1.4 Proteins

Question 1

What is the general structure of an amino acid?

Answer 1

• COOH carboxyl / carboxylic acid group
• R variable side group consists of carbon chain and may include other functional groups
• NH2 amine / amino group

Question 2

How many amino acids are there and how do they differ from one another?

Answer 2

20
Differ only by side ‘R’ groups

Question 3

How do dipeptides form?

Answer 3

A condensation reaction between 2 amino acids and eliminates molecule of water

Question 4

How do polypeptides form?

Answer 4

A condensation reaction between 3 or more amino acids forming a peptide bond and eliminates a molecule of water

Question 5

How many levels of protein structure are there?

Answer 5

4

Question 6

Define ‘primary structure’ of a protein

Answer 6

• Sequence, number and type of amino acids in the polypeptide
• Determined by sequence of codons on mRNA

Question 7

Define ‘secondary structure’ of a protein

Answer 7

Hydrogen bonds form between O - (slightly negative) attached to -C=O and H + (slightly positive) attached to -NH

Question 8

Describe the 2 types of secondary protein structure

Answer 8

Alpha helix:
- all N-H bonds on same side of protein shape
- spiral shape
- H-bonds parallel to helical axis

Beta-pleated sheet:
- N-H and C=O groups alternate from one side to the other

Question 9

Define ‘tertiary structure’ of a protein

Answer 9

3D structure formed by further folding of polypeptide
Bonds present:
- Disulfide bridges
- Ionic bonds
- Hydrogen bonds

Question 10

Describe each type of bond in the tertiary structure of proteins

Answer 10

• Disulfide bridges: strong covalent S-S bonds between molecules of the amino acid cysteine
• Ionic bonds: relatively strong bonds between charged R groups (pH changes cause these bonds to break)
• Hydrogen bonds: numerous and easily broken

Question 11

Define ‘quaternary structure’ of a protein

Answer 11

• Functional proteins may consist of more than one polypeptide
• Precise 3D structure held together by the same types of bond as tertiary structure
• May involve addition of prosthetic groups, e.g. metal ions or phosphate groups

Question 12

Describe the structure and function of globular proteins

Answer 12

• Spherical and compact
• Hydrophilic R groups face outwards and hydrophobic R groups face inwards = usually water-soluble
• Involved in metabolic processes, e.g. enzymes and haemoglobin

Question 13

Describe the structure and function of fibrous proteins

Answer 13

• Can form long chains or fibres
• Insoluble in water
• Useful for structure and support, e.g. collagen in skin

Question 14

Outline how chromatography could be used to identify the amino acids in a mixture

Answer 14

1. Use capillary tube to spot mixture onto pencil origin line and place chromatography paper in solvent
2. Allow solvent to run until it almost touches other end of paper. Amino acids move different distances based on relative attraction to paper and solubility in solvent
3. Use revealing agent or UV light to see spots
4. Calculate Rf values and match to database

Question 15

Describe how to test for proteins in a sample

Answer 15

Biuret test confirms presence of peptide bond
1. Add equal volume of sodium hydroxide to sample at room temperature
2. Add drops of dilute copper (II) sulfate solution. Swirl to mix - makes biuret reagent
3. Positive result: colour changes from blue to purple
Negative result: solution remains blue