Proteins

Question 1

When carrying out the buriet test for proteins. Suggest why scientist would de- ionise water?
1 mark

Answer 1

As a control

Question 2

What is a amino acid?

Answer 2

A monomer in which proteins are made

Question 3

What is the structure and function of antibodies?

Answer 3

Two light pp chain (short)
Two heavy pp chain (long)
Bonded together
Found in blood

Immune response.
Have variable regions which bind to antigens invading cells

Question 4

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

Answer 4

Different sequence of amino acids
OR
Different primary structure;
If assumed proteins are the same, accept effect of different pH/ temperature

Forms ionic / hydrogen / disulfide bonds in different places;

Question 5

Describe how a peptide bond is formed between two amino acids to form a dipeptide.
2 marks

Answer 5

Condensation (reaction) / loss of water;

Between amine / NH and carboxyl / COOH;
Accept between amino (group) and carboxylic acid group

Question 6

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.
3 marks

Answer 6

Similarity
Amino group NH3 (group at end)
Carboxyl/COOH (group at end)
Two R groups;
All contain C and H and N and O;

Difference
Variable/different R group(s);

Question 7

When carrying out chromatography with different amino acid using electrodes
explain why they would be in different positions?
3 marks

Answer 7

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

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

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

Question 8

Enzymes are a protein. What is their structure?

Answer 8

Spherical and compact with a tightly folded polypeptide chain

Question 9

Describe how structure of a protein depends on amino acid it contains.
5 marks

Answer 9

1. Structure is determined by (relative) position of amino acid/R
   group/interactions;

Accept for ‘interactions’, hydrogen bonds / disulfide bridges / ionic bonds / hydrophobichydrophilic interactions

2. Primary structure is sequence/order of amino acids;
3. Secondary structure formed by hydrogen bonding (between amino acids);

Accept alpha helix/ß-pleated sheet for ‘secondary structure’

4. Tertiary structure formed by interactions (between R groups);
   Accept for ‘interactions’, hydrogen bonds / disulfide bridges / ionic bonds / hydrophobichydrophilic interactions
5. Creates active site in enzymes
   OR
   Creates complementary/specific shapes in antibodies/carrier proteins/receptor (molecules);
6. Quaternary structure contains >1 polypeptide chain
   OR
   Quaternary structure formed by interactions/bonds between polypeptides;

Accept for intereactions’, hydrogen bonds/ disulfide bridges/ionic bonds/hydrophobichydrophilic interactions
Accept prosthetic (group)

Question 10

A change from Glu (negative charge) to Lys at amino acid 300 had no effect on the rate of reaction catalysed by the enzyme.
The same change at amino acid 279 significantly reduced the rate of reaction catalysed by the enzyme.

Use all the information and your knowledge of protein structure to suggest reasons for the differences between the effects of these two changes.

3 marks

Answer 10

1. Both) negatively charged to positively charged change in amino acid;
2. Change at amino acid 300 does not change the shape of the active site
   OR
   Change at amino acid 300 does not change the tertiary structure OR
   Change at amino acid 300 results in a similar tertiary structure;
   Reference to ‘shape’ of active site only needed once.
3. Amino acid 279 may have been involved in a (ionic, disulfide or
   hydrogen) bond and so the shape of the active site changes
   Or
   Amino acid 279 may have been involved in a (ionic, disulfide or hydrogen) bond and so the tertiary structure changed;
   Amino acid 279 may be in the active site and be required for binding the substrate;

Reference to ‘shape’ of active site only needed

Both parts are required for each mark option.
For ‘a bond’ reject peptide bond.

Question 11

Suggest two variables the biochemist controlled when investigating the effect of temperature on the rate of breakdown of a protein by the protease.
1 mark

Answer 11

Intial/ starting substrate concentration

Enzyme conc

Ph

Question 12

The genetic code is described as degenerate.
What is meant by this?

Answer 12

More than one codon codes for a single amino acid;

Question 13

Describe the primary structure of all proteins.
2 marks

Answer 13

1. Sequence / order of amino acids
2. (Joined by) peptide bonds