1.4 Proteins

Question 1

What is general structure of an amino acid?

Answer 1

-COOH carboxyl group
-R variable side groups consists of carbon chain & may include other functional groups e.g. benzene ring or -OH group
-NH2 amine group

Question 2

Describe how to test for proteins in a sample

Answer 2

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

Question 3

How many amino acids are there? How do they differ?

Answer 3

20
Differ by R group

Question 4

How do dipeptides and polypeptides form?

Answer 4

• condensation reaction forms peptide bond (-CONH-) & eliminates molecule of water
• Dipeptide: 2 amino acids
• Polypeptide: 3 amino acids

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 & 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 & H (slightly positive) attached to -NH

Question 8

Describe the 2 types of secondary protein structure

Answer 8

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

b-pleated sheet:
- N-H & C=O groups alternate from one side to another

Question 9

Define tertiary structure of a protein. Name bonds present.

Answer 9

3D structure formed by further folding of polypeptide
- disulfide bridges
- ionic bonds
- hydrogen bonds

Question 10

Describe each type of bond in tertiary structure of proteins

Answer 10

• Disulfide bridges: strong covalent S-S bonds between molecules of amino acid cysteine
• Ionic bonds: relatively strong bonds between charged R groups (pH changes cause these bonds to break)
• Hydrogen bonds: numerous & 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 structure and function of globular proteins

Answer 12

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

Question 13

Describe 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