3.1.4 Proteins

Question 1

What groups do all amino acids have in common?

Answer 1

All amino acids have in common:

• a carboxyl group (-COOH)
• an amine/amino group (-NH2)
• an R group (variable)

Question 2

Give three functions of protein.

Answer 2

Proteins form:

• enzymes (break down and synthesise large molecules)
• antibodies (involved in the immune system)
• channel proteins (transport molecules and ions across membranes)
• structural proteins (form the protective covering)

Question 3

Describe how you would test for the presence of protein in a sample.

Answer 3

• Use the biuret test.
• Add a few drops of sodium hydroxide solution.
• Then add some copper(II) sulfate solution.
  
  • If protein is present the solution turns purple.
  • If there’s no protein, the solution will stay blue.

Question 4

Leucyl-alanine is a dipeptide. Describe how a dipeptide is formed.
[3 marks]

Answer 4

• A peptide bond…
• …forms between carboxyl group of one amino acid and the amino group of the other amino acid.
• A molecule of water is released / a condensation reaction takes place.

Question 5

Myoglobin is a protein formed from a single polypeptide chain. Describe the tertiary structure of a protein like myoglobin.
[2 marks]

Answer 5

• The second structure is coiled and folded further to form the protein’s final 3D structure.
• More bonds, including hydrogen bonds, ionic bonds and disulphide bridges, form between different parts of the polypeptide chain.

Question 6

Describe the primary structure of a protein.

Answer 6

A sequence of amino acids in the polypeptide chain.

Question 7

Describe the secondary structure of a protein.

Answer 7

• Hydrogen bonds form between the amino acids in the chain.
• This makes it automatically:
  
  • coil into an alpha helix.
    or
  • fold into a beta pleated sheet.

Question 8

Describe the tertiary structure of a protein.

Answer 8

• The coiled or folded chain is often coiled and folded further.
• More bonds form between different parts of the polypeptide chain, including hydrogen bonds and ionic bonds.
• Disulfide bridges also form whenever two molecules of the amino acid cysteine come closer together.

Question 9

Describe the quaternary structure of a protein.

Answer 9

Proteins made of several different polypeptide chains held together by bonds.

Question 10

What is an enzyme?

Answer 10

Enzymes are biological catalysts that speed up chemical reactions without being used up in the reaction itself.

Question 11

What is the name given to the amount of energy needed to start a reaction?

Answer 11

Activation energy

Question 12

What do enzymes do during reactions?

Answer 12

Enzymes lower the amount of activation energy that’s needed, often making reactions happen at a lower temperature. This speeds up the rate of reaction.

Question 13

What is an enzyme-substrate complex?

Answer 13

When a substrate fits into the enzymes’s active site it forms an enzyme-substrate complex.

Question 14

Why can an enzyme only bind to one substance?

Answer 14

Enzymes have an active site, which has a specific shape. Only substrates with the right shape can bind to the active site.

Question 15

Describe the ‘lock and key’ model of enzyme action.

Answer 15

Enzymes only bind with substrates that fit their active site.

Question 16

Describe the ‘induced fit’ model of enzyme action.

[4 marks]

Answer 16

• The complementary substrate binds to the active site of the enzyme…
• …to form an enzyme-substrate complex.
• As the substrate binds, the active site changes shape slightly, which provides a better fit.
• The substrate is broken down/joined together to form the product(s).

Question 17

Explain how a change in the amino acid sequence of an enzyme may prevent it from functioning properly.
[2 marks]

Answer 17

• A change in the amino acid sequence of an enzyme may alter its tertiary structure.
• This changes the shape of the active site so that the substrate can’t bind to it.

Question 18

What are the factors affecting enzyme activity?

Answer 18

• Temperature
• pH
• Enzyme concentration
• Substrate concentration

Question 19

When proteins are heated to a high temperature, their tertiary structure is disrupted. Explain how this occurs.
[3 marks]

Answer 19

• Heating increases kinetic energy of atoms
• Atoms in molecules vibrate
• Weaker bonds are broken
• Permanent distortion of shape (change in 3D shape)
• Protein denatures

Question 20

Explain how a competitive inhibitor works.

[3 marks]

Answer 20

• Competitive inhibitor molecules have a similar shape to the substrate molecules.
• They compete with the substrate molecules to bind to the active site of an enzyme.
• When an inhibitor molecule is bound to the active site it stops the substrate molecule from binding.

Question 21

Explain how a non-competitive inhibitor works.

[2 marks]

Answer 21

• Non-competitive inhibitor molecules bind to enzymes away from their active site.
• This causes the active site to change shape so the substrate molecule can no longer fit.

Question 22

Describe how the scientists could obtain data to produce a calibration curve and how they would use the calibration curve to find the concentration of protein in a sample of blood plasma.
[3 marks]

Answer 22

Any three from:

• Produce known concentrations of protein.
• Measure absorbance of each concentration.
• Plot a graph of absorbance on the y-axis against concentration (on the x-axis) and draw a curve.
• Use absorbance of the sample to find protein concentration from the curve.

Question 23

Older people are more likely to suffer from infectious diseases.
Suggest how this may be linked to the decrease in the mean concentration of protein in the blood as people get older.
[1 mark]

Answer 23

(lower plasma protein concentration suggests) fewer antibodies.