3.6- Structure Of Proteins

Question 1

What are Proteins made up of?

Answer 1

Peptides are polymers made up of amino acid molecules.
Proteins consist of one or more polypeptides arranged as complex macromolecules with specific biological functions.

Contain carbon, hydrogen, oxygen, nitrogen.

Question 2

Amino acids

Answer 2

All amino acids have the same basic structure- a carboxyl group (-COOH) and an amino group (-NH2) attached to a carbon atom.
The difference between different amino acids is the R-group (variable group).

Question 3

Explain the formation of peptide bonds.

Answer 3

Amino acids join when the hydroxyl in the carboxylic acid group of one amino acid reacts with a hydrogen in the amine group of another.
A peptide bond is formed between the amino acids and water is produced = condensation reaction.
Resulting compound = dipeptide.

Question 4

What is the importance of the R-group?

Answer 4

• gives amino acids the ability to interact (form bonds) with other amino acids in specific ways.
• results in complex structures as proteins fold in different ways.

Question 5

What is the first level of protein structure?

Answer 5

1. Primary structure:
   - the sequence in which amino acids are joined.
   - directed by info carried within DNA.
   - the amino acids in the sequence will influence how it folds which therefore determines its function.
   - peptide bonds (only bond involved at thus stage)

Question 6

What is the second level of protein structure?

Answer 6

2. Secondary structure:
   - the O H and N atoms of the amino acids interact.
   - hydrogen bonds may form causing the chain to either coil into an alpha helix or fold into a beta pleated sheet.

Question 7

What is the third level of protein structure?

Answer 7

3. Tertiary structure:
   -the folding of a protein into its final shape, often includes the secondary structure.
   -folding/coiling brings R-groups close enough to interact so further folding occurs.
   -following interactions happen:
   > hydrophilic/ hydrophobic interactions (weak interactions between polar + non-polar R-groups)
   > hydrogen bonds (weakest bonds formed)
   > ionic bonds ( form between oppositely charged R-groups)
   > disulfide bonds ( covalent and form between R-groups containing sulfur atoms)

Question 8

What is the final level of protein structure?

Answer 8

4. Quaternary structure:
   - results from the association of 2 or more individual proteins (subunits).
   - the interactions between subunits are the same as in the tertiary structure except that they are between different protein molecules rather than within one molecule.

Question 9

How does the breakdown of peptides take place?

Answer 9

Hydrolysis reaction- reforms the amine and carboxylic acid groups.
Proteases catalyse the reverse reaction.

Question 10

How are proteins identified?

Answer 10

Biuret test:

1. Mix liquid sample with equal volume of 10% NaOH.
2. Add drops of 1% copper sulfate until it turns blue.
3. Leave to stand for 5 mins.

Peptide bonds form violet coloured complexes with copper ions in alkaline solutions.
Biuret reagent= mixture of alkali + copper sulfate