions.

Her results are shown in the diagram.

(c)  Explain what the positions of the spots in the diagram show about these amino acids.

Answer 4

  Moved to negative (electrode) because positive(ly charged);

2.   (Spots move) different distances/rates because (amino acids) different charge/mass;

Accept size for mass.

3.   Two spots (not three) because (amino acids) same charge/mass

OR

One spot has 2 amino acids because (amino acids) same charge/mass;